Modern Concept Science and Technology 10

346 ClassifiCation of ElEmEnts Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 34711. Groups : In the modern periodic table, there are a total of 18 vertical columns (IUPAC system), which are called groups. In a group, elements having similar properties are placed one below the other. 12. Sub-shell : The regions in the orbits of an atom where the probability of finding an electron is maximum are called subshells or orbitals. 13. Aufbau principle : Aufbau principle states, \"The filling of electrons always occurs from the sub-shells of lower energy level to the sub-shells of higher energy level. 14. s-Block elements : The elements whose last electron enters the s-orbital of the outermost shell are called s-Block elements. 15. p-Block elements: The elements whose last electron enters the p-orbital of the outermost shell are called p-Block elements. 16. d-Block elements : Elements whose last electron enters the d-orbital are called d-Block elements. 17. f-Block elements : Elements whose last electron enters the f-orbital are called f-Block elements.18. Atomic radius : Atomic radius is the distance between the outermost shell and the center of the nucleus of an atom.19. Ionization energy : The amount of energy required to remove the outermost loosely bound electron of an isolated gaseous atom is called ionization energy. 20. Valence electrons : The electrons present in the outermost shell (or valence shell) of an atom are called valence electrons.21. Valency : The total number of electron(s) gained, lost or shared by atoms during a chemical reaction is called valency. 22. Electronegativity : The amount of energy required for an atom to attract the outer electron of another atom is called electronegativity (EN).IntroductionIn the history of chemistry, grouping of elements into blocks, periods, and groups based on their properties is considered a big achievement. Previously very few elements were discovered. So, individual study of these elements was possible. With the development of science and human knowledge, many new elements were discovered. Thereafter, the study of an individual element became difficult. To make the study easy, fast and clear, chemists felt the need of grouping these elements. About 118 elements have been discovered till today and they have similarities as well as dissimilarities among one another. So, the grouping of elements on the basis of their similarities and dissimilarities is the called classification of elements. Many chemists have made attempts to classify the known elements. Among them Antoine Lavoisier was the first person who classified all the 32 known elements into metals and nonmetals. Since then many systematic classifications have been given by various scientists.Periodic Table The table, or chart, which is obtained after arranging elements on the basis of similarities and dissimilarities is called the periodic table. It is called the periodic table because elements having similar characteristics are repeated after a certain interval in the table. In the periodic table, there are horizontal rows, i.e., periods and vertical columns, i.e., groups.

348 ClassifiCation of ElEmEnts Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 349Mendeleev's Periodic LawRussian chemist Dmitri Ivanovich Mendeleev studied the physical and chemical properties of 63 known elements and their compounds. After the study, he arranged all the known elements on the basis of increasing atomic weights. As a result, he found that elements with similar properties occur at regular intervals. On the basis of the above observation, Mendeleev formulated the periodic law in 1869 AD. According to Mendeleev's periodic law, \"The physical and chemical properties of elements are a periodic function of their atomic weights.\" Periodic function means, if elements are arranged in the order of increasing atomic weights, the properties of the elements go on changing with their atomic weights. But after a certain interval, they repeat the properties of the previous elements and fall one below another in the same vertical column. Mendeleev's Periodic TableWith the help of above mentioned periodic law, Mendeleev arranged elements according to increasing atomic weights. Hence, he found a table known as Mendeleev's periodic table. In Mendeleev's periodic table, elements are classified into horizontal rows called periods and vertical columns called groups. Elements with similar properties were in the same group and elements with gradual change in properties were in the same period. In Mendeleev's periodic table, the periods 4, 5 and 6 were divided into two halves to place more elements. Half of the elements were placed in the upper left corner and half of the elements were placed in the lower right corner of each box.Many elements were not discovered when Mendeleev formed the periodic table. So, he left some gaps for the undiscovered elements and proposed names for them, e.g., Eka–Boron, Eka–Aluminium, Eka–Silicon, Eka–Manganese, etc.Fig: A part of Mendeleev's periodic table

348 ClassifiCation of ElEmEnts Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 349Merits of Mendeleev's periodic table1. Mendeleev's periodic table was the first scientific and systematic study of all known 63 elements.2. In Mendeleev's periodic table, there were many gaps for undiscovered elements. After their discovery, they got their proper place in the table.3. Mendeleev's periodic table helped to correct the doubtful atomic weights of some elements on the basis of their position in the periodic table, e.g., gold (Au), platinum (Pt), uranium (U), etc.4. With the help of Mendeleev's periodic table, properties of several elements were studied correctly.Demerits of Mendeleev's periodic table1. Mendeleev could not arrange hydrogen properly because it shows the properties of both alkali metals and halogens.2. Mendeleev could not arrange isotopes because they have different atomic weights but the same atomic number, e.g., 12 C⁶, 13 C⁶, 14 C⁶.3. Mendeleev could not arrange Lanthanides and Actinides.4. In Mendeleev's periodic table, some elements do not obey the periodic law. In some places, elements having more atomic weight were placed before those of less atomic weight. For example, 58.9Co 27 was placed before 58.6 Ni 28.5. In Mendeleev's periodic table, more reactive alkali metals (Li, Na, K, etc.) and less reactive coinage metals (Cu, Ag, Au) were placed together in the same group.6. The eighth group of Mendeleev's periodic table was not divided into sub-groups.7. Inert gases or noble gases were absent.Modern Periodic LawA group of chemists led by Henry Moseley studied the chemical and physical properties of all known elements. They found that properties of elements depended more correctly on the atomic number than on atomic weight. Moseley said that atomic number is the fundamental property of the atom. Afterward, they proposed a new law called the modern periodic law.Modern periodic law states that, \"Physical and chemical properties of the elements are a periodic function of their atomic numbers.\" Modern Periodic TableAfter the discovery of the modern periodic law, elements were arranged on the basis of increasing atomic numbers. As a result, they found that elements having similar properties fall one after another in the same vertical column called group. Elements having gradual change in the properties were in the horizontal row called period. Hence, a more correct table than Mendeleev's was obtained, which is known as the modern periodic table. So, \"The table which is obtained after arranging elements on the basis of increasing atomic numbers is called the Modern Periodic Table.\" Correction of Defects of Mendeleev's Periodic Table by Modern Periodic TableIn the Modern periodic table, elements have been arranged on the basis of increasing atomic numbers. As a result, all the defects of Mendeleev's periodic table were removed. The defects

350 ClassifiCation of ElEmEnts Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 351and their reasonable corrections are given below. 1. Position of Hydrogen: Since hydrogen has the least atomic number, i.e., 1, it is kept in group 1 of the modern periodic table, but its position is still controversial due to its dual characteristics, i.e., characteristics of group 1 and group 17.2. Position of Isotopes: Isotopes are elements having the same atomic number but different atomic weights, e.g., C, C, C 126136146 . So, without any doubt, all isotopes of one element are kept in one place. 3. Position of Lanthanides and Actinides: Elements of Lanthanide series and elements of Actinide series are kept below the main block of the periodic table as they have different properties from other elements. 4. Correction of Periodic Law: Some elements do not obey Mendeleev's periodic law, but when they are arranged according to atomic number they obey the modern periodic law. For example, Ar 39.918 is placed before K 39.119 , Co 58.927 is placed before Ni 58.618 .5. Position of Alkali metals and Coinage metals: In the modern periodic table, alkali metals (Li, Na, K, etc.) are kept in group IA and coinage metals (Cu, Ag, Au) are in group IB.6. In the modern periodic table, the eighth group is divided into three vertical columns.Differences between Modern Periodic Table and Mendeleev's Periodic TableS.N. Modern Periodic Table S.N. Mendeleev's Periodic Table1. Position of isotopes is clear because all of them are kept in same cell due to same number of protons. 1. Position of isotopes in not clear. 2. In this table, there are eighteen vertical columns. 2. In this table, there are eight vertical columns. 3. Modern periodic table is made based on increasing atomic number of the elements. 3. Mendeleev's periodic table is based on increasing atomic weight of the elements. Position of elements in modern periodic table1. Position of hydrogenHydrogen having the least atomic number, i.e., 1 is placed in group 1 of the modern periodic table. But it also shows the properties of group 17 of the periodic table.FACTS WITH REASONSHydrogen is Kept at the top of the IA (1) group. Hydrogen is kept at the top of the IA (1) group because its atomic number is 1. It also loses 1 electron similar to alkali metals. Alkali metals are more reactive than other metals in their period.Alkali metals are more reactive than other metals in their period because they are the largest metal atoms in their period. Their valance electrons are farther than the other elements of the period. They have more electro positivity and less electronegativity. So, they lose electrons easily and react faster.

350 ClassifiCation of ElEmEnts Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 351

352 ClassifiCation of ElEmEnts Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 3532. Position of metalsMetals are placed on the left side of the modern periodic table. For example, Alkali metals (Li, Na, K, Rb, Cs, and Fr) are kept in group 1(IA). Alkaline earth metals (Be, Mg, Ca, Sr, Ba and Ra) are kept in group 2(IIA). The metals of group 1 are called alkali metals because they react with water and produce alkalis. Similarly, the metals of group 2 are called alkaline earth metals because they are found on the surface of the earth in the form of oxides and react with water to produce alkalis. 3. Position of non-metalsNon-metals are placed on the right side of the modern periodic table. For example, halogens (F, Cl, Br, I and At) are placed in group 17. The elements of group 17 are called halogens because they react with hydrogen to form acids, from which simple salts like NaCl, KCl, KBr, etc. can be produced. 4. Position of metalloidsMetalloids (like Si, Ge, As, Te, etc.) are placed in between metals and non-metals. 5. Position of Lanthanides and ActinidesElements of Lanthanides and Actinides series, having different properties from other elements of the periodic table, are kept in f-Block below the main table in a separate box. The 15 radioactive, inner-transition elements from cerium (58Ce) to lutetium (71Lu) along with lanthanum (57La) are called Lanthanides and other 15 radioactive, innertransition elements from thorium (90Th) to lawrencium (103Lr) along with actinium (89Ac) are called Actinides. 6. Position of inert gasesInert gases (He, Ne, Ar, Kr, Xe and Rn) are kept in the zero group (18) at extreme right side of the periodic table. These elements have zero valency as they have complete octet or duplet in their valence shell.Characteristics of Modern Periodic Table1. Modern periodic table is based on increasing atomic number.2. In modern periodic table there are seven horizontal rows called periods and 18 vertical columns called groups.3. In modern periodic table elements from group IA to group VIIA are called normal elements or representative elements.4. In modern periodic table, metals are kept in left hand side, non-metals are kept in right hand side and metalloids are kept in between them.5. In modern periodic table inert gases are kept at the extreme right hand side of the table (zero group or 18).6. In modern periodic table lanthanides and actinides are kept below the main table.7. In modern periodic table most reactive alkali metals are kept in IA group and most reactive non-metals (halogens) are kept at VIIA group. 8. Transitional metals (elements from IB to VIIB and VIII ) are kept between metals and non-metals.

352 ClassifiCation of ElEmEnts Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 353Periods and Groups of Modern Periodic Table PeriodsIn the modern periodic table, there are seven horizontal rows, which are called periods. In a period, elements having gradual change in properties are placed one after another. According to the number of elements, periods of the modern periodic table are divided into four groups. They are very short, short, long and very long.S.N. Period Number of elements Types of period1. First 2 Very short period 2. Second 8 Short period 3. Third 8 Short period4. Fourth 18 Long period 5. Fifth 18 Long period 6. Sixth 32 Very long period 7. Seventh 32 Very long periodGroupsIn the modern periodic table, there are a total of 18 vertical columns (IUPAC system), which are called groups. In a group, elements having similar properties are placed one below the other. In the modern periodic table, representative elements are kept in groups 1, 2, 13, 14, 15, 16 and 17. These elements are called representative elements, or normal elements, as they have only one incomplete outer shell. Noble gas elements are kept in 0 (zero) group of the Modern periodic table. Similarly, transitional elements are kept in groups 3, 4, 5, 6, 7, 8, 9, 10, 11 and 12. Note: The groups are written in two different methods. They are given in the table.1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18IA IIA IIIB IVB VB VIB VIIB VIIIB VIIIB VIIIB IB IIB IIIA IVA VA VIA VIIA 0Differences between Periods and Groups.S.N. Periods S.N. Groups1. Horizontal rows of the periodic table are called periods. 1. Vertical columns of the periodic table are called groups. 2. In periods, the atomic size of elements decreases while moving from left to right. 2. In groups, the atomic size of elements increases while moving from top to bottom. 3. Valency of elements increases first (upto group IV) and then decreases (upto group 0).3. Valency of elements remains the same in a group. 4. The elements of the same period have different properties.4. The elements of the same group have similar properties.

354 ClassifiCation of ElEmEnts Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 355Sub-shell After the scientific research, it is found that the main shell is not the ultimate structure of an atom. It has one or more sub-shells. It means that each main shell contains one or more than one sub-shell, which are denoted by s, p, d and f. So, sub-shell is a region of orbit where probability of finding an electron is maximum.The main shells along with their sub-shells are listed in the given box.Main Shells Sub-shells (orbitals)K (n=1) sL (n=2) s and p M (n=3) s, p and dN (n=4) s, p, d and fThe maximum number of electrons that can be accommodated by each sub-shell is given below: Sub-shells (orbitals) Maximum number of electronss (sharp) 2p (principal) 6d (diffuse) 10f (fundamental) 14The above data shows that K shell (n = 1) contains only one sub-shell (1s) with maximum two electrons. L shell (n = 2) contains two sub-shells (2s and 2p) with maximum eight electrons. M shell (n = 3) contains three sub-shells (3s, 3p and 3d) with maximum eighteen electrons. N shell (n = 4) contains four sub-shells (4s, 4p, 4d and 4f) with maximum thirty two electrons. The last electron present in a sub-shell determines the block of an element. Aufbau PrincipleThis principle was given by Wolfgang Pauli and Niels Bohr in the early 1920s. The different sub-shells of an atom have different energy. Electrons always try to enter the sub-shell which has less energy. Aufbau principle states, \"the electrons in an atom are so distributed that they occupy shells in the order of their increasing energy.\"It means that the shells having low energy are filled faster than the shells having high energy in the following sequence.1s<2s<2p<3s<3p<4s<3d<4p<5s<4d<5p<6s<4f<5d<6p<7s<5f<6d<7p.1s2s 2p3s 3p 3d4s 4p 4d 4f5s 5p 5d 5f6s 6p 6d7s 7pMEMORY TIPSAufbau principle describes that electrons goes first to that orbit and sub-shell which has less energy level.

354 ClassifiCation of ElEmEnts Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 355Electronic configuration of some elements on the basis of sub-shells (s, p, d, and f) is given below:S.N. Element Atomic no.Electronic configuration based on shells (K, L, M, N)Electronic configuration based on sub-shells (s, p,d,f)Valency1 Hydrogen 1 1 1s1 12 Helium 2 2 1s2 03 Lithium 3 2, 1 1s2, 2s1 14 Beryllium 4 2, 2 1s2, 2s2 25 Boron 5 2, 3 1s2, 2s2 2p1 36 Carbon 6 2, 4 1s2, 2s2 2p2 47 Nitrogen 7 2, 5 1s2, 2s2 2p3 38 Oxygen 8 2, 6 1s2, 2s2 2p4 29 Fluorine 9 2, 7 1s2, 2s2 2p5 110 Neon 10 2, 8 1s2, 2s2 2p6 011 Sodium 11 2, 8, 1 1s2, 2s2 2p6, 3s1 112 Magnesium 12 2, 8, 2 1s2, 2s2 2p6, 3s2 213 Aluminium 13 2, 8, 3 1s2, 2s2 2p6, 3s2, 3p1 314 Silicon 14 2, 8, 4 1s2,2s2 2p6, 3s2, 3p2 415 Phosphorus 15 2, 8, 5 1s2,2s2 2p6, 3s2, 3p3 3, 516 Sulphur 16 2, 8, 6 1s2,2s2 2p6, 3s2, 3p4 2, 617 Chlorine 17 2, 8, 7 1s2,2s2 2p6, 3s2, 3p5 118 Argon 18 2, 8, 8 1s2,2s2 2p6, 3s2, 3p6 019 Potassium 19 2, 8, 8, 1 1s2,2s2 2p6, 3s2, 3p6, 4s1 120 Calcium 20 2, 8, 8, 2 1s2,2s2 2p6, 3s2, 3p6, 4s2 2Project workBased on sub-shells, show electronic configuration of first 20 elements in a chart paper and display in the class.Classification of Elements Based on Electronic ConfigurationAfter the discovery of sub-shells, elements were divided into four groups. They are: i. s - Block elements ii. p - Block elementsiii. d - Block elements iv. f - Block elements

356 ClassifiCation of ElEmEnts Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 357i) s-Block ElementsThe elements whose last electron enters into s-orbital are called s-Block elements. The elements of groups 1 and 2 are s-Block elements. Either one or two valence electrons are filled in s-orbital of s-Block elements. In first group elements, the general electronic configuration is represented by ns1 and in second group elements the general electronic configuration is represented by ns2. s-Block is located on the left side of the Modern periodic table. The elements of s-Block form electropositive ions by losing one or two electrons. All the elements of s-Block are metals except H and He. Example: Sodium (Na) is kept in s-Block as its last electron enters the s-orbital, i.e., 11Na: 1s2, 2s2 2p6, 3s1.ii) p-Block ElementsThe elements whose last electron enters the p-orbital of the outermost shell are called p-Block elements. The elements of groups 13, 14, 15, 16, 17 and elements of zero (0) group or group 18 except Helium are p-Block elements. p-Block is situated on the right side of Modern periodic table. This block consists of metals, metalloids and non-metals. Example: Aluminium (Al) is kept in p-Block as its last electron enters into p-orbital, i.e. 13Al: 1s2, 2s2 2p6, 3s2 3p1. FACTS WITH REASONSNeon can exist freely in nature. Neon has a stable electronic configuration of 8 valance electrons. So, it neither loses electrons nor gains electrons. So, it does not do reaction. Therefore, it exists freely. iii) d-Block ElementsElements whose last electron enters the d-orbital of the penultimate shell are called d-Block elements. This block consists of elements of group 3 to 12. d-Block is located between s-Block and p-Block. The elements of d-Block are called transitional metals as they have partially filled d subshell but can form stable cations. Example: Copper (Cu) is kept in d-Block as its last electron enters the d-orbital, i.e., 29Cu: 1s2 , 2s2, 2p6 , 3s2 , 3p6,4s1, 3d10.iv) f-Block ElementsElements whose last electron enters the f-orbital of the ante-penultimate shell are called f-Block elements. The elements of lanthanide and actinide series are kept in f-Block. This block is located at the bottom of Modern periodic table just below the d-Block elements. The properties of f-Block elements are similar to the transition metals of MEMORY TIPSElements having general electronic configuration ns1 and ns2 are kept in s-block.MEMORY TIPSIn the 17 (VIIA) group, the general electronic configuration is represented by ns2 np5

356 ClassifiCation of ElEmEnts Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 357d-Block. However, their last electron enters f-orbital. Therefore, f-Block elements are kept separately below the d-Block elements or transitional elements. So, f-Block elements are also called inner-transition elements.Characteristics of Periods and Groups 1. Atomic RadiusAtomic radius is the distance between the outermost shell and the center of the nucleus of an atom. It determines the size of an atom. It is measured in picometer (pm) [1m = 10-12 pm]. The atomic radius (atomic size) decreases on moving from left to right in a period of the Modern periodic table.Elements of 3rd period Na(2, 8, 1)Mg(2, 8, 2)Al(2, 8, 3)Si(2, 8, 4)P(2, 8, 5)S(2, 8, 6)Cl(2, 8, 7)Atomic radius (pm) 157 136 125 117 110 104 99Na Mg Al Si P S ClAtomic size decreasesThe atomic number of elements increases as we move from left to right in a period. It means that the number of protons and electrons increases, but extra electrons are added into the same shell. As a result, the electrons are pulled towards the nucleus due to more powerful positive charge in the nucleus of the atom. Due to this, the atomic size of elements decreases on moving from left to right in a period.The atomic radius of elements increases gradually on moving from top to bottom in a group of the Modern periodic table.Group 1 Atomic radius (pm) Atomic size increases while moving down in a groupLi 123 Li The smallest atomNa 157 NaK 202 KRb 216 RbCs 235 CsFr 256 Fr The biggest atomAs we move down in a group, a new shell is added to the atom at every step. As a result, the size of the atom increases gradually from top to bottom of the same group. However, as we move down in a group, the nuclear charge also increases, but the nuclear attraction becomes less as compared to the increase in atomic size. The smallest atomic size is found at the top of a group and the biggest atomic size is found at the bottom of the same group.

358 ClassifiCation of ElEmEnts Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 3592. Ionization Energy or Nuclear PowerThe amount of energy required to remove the outermost loosely bound electron of an isolated gaseous atom is called ionization energy. It increases in a period on moving from left to right but decreases in a group on moving from top to bottom.PeriodGroupIonization energy decreases from top to bottom in groupIonization energy increases from left to right in a period3. Valence Electrons The electrons present in the outermost shell (or valence shell) of an atom are called valence electrons. The number of valence electrons remains the same in a group, but it increases continuously in a period. PeriodGroupNumber of valence electrons remains the same in a groupNumber of valence electrons increases from left to right in a period4. ValencyThe total number of electron(s) gained, lost or shared by atoms during a chemical reaction is called valency. In a period, the valency increases from 1 to 4 and then decreases to zero (0). It is because the number of valence electrons increases from 1 to 8 on moving from left to right in a period. But in a group, the valency of all elements remains the same as they have equal number of valence electrons in the same group.PeriodGroupValency of elements remains the same in a groupValency increases from 1 to 4 and then decreases to zero5. Electronegativity The amount of energy required for an atom to attract the outer electron of another atom is called electronegativity (EN). It increases on moving from left to right in a period and decreases from top to bottom in a group. PeriodGroupElectronegativity decreases from top to bottom in a groupElectronegativity increases from left to right in a period

358 ClassifiCation of ElEmEnts Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 359Electro-positivity: The tendency of an element to lose an electron to form a cation is called electropositivity. Electro-positivity is more in bigger elements and less in smaller elements. It increases from top to bottom in a group and decreases from left to right in a period. 6. Metallic Character (electro positivity)Metallic character is the tendency of an atom to lose one or more electrons. It increases when we move from top to bottom in a group and decreases from left to right in a period.PeriodGroupMetallic character increases from top to bottom in a groupMetallic character decreases from left to right in a period7. Non-metallic CharacterNon-metallic character is the tendency of an atom to gain one or more electrons when supplied with energy. It increases in a period on moving from left to right and decreases on moving from top to bottom in a group.PeriodGroupNon-metallic character decreases on moving from top to bottom in a groupNon-metallic character increases on moving from left to right in a periodReactivity of ElementsMetals have a tendency to lose electrons, and non-metals have a tendency to gain or share electrons. A metal which loses electrons easily is called an active metal and a non-metal which gains electrons easily is called an active non-metal. There are several factors on which the reactivity of elements depend. Among them, some common factors are the atomic size, nuclear charge and number of electrons present in the valence shell. Reactivity of Metals in a GroupThe chemical reactivity of metals increases on moving down in a group of modern periodic table. When we move from top to bottom in the groups of metals, the atomic size increases gradually. Due to this, nuclear attraction to the valence electron decreases. As a result, the tendency of metal atoms to lose electron/s increases. Therefore, chemical reactivity of metals increases on moving from top to bottom in a group.Example: In group 1, the chemical reactivity of alkali metals increases from lithium (Li) to francium (Fr).Li Least reactive metalMost reactive metalChemical reactivity of metals increases on moving down in a groupNaKRbCsFr Group 1

360 ClassifiCation of ElEmEnts Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 361FACTS WITH REASONSThe valency of sodium is one and the valency of chlorine is also one. However, Sodium is kept in the IA group but chlorine is kept in the VIIA group. Even though sodium and chlorine both have one valency, they are not kept in the same group. This is because the group of elements depends upon the number of valance electrons. Sodium belongs to the IA group because it has 1 valance electron. Chlorine belongs to the VIIA group because it has 7 valance electrons. Reactivity of Non-metals in a GroupThe chemical reactivity of non-metals decreases on moving from top to bottom in a group of the modern periodic table. When we move from top to bottom in a group of non-metals, the atomic size increases gradually. Hence, the nuclear attraction for incoming electron/s decreases. As a result, the tendency of the non-metallic atom to gain electron/s decreases. Therefore, the chemical reactivity of non-metals decreases on moving from top to bottom in a group. Most reactive non-metalLeast reactive non-metalReactivity of non-metals decreases on moving from top to bottom in a groupFClBrIAtGroup VIIA/17Example: In group VIIA/17, the chemical reactivity of halogens decreases from fluorine (F) to Astatine (At).Reactivity of elements in a periodThe atomic size of elements decreases gradually on moving from left to right in a period of the Modern periodic table. So the reactivity of metallic elements decreases and the reactivity of non-metals increases on moving from left to right in a period.Elements of 3rd period Na Mg Al Si P S ClChemical reactivity Most reactiveLeast reactiveMost reactive → Reactivity decreases → → Reactivity increases →

360 ClassifiCation of ElEmEnts Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 361In the given table, Na is the most reactive metal among Na, Mg, and Al. Si is the least reactive element. Similarly, Cl is the most reactive non-metal among P, S and Cl.FACTS WITH REASONSChlorine is more reactive than bromine, why?Chlorine is more reactive than bromine because the atomic size of chlorine is smaller than that of bromine. Due to the smaller atomic radius of the chlorine atom, nuclear attraction is greater on the valence shell of chlorine than that of bromine. Therefore, chlorine is more reactive than bromine. Variation in periodic properties in periods and groupsS.N. PropertiesAlong a period (While moving from left to right) Down in a group (While moving from top to bottom)1. Atomic size (radius) Decreases Increases 2. Valency 1 to 4 and 4 to zero Remains the same 3. Metallic character Decreases Increases 4. Non-metallic character Increases Decreases 5. Ionization potential Increases Decreases 6. Electron affinity Increases Decreases 7. Electronegativity Increases Decreases HOT SKILL HIGHER ORDER THINKING SKILL1. Despite lots of drawbacks, Mendeleev’s periodic table is very important. Despite lots of drawbacks, Mendeleev’s periodic table is very important because:i. It classified all elements available at that time into a scientific table. ii. It helped to rectify the incorrect atomic weight of some elements.iii. Table could predict new elements.iv. Study of elements got easier. 2. Classification of elements is very important. Classification of elements is very important because it simplifies their studies, helps to predict properties of new elements to be found, tells general properties etc. 3. Sodium is more reactive than lithium. Sodium has three orbits but lithium has two orbits only. So, sodium is bigger than lithium. The valance electrons of sodium are farther from the nucleus, held loose and can be lost more easily than the valance electron of lithium. Hence, sodium is more electropositive and more reactive than lithium. 4. Describe the exact location of potassium in the modern periodic table.Side : Left Group : IAPeriod : 4th Block : s-block

362 ClassifiCation of ElEmEnts Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 3635. Answer the following questions based on the given table.Name of element Electronic configurationX 1s2 , 2s22p6, 3s1Y 1s2, 2s22p6, 3s23p5Z 1s2, 2s22p6, 3s23p6i. Write the block of the elements X and Y.Ans: X belongs to s-block and Y belongs to p-block.ii. Write the chemical nature of element Z.Ans: Z is a chemically inert element.iii. Write the molecular formula of the compound formed by the chemical reaction between elements X and Y.Ans: NaCl 6. Chlorine is smaller than sodium even though both have three orbits.Chlorine is smaller than sodium even though both have three orbits because chlorine has 17 protons and 18 neutrons whereas sodium has 11 protons and 12 neutrons only. Since the nuclear charge of chlorine is greater than that of sodium it holds valance electrons strongly compared to sodium. So, the chlorine atom shrinks and gets smaller than sodium. 7. The molecular formula of a certain ionic compound is X2Y3 and ‘X’ is a metal. State group and block to which elements X and Y belong to the periodic table. X belongs to the III A group, p block. Y belong to the VIA group and p-block. 8. Compare between electronegativity and electro-positivity.The comparison between electronegativity and electro-positivity are:SN Electronegativity SN Electro positivity 1 Electronegativity is the property of an element or ion by the virtue of which it tends to attract electrons to itself.1 The electro positivity is the property of an element or ion by the virtue of which it can donate electrons to other atoms. 2 The electronegativity of an atom decreases if we observe from top to bottom in a group. 2 The electro-positivity of an atom increases if we observe from top to bottom in a group. 9. On what basis groups and periods are classified in the modern periodic table.Groups are classified based on the number of valance electrons. Periods are classified based on the number of orbits. For example: Lithium has electronic configuration= 2,1. It has two orbits K and L. So, it belongs to the second period. It has 1 valance electron in the valance shell (L). So, it belongs to the IA group. 10. The reactivity of metals increases down the group but the reactivity of non-metals decreases down the group.Metals are elements that should lose electrons. If an atom is bigger and valance electrons will be farther away from the nucleus then it can lose electron easily. So bigger atoms of metals can lose electrons easily and are more reactive. Since elements down the group are bigger, the reactivity of metals must increase down the group.

362 ClassifiCation of ElEmEnts Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 363Non-metals are elements that should take electrons. If an atom is bigger, valance electrons will be farther away from the nucleus and cannot be held properly. So, the atom cannot pull electrons from other elements. So bigger atoms of elements cannot pull electrons effectively and are less reactive. Since elements down the group are bigger, the reactivity of non-metals must decrease down the group. 11. Write some short-comings of modern periodic table. i. Hydrogen is nonmetal but kept in group of alkali metal. ii. Helium is kept in p block even if its last electron enters in s block.iii. Lanthanides and actinides are not kept in main body of periodic table.4 STEPS EXERCISE EXERCISESTEP11. Multiple Choice Questions a. What is the primary basis for the formation of a group of modern periodic tables?i. vertical columns ii. number of orbitsiii. atomic radius iv. number of valance electronb. Why does silicon lie in period 4?i. it has one valance electron. ii. its valency is 4.iii. it has four shells iv. it has four valance electrons c. Arrange the elements fluorine, chlorine, bromine and iodine in the increasing order of electronegativity. i. Fluorine > chlorine> iodine > bromine ii. Iodine > bromine > chlorine > fluorine iii. Fluorine < bromine < chlorine < iodine iv. Iodine < bromine < chlorine < fluorine d. 3s, 3p, 3d, 4s are few subshells of an atom. Which one is the correct order of energy level?i. 3p < 4s < 3d ii. 3p < 3d < 4s iii. 3s < 4s < 3p iv. 4s < 3s < 3p e. Hydrogen is kept with metals although it is a non-metal, why? i. one valance electron, electropositive, valency is 1 ii. light metal, soluble in water, can pass electricity iii. atomic number 1, valency 1, light metaliv. one valence electron, good conductor, light metalf. The electronic configuration of the valance orbit of certain elements is 3s23p5. Write the atomic number of that element.i. 2 ii. 5iii. 7 iv. 17 g. Atomic weight of argon is more than that of potassium but also it is kept before the potassium in the modern periodic table. Why?i. because it is a defect of Mendeleev’s periodic table ii. because argon is an inert gas

364 ClassifiCation of ElEmEnts Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 365iii. because the atomic number of Argon is 18 and that of potassium is 19 iv. because they are arranged in alphabetical orderh. Three friends are observing this part of the periodic table and speaking their opinions. Who is correct and who is incorrect?VIA VIIASecond period 8 O 9 FThird period 17 ClToshima : Fluorine is the most reactive element among these three elements.Sneha : Smaller atoms are more reactive. Therefore, oxygen is the most reactive among them because it is the smallest as we can see in the table.Khushi : Chlorine is the most reactive element here because it is in the third period, atomic number is 17. It has more orbits. It is bigger. Valance electrons are far from the nucleus and can be lost more easily than other elements.i. Toshima is correct. Sneha and Khushi are wrong.ii. Sneha is correct. Toshima and Khushi are wrong. iii. Khushi is correct. Toshima and Sneha are wrong.iv. Toshima is incorrect. Sneha and Khushi are correct. i. Which one makes the acidic oxide?Group IAGroup IIAGroup IIIAGroup IV AGroup VAGroup VIAGroup VIIA Group 0X Y Z i. X ii. Y iii. Z iv. X and Z j. Which of the following is the set of non-metals?i. iron, silver, sodium and aluminiumii. copper, silver, gold, sulphur iii. oxygen, carbon, sulphur, copper iv. carbon, nitrogen, chlorine, oxygena. iv b. iii c. iv d. i e. i f. iv g. iii h. i i. ii j. iv Ans of MCQSTEP22. Very short questions a. What happens to the electronegativity of an element if a new orbit is added to its structure?b. In which periodic table are the elements arranged based on increasing atomic number? c. State the Modern Periodic Law. d. Give two examples of alkali metal, alkaline earth metal, metalloid, halogens and transition metals.

364 ClassifiCation of ElEmEnts Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 365e. Some observations in elements of the third period are listed below. Which one is the most electronegative among them?No. of valance electrons less than four No. of valance electrons more than four Atomic size is more A BAtomic size is less C D f. Which imaginary element from the above concept map is more electropositive than the imaginary element M and why?A B One element (M) of third periodContains one more Contains one lessProton and electron Proton and electrong. Write down the common name of the elements shown in the given table.h. The electronic configuration of some elements is as given below: A–(2, 8, 1), B–(2,8,2), C–(2,8,3) and D–(2,8,4)Which one is more electropositive among these? i. Identify the group and period of this element by observing the diagram given below.STEP33. Short questions a. Write any two features of the Modern Periodic Table. b. Hydrogen is a non-metal but is placed in the group of metals. Give reason. c. Why does the valency remain the same in a group while we move from top to bottom?d. Write the name and group of the element whose valance orbit has an electronic configuration of 4s2.e. Answer the following questions based on the information that the atomic number of sodium is 11. i) Write the electronic configuration of it based on the subshell.ii) Write its group and period in the periodic table.f. The molecular formula of a certain ionic compound is XY2. ‘X’ is a metal. State the group to which elements X and Y belong in the periodic table. g. Sulphur belongs to VI A group and period 3. What does it mean?h. Why is sodium larger than aluminium?HLiNaK16 p+16 n°

366 ClassifiCation of ElEmEnts Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 367STEP44. Long questions a. Answer the following questions from the given electronic configuration.Electronic configurationA = 1s2 , 2s22p6, 3s1B =1s2 , 2s22p6, 3s23p6, 4s1C = 1s2 , 2s22p6, 3s23p5D = 1s2, 2s22p5i. Name the elements A, B and C. ii. Identify the period, group, valency, metal or non-metal and block of the given elements. iii. Which compounds will be formed due to the chemical reaction between A with C and B with C?iv. Which is more reactive out of A and B and why? Similarly, which is more reactive out of C and D and why?b. A part of the periodic table with the elements placed in a suitable order is shown below. Answer the following questions based on the table.(i) What is the proper name given to these two groups in a periodic table? Explain how you concluded.(ii) Compare the reactivity of the elements of each group with a suitable reason.c. Clarify the relationship between the size of the atoms of very active metals and very active non-metals with their reactivity. d. A, B, C, D, E, F, G and H are eight different elements of IA, IIA, IIIA, IVA, VA, VIA, VIIA and zero group respectively. All of them have same number of orbits. If the element A and element G make common salt, write the symbol of element H. Which one is the largest atom among them? Write the position of element D in the modern periodic table. e. There are 118 types of elements. They are arranged based on their ……….. in the modern periodic table. Henry Mosley believed that ………….. is the main factor that determines the chemical and physical properties of the elements. He arranged all available elements based on …………… of the element. The modern periodic table was thus formed. Answer the following question based on the above text.i. On what basis did Henry Mosley arrange the modern periodic table? ii. Carbon-12 lies in the second period and IVA group. Where should carbon-13 be placed in the modern periodic table? Why?♦♦♦Group X Group YLi FNa ClK Br

366 ClassifiCation of ElEmEnts Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 367Key terms and terminologies of the unit1. Chemical reaction : The combination, decomposition or displacement that occurs in the molecules of matters during a chemical change is called a chemical reaction. 2. Word equation : A chemical reaction expressed by writing the full names of reactants and products is called word equation. 3. Chemical equation : A chemical reaction expressed by writing the symbols and molecular formulae of reactants and products is called a chemical equation. 4. Balanced chemical equation: A chemical equation written by balancing the total number of atoms of each element on the reactant side and product side is called a balanced chemical equation. 5. Reversible reaction : A chemical reaction in which the products can recombine to give back the reactants is called a reversible reaction. 6. Irreversible reaction : A chemical reaction in which the products cannot recombine to give back the reactants is called an irreversible reaction. 7. Exothermic reaction : A chemical reaction which evolves heat during the chemical change is called an exothermic reaction. 8. Endothermic reaction : A chemical reaction which absorbs heat during the chemical change is called an endothermic reaction. 9. Catalyst : A catalyst is a chemical substance which increases or decreases the rate of chemical reaction but itself remains chemically unchanged.10. Positive catalyst : A catalyst which increases the rate of chemical reaction is called a positive catalyst. 11. Negative catalyst : A catalyst which decreases the rate of chemical reaction is called a negative catalyst.Sequence of Curriculum Issued by CDC Types of chemical reaction: composition, decomposition, displacement and acid-base reaction.  Rate of chemical reaction Factors affecting the rate of chemical reaction(heat, pressure, catalyst, light and surface area)UNITChemical Reaction15Estimated teaching periods Theory Practical4 2UdayarajKhanal, born on January 14, 1955, in Kathmandu, is a distinguished Nepalese physicist renowned for his expertise in cosmology and science communication. With over three decades of dedication to studying the origins and structure of the universe, Khanal's research, conducted at Delhi University and Florida Atlantic University, has significantly contributed to our understanding of cosmology. Udayaraj Khanal About the Nepalese Scientist

368 ChEmiCal REaCtion Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 36912. Addition reaction : A chemical reaction in which two or more reactants combine together to give a single product is called addition reaction.13. Decomposition reaction : A chemical reaction in which a single reactant is broken down into two or more products is called decomposition reaction. 14. Displacement reaction : A chemical reaction in which an atom or a radical of a compound is displaced by another element is called displacement or replacement reaction.15. Single displacement reaction :A chemical reaction in which one atom or one radical is displaced by another element is called single displacement reaction. 16. Double displacement reaction: A chemical reaction in which an element or a radical of a compound is mutually displaced by an element or a radical is called double displacement reaction.17. Acid-base reaction : A chemical reaction in which an acid and a base react together to give salt and water is called acid-base reaction.18. Rate of chemical reaction : The positive change in the concentration of a reactant or a product per unit time is called the rate of chemical reaction.19. Reactants: The elements or compounds that take part in the reaction are called reactants.20. Products: The elements or compounds formed when the reactants react are called products. IntroductionWater (H₂O) is made when hydrogen gas (H2) burns in air (O2). In this case, hydrogen and oxygen join together to make water. Calcium carbonate (CaCO₃) breaks down into calcium oxide (CaO) and carbon dioxide (CO₂) when it is heated. Similarly, when iron (Fe) powder is kept in copper sulphate solution (CuSO4), copper (Cu) and iron sulphate (FeSO4) are formed. Here, iron displaces copper from copper sulphate solution. These are some examples of chemical changes.H2 + O2 burn H2O [Combination]CaCO3 heat CaO + CO2 [Decomposition]Fe + CuSO4 FeSO4 + Cu [Displacement]The combination, decomposition or displacement that occurs in the molecules of matters during a chemical change is called a chemical reaction. It can be represented by an equation. The elements or compounds that take part in a chemical reaction are called reactants, whereas the elements or compounds that are formed as a result of chemical change are called products. Reactants are written on the left side of an arrow and products are written on the right side of the arrow. The direction of the arrow indicates the reactants and the products. Examples: Reactants ProductsZinc + Sulphuric acid Zinc sulphate + HydrogenZn + H2SO4 ZnSO4 + H2MEMORY TIPSThe chemical reaction is represented by word equation and formula (chemical) equation.

368 ChEmiCal REaCtion Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 369Word EquationA chemical reaction expressed by writing the full names of reactants and products is called word equation. Examples: Magnesium + Oxygen Magnesium oxideHydrochloric acid + Sodium hydroxide Sodium chloride + Water Chemical EquationA chemical reaction expressed by writing the symbols and molecular formulae of reactants and products is called a chemical equation. Examples: 2Mg + O2 2MgOHCl + NaOH NaCl + H2OBalanced Chemical Equation A chemical equation written by balancing the total number of atoms of each element on the reactant side and product side is called a balanced chemical equation. Examples: 2KClO3 ∆ 2KCl + 3O2H2SO4 + 2KOH ∆ K2SO4 + 2H2O Types of Chemical Reaction There are basically four types of chemical reactions on the basis of their nature. 1. Combination or Addition or Synthesis Reaction A chemical reaction in which two or more reactants combine together to give a single product is called addition reaction. This type of chemical reaction occurs either in the presence or absence of heat, light, pressure, electricity, catalyst, etc.Examples:N2 + 3H2 2NH3C + O2 CO2C + 2H2 CH42 Na + Cl2 2NaClFe + S FeSCaO + H2O Ca (OH)2CaCO3 + H2O + CO2 Ca(HCO3)22Mg + O2 2MgO

370 ChEmiCal REaCtion Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 3712. Decomposition or Dissociation or Analysis Reaction A chemical reaction in which a single reactant is broken down into two or more products is called decomposition reaction. Such type of chemical reaction occurs in the presence of heat, light, catalyst, electricity, etc.Examples:CaCO3 ∆ CaO + CO22KClO3∆ 2KCl + 3O2CuCO3∆ CuO + CO22Ag2O ∆ 4Ag + O2 2HgCO3 ∆ 2Hg + 2CO2 + O22Pb(NO3)2 ∆ 2PbO + 4NO2 + O22Cu(NO3)2∆ 2CuO + 4NO2 + O22 AgNO3 ∆ 2Ag + 2 NO2 + O23. Displacement Reaction or Replacement ReactionA chemical reaction in which an atom or a radical of a compound is displaced by another element is called displacement or replacement reaction. It is of two types.i. Single displacement reaction : A chemical reaction in which one atom or one radical is displaced by another element is called single displacement reaction. Examples:Zn + 2HCl ZnCl2 + H2Zn + H2SO4 ZnSO4 + H22KBr + Cl2 2KCl + Br22KI + Cl2 2 KCl + I2Zn + CuSO4 ZnSO4 + CuMg + ZnCl2 MgCl2 + Zn Fe + CuSO4 FeSO4 + CuCu + 2AgCl CuCl2 + 2Agii. Double displacement reaction : A chemical reaction in which an element or a radical of a compound is mutually displaced by an element or a radical is called double displacement reaction.Examples:AgNO3 + NaCl NaNO3 + AgCl CaCl2 + 2AgNO3 Ca(NO3)2 + 2AgCl

370 ChEmiCal REaCtion Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 371HgCl2 + 2KI 2KCl + HgI2Pb (NO3)2 + Na2SO4 PbSO4 + 2NaNO3FeCl2 + 2NaOH Fe (OH)2 + 2NaCl 4. Acid-base Reaction A chemical reaction in which an acid and a base react together to give salt and water is called acid-base reaction. Here, acidic and basic nature of the compounds is neutralized during the chemical reaction, so it is also called neutralization reaction. However, all acid-base reactions are not neutralization reactions.Examples:Acid + Base Salt + Water HCl + NaOH NaCl + H2OH2SO4 + CaO CaSO4 + H2O H2SO4 + 2NaOH Na2SO4 + 2H2O 2HCl + CuO CuCl2 + H2O2HNO3 + Ca(OH)2 Ca(NO3)2 + 2H2O H2SO4 + 2KOH K2SO4 + 2H2OFACTS WITH REASONSWhy is acid-base reaction called neutralization reaction?In acid-base reaction, both acid and base lose their properties during the chemical reaction. As a result, a neutral substance (i.e., salt and water) is formed. So, acid-base reaction is also called neutralization reaction.Rate of Chemical ReactionThe positive change in the concentration of a reactant or a product per unit time is called the rate of chemical reaction. It is calculated by the given formula.Rate of chemical reaction = Change in concentration of a reactant or a productTime taken for the changeThe SI unit of the rate of chemical reaction is moles per liter per second (mol/ls).FACTS WITH REASONSWrite down the effect of concentration of reactants on the rate of chemical reaction.The rate of a chemical reaction increases on increasing the concentration of reactants. It is because increasing the concentration of reactants increases the number of reacting molecules. There will be more collision between the reacting molecules and hence the rate of chemical reaction increases.Various factors affect the rate of chemical reaction. Some major factors are:i. Concentration of reactants ii. Heat and temperature

372 ChEmiCal REaCtion Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 373iii. Catalyst iv. Chemical nature of reactantsv. Physical nature of reactants vi. Surface area of the reactants vii. Pressure viii. Lightix. Electricityi) Concentration of ReactantsThe rate of chemical reaction increases on increasing the concentration of reactants. It is because increasing the concentration of reactants increases the number of reacting molecules. There will be more collision between the reacting molecules, and hence the rate of chemical reaction increases.ii) TemperatureHeat increases the thermal energy and kinetic energy of reactants. The temperature of the reactants increases. The molecules will start to vibrate faster and farther. This increases the capacity to react by colliding with other atoms. Some reactions occur only in the presence of heat. An increase in temperature increases the rate of a chemical reaction, whereas a decrease in temperature decreases the rate of the chemical reaction. For example,2KClO3Heat 2KCl + 3O2iii) CatalystA catalyst is a chemical substance which increases or decreases the rate of chemical reaction but itself remaining chemically unchanged. The mass and chemical nature of a catalyst does not change during the chemical reaction. There are two types of catalysts. a. Positive catalyst : A catalyst which increases the rate of chemical reaction is called a positive catalyst. For example, manganese dioxide (MnO2) acts as a positive catalyst during the decomposition of hydrogen peroxide. 2H2O2 MnO2(catalyst) 2H2O + O2↑b. Negative catalyst : A catalyst which decreases the rate of chemical reaction is called a negative catalyst. For example, glycerine acts as a negative catalyst and decreases the rate of the given chemical reaction.2H2O2 Glycerine 2H2O + O2↑FACTS WITH REASONSPhosphoric acid is called a negative catalyst during laboratory preparation of oxygen gas without heating.Phosphoric acid is called a negative catalyst during laboratory preparation of oxygen gas without heating because when phosphoric acid is mixed with hydrogen peroxide, oxygen gas is produced slowly. It decreases the rate of reaction.

372 ChEmiCal REaCtion Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 373iv) Physical nature of reactantsThe reaction takes place in the contact area of the reacting molecules. So the rate of chemical reaction can be increased by increasing the area of contact between the reactants. Similarly, the rate of chemical reaction increases by using a common solvent if the reactants are not soluble in one another. The common solvent helps bring the reacting molecules closer, and hence the rate of the reaction increases.v) Chemical nature of reactantsChemical nature of reactants also determines the rate of chemical reaction. Some reactants are more active and some are less reactive. The elements of group IA and VIIA are more reactive than the elements of other groups. Similarly, the reactions between ionic compounds are faster than the reactions between covalent compounds. vi) Surface area of the reactants The surface area of a reactants can affect the rate of a chemical reaction. If surface area of reactants increases, it increases the rate of collisions between reactant molecules and increases the rate of reaction. Similarly, when the surface area of a reactant is decreased, the rate of reaction may decrease as there are fewer chances for successful collisions to occur. For example, powdered calcium carbonate reacts faster with hydrochloric acid to produce carbon dioxide. Powdered vitamin C capsules react faster than solid capsules.vii) PressurePressure can affect the rate of a chemical reaction if at least one of the reactants is a gas. When pressure is increased, the gas molecules are compressed and more densely packed. It increases the frequency of collisions between gas molecules and possibly increases the rate of reaction.For example, powdered calcium carbonate reacts faster with hydrochloric acid to produce carbon dioxide. Powdered vitamin C capsules react faster than solid capsules.The reactant molecules having more volume than product cannot collide easily with each other to give a product. Hence, to accelerate such a reaction, high pressure is needed. For example, when pressure is applied on a mixture of potassium chloride and sulphur, explosion take place. A fire cracker explodes on applying pressure. Similarly, the synthesis of ammonia by Haber's process needs high pressure, i.e., about 200-500 atmospheric pressure. N2 + 3H2 200-500 atm, 600 0CFe/Mo 2NH3↑MEMORY TIPSExamples of positive catalyst are: MnO2, V2O5, Fe, etc. Similarly, examples of negative catalyst are H3PO4,H2O2, glycerine, etc.

374 ChEmiCal REaCtion Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 375viii) SolutionAcid doesn't show any effect without water. Similarly, salt doesn't give ions without water. This shows that a solution also plays an important role in many chemical reactions. When silver nitrate and calcium chloride are brought in contact, they do not show any reaction. But if they are dissolved in water, they give corresponding ions; and after exchanging ions, products are formed. AgNO3 (aq) + NaCl (aq) NaNO3 (aq) + AgCl↓ 2AgNO3 (aq) + CaCl2 (aq) Ca(NO3)2 (aq) + 2AgCl↓ix) Light:Some reactions occur in the presence of light only. For example, decomposition of silver bromide and hydrogen peroxide. Light energy increases the kinetic energy of molecules of reactants and helps them to react faster. Photosynthesis occurs only in the presence of sunlight 6CO2 + 6H2O Chlorophyll, Light C6H12O6 (Glucose) + 6O2X) ElectricityElectricity is also one of the important factors that brings about chemical reaction. When electricity is passed in acidified water, it decomposes and gives hydrogen (H2) and oxygen (O2) gas. H2O Electricity 2H2 + O2xi) Contact: Some chemical reactions occur faster when reactants come in direct contact. Sodium comes in contact with chlorine to produce sodium chloride. 2Na + 2H2O 2NaOH + H2

374 ChEmiCal REaCtion Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 375HOT SKILL HIGHER ORDER THINKING SKILL1. If AB and CD are a strong acid and a strong base respectively, what is the nature of the compound AD from their reaction?If AB is a strong base and CD is a strong base respectively, the compound AD is a neutral substance. 2. The rate of different types of chemical reactions will be different. The rate of reaction depends upon the nature of bonds to be formed or broken, concentration of reactants, state of reactants, temperature etc. hence based on the type of reaction the rate of reaction must be different. 3. Hydrogen peroxide is stored in a black dark vessel. Hydrogen peroxide is stored in a black dark bottle to prevent light from entering the bottle. If the light reaches the hydrogen reaction, it will decompose into water and oxygen. Word equation : Hydrogen peroxide Light Water + OxygenBalanced chemical equation : 2H2O2Light 2H2O + O2 4. Show by an activity that the rate of reaction is directly proportional to surface area. Activity: To show that the rate of reaction is directly proportional to surface area. Materials required: two test tubes, 5-gram marble chip, 5-gram calcium carbonate powder, dilute hydrochloric acid Procedure: We should put marble chips in the first test tube and calcium carbonate powder into the second test tube. Then pour 5 ml of dilute hydrochloric acid in both test tubes. Observation: The 5-gram calcium carbonate powder in the second test tube finishes earlier than the 5-gram marble chip in the first test tube. Analysis: The 5-gram calcium carbonate powder has more surface area than the 5-gram marble chips. When an equal volume of dilute hydrochloric acid was put in both test tubes, the powder calcium carbonate was finished earlier. So, we can assume that the rate of reaction is faster if the reactants have more surface area. Result: The rate of reaction is higher if the reactants have more surface area. The rate of reaction is less if the reactants have less surface area. Conclusion: The rate of reaction is directly proportional to the surface area. 5. How does the concentration of sodium thiosulphate affect the rate of reaction between hydrochloric acid and sodium thiosulphate? The rate of reaction is directly proportional to the concentration of the reactants. So, if the concentration of sodium thiosulphate is increased the rate of reaction will be faster because there will be a greater number of molecules of reactants, a greater number of molecules of reactants will come in contact, the rate of collision will increase and the product will be formed faster. If the concentration of sodium thiosulphate is decreased the rate of reaction will be decreased.

376 ChEmiCal REaCtion Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 3776. Silver nitrate and hydrogen peroxide cannot be stored in a transparent container. Silver nitrate and hydrogen peroxide are sensitive to light. When light enters from a transparent container these chemicals decompose to produce respective products. So these chemicals won’t be useful. Therefore, silver nitrate and hydrogen peroxide cannot be stored in a transparent container. 7. What happens to the rate of chemical reaction, when yeast is added to the hydrogen peroxide? The yeast releases the chemicals that force hydrogen peroxide to decompose. Therefore when yeast is added to hydrogen peroxide, the rate of chemical reaction will increase. 8. Observe the given chemical reaction and answer the following questions.Zn + dil.HCl →ZnCl2 + H2i. What type of reaction is it? It is a single displacement reaction. ii. Is it an exothermic or endothermic reaction?It is an exothermic reaction. iii. Write the names of reactants and products.Reactants : zinc and hydrochloric acid Products : zinc chloride and hydrogen gas iv. Balance the chemical reaction. Zn + dil.2 HCl → ZnCl2 + H29. Rimon put an iron nail in a test tube which contained copper sulphate solution. One day later the iron nail was already changed into copper nail. Describe the chemical change that occurred in the test tube. Chemical change: Iron nail changed into copper nail.Chemical reaction: Iron nail reacted with copper sulphate solution to produce copper nail and iron sulphate salt. Word equation: Iron + Copper sulphate → Copper + Iron sulphate Balanced chemical equation: Fe + CuSO4→ Cu + FeSO4 Type of reaction: Single displacement reaction Explanation: Since iron is more reactive than copper, it displaces the copper radicals from copper sulphate salts. As a result, iron nails lost iron, collected copper and changed into copper nails. 10. Ammonia gas and hydrogen gas are kept in a hot pressurised iron tank in the presence of molybdenum. Write the name of the product and mention the type of reaction. Ammonia is the product formed during this reaction. It is a combination reaction. Nitrogen + Hydrogen → Ammonia N2 + 3H2200atm, 500°CFe, Mo 2NH3 ↑

376 ChEmiCal REaCtion Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 3774 STEPS EXERCISE EXERCISESTEP11. Multiple Choice Questions a. Identify the type of chemical reaction given below.Ca + H2SO4 → CaSO4+ H2i. single displacement reaction ii. double displacement reaction iii. acid-base reaction iv. combination reactionb. What is the type of reaction if hydrogen combines with nitrogen under high pressure to form ammonia?i. reversible, combination, exothermic ii. double displacement, reversible, endothermic iii. combination, irreversible, endothermic iv. synthesis, irreversible, exothermic c. Which one of the following is a neutralization reaction?The reaction between two compounds The reaction between a metal and one compound Products are salts A BProducts are salts and water C Di. A ii. B iii. C iv. D d. Which one of the following is a combination reaction?i. Reactants → breaking of bonds → productsii. Reactants → exchange of radicals → products iii. Reactants → formation of bonds → products iv. Reactants → exchange of radicals → acid and base e. Write the names of factors that affect the chemical reaction i. concentration of reactant, temperature, pressure, catalyst ii. concentration of reactant, colour, pressure, catalyst iii. concentration of reactant, temperature, odour, catalyst iv. concentration of reactant, temperature, taste, catalyst f. Who is correct and who is wrong in the given conversation?Srijana : The sourness of vinegar decreases if limewater is mixed in it. Krishna : Yes! The limewater has water. The addition of water makes the vinegar dilute and sourness decreases.Anjan: I believe the neutralization reaction occurs between vinegar and limewater to form salt and water. Hence, the sourness decreases. i. Srijana and Krishna are correct. Anjan is wrong. ii. Krishna and Anjanare correct. Srijana is wrong.iii. Anjan and Srijana are correct. Krishna is wrong. iv. Anjan and Srijana are wrong. Krishna is correct.

378 ChEmiCal REaCtion Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 379g. What are the set of positive catalysts? i. MnO2 (Manganese dioxide), V2O5 (Vanadium Pentoxide), Fe ii. Potassium dichromate, manganese dioxide, molybdenum iii. H3PO4 (Phosphoric acid), H2O2 (Hydrogen peroxide), glycerine iv. Phosphoric acid, glycerine, molybdenum h. Which substance is X in the given chemical reaction? HCI + X → NaCl + H2Oi. NaOH ii. Na iii. NaH iv. Na + H2Oi. Mention the type of change and type of reaction shown in the diagram. i. Physical change, combination reaction ii. Physical change, decomposition reaction iii. Chemical change, combination reaction iv. Chemical change, combination reaction j. Utshav kept hydrogen peroxide in the transparent glass bottle a month ago. Today, he tried to use it to produce oxygen gas but failed. On testing, he noticed that the bottle contained water only. What might have happened?i. Light entering the bottle started the decomposition reaction of hydrogen peroxide.ii. Moisture entering the bottle started the decomposition of hydrogen peroxide.iii. In a month, hydrogen peroxide always changes into water.iv. Hydrogen peroxide reacts with a glass bottle to change into water. a. i b. i c. iii d. iii e. i f. iii g. i h. i i. iii j. i Ans of MCQSTEP22. Very short questions a. Which container will be filled with oxygen faster? One element (M) of third periodContainer A Container BAdd H3PO4 Add MnO2b. What is the value of X in the given chemical equation? 2KClO3 ∆ X KCl + 3O2c. During laboratory preparation of carbon dioxide gas in the laboratory, small marble pieces are used instead of large pieces of marble. Why? d. Acid-base reaction is called a neutralization reaction. Why?e. Identify the type of chemical reaction given below.Ca + H2SO4 CaSO4+ H2f. If a compound breaks into two or more products, what type of reaction is it?g. Write the relationship between the rate of the chemical reaction and the surface area of the reactants.h. If AB and CD are acid and base respectively, what is the nature of the compound BC from their reaction?

378 ChEmiCal REaCtion Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 379i. Write the names of the reactants from the given equation. Zn + dilute HCl ZnCl2 + H2STEP33. Short questions a. How does the concentration of sodium thiosulphate affect the rate of the chemical reaction between hydrochloric acid and sodium thiosulphate? b. Differentiate between composition reaction and decomposition reaction.c. How can you prove that the rate of the chemical reaction increases with the increase in temperature? Explain with an example.d. Write the relation between the concentration of reactants and the rate of reaction.e. If an iron nail is placed in copper sulphate solution for a few days, what will happen? What type of reaction is it? Write a balanced chemical equation for the reaction.f. Write the formula equation for the chemical reaction between a very active metal and a very active non-metal of third period. What type of chemical reaction is it? g. Change the given word equation into a balanced chemical equation. What type of reaction is it?Hydrogen peroxide Water + Oxygenh. The starch test experiment was carried out in two leaves. The observations are shown in the table below. What can be concluded from this experiment about the photosynthesis reaction? Show it in a balanced chemical equation.A plant leaf was covered with black tape for a week. A plant leaf not covered with anything for a week.A negative result for the starch test. A positive result for the starch test.STEP44. Long questions a. Write with balanced chemical equations. i. A piece of potassium metal is kept in water.ii. Magnesium is kept in hydrochloric acid.iii. Calcium + Oxygen → Calcium oxide iv. Magnesium + Copper nitrate → Magnesium nitrate + Copperb. In which condition do sodium chloride and silver nitrate react? Write the balanced chemical reaction between them. Also, mention the type of chemical reaction. Define this type of reaction. c. Explain four ways to increase the rate of the chemical reaction.d. A farmer is suggested to use the solid compound obtained after heating calcium carbonate in her/his field. Answer the following questions based on this short piece of information.(i) State, with a suitable reason, the problem of the farmer.(ii) How does the use of a given compound help to solve her/his problem? Explain with a suitable reason.e. Why we should not store dilute hydrochloric acid in a zinc bottle. Support your answer with a balanced chemical equation. What type of reaction is it?

380 somE GasEs Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 381Key terms and terminologies of the unit1. Laboratory preparation of ammonia : In the laboratory, ammonia is prepared by heating a mixture of ammonium chloride and calcium hydroxide (slaked lime) in the ratio 2:1. 2NH4Cl + Ca(OH)2 CaCl2 + 2H2O + 2NH3↑2. Chemicals required to prepare ammonia : Ammonium chloride [NH4Cl] and Calcium hydroxide [Ca(OH)2]3. Haber's process of ammonia manufacturing : In this process, a mixture of nitrogen and hydrogen (1:3) is combined directly at 500 0C temperature and 200 to 500 atmospheric pressure. For this reaction, finely powdered iron is taken as a catalyst and molybdenum as a promoter. N2 + 3H2 2NH34. Promoter : A promoter is a chemical substance that makes a catalyst more active during a chemical reaction, e.g., Molybdenum (Mo).5. Laboratory preparation of carbon dioxide : In the laboratory, carbon dioxide gas is prepared by the reaction of dilute hydrochloric acid with calcium carbonate (limestone or marble). CaCO3 + 2HCl CaCl2 + H2O + CO2↑6. Chemicals required to prepare carbon dioxide :Calcium carbonate [CaCO3] and dilute hydrochloric acid [HCl].7. Carbogen : The mixture of 95% oxygen and 5%carbon dioxide is called carbogen. It is used in the artificial respiration of pneumonic patients.Sequence of Curriculum Issued by CDC Laboratory preparation of carbon dioxide and ammonia gases Physical properties, chemical properties and application Greenhouse effect: causes, effects and control measures Acid rain: causes (reaction of carbon dioxide and sulphur dioxide with water to produce respective acid) Effects and control measures UNITSome Gases16Estimated teaching periods Theory Practical5 3Niranjan Parajuli, born in August 1972 in Kathmandu, is a prominent Nepali chemist, biotechnologist and biochemist who notably served as President of the Nepal Chemical Society from 2019 to 2021. As a Professor at Tribhuvan University, he has made significant contributions to chemistry education and research. Niranjan Parajuli, About the Nepalese Scientist

380 somE GasEs Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 3818. Dry ice : When carbon dioxide is cooled to below -78°C, it is converted into solid form, which is known as dry ice. It is known as dry ice because it melts without wetting the surface. 9. Fire extinguisher: A fire extinguisher is a protective device which is used to extinguish fire. It has a red metallic vessel containing a sodium bicarbonate solution and a bottle of sulphuric acid.10. Greenhouse: Green house is either a natural or an artificial structure which can trap heat energy and maintain particular temperature. 11. Artificial greenhouse: An artificial greenhouse is a structure covered with transparent polyethylene (plastic) or glass that traps solar rays inside. 12. Natural greenhouse: The earth is surrounded by several layers of gases called atmosphere that traps the heat from the sun increasing temperature of the earth similar to the glass or plastics house. It is called natural greenhouse.13. Acid rain: The rain fall whose pH value ranges from 3 to 5 is known as acid rain.14. Greenhouse effect: The phenomenon of trapping of solar heat inside the earth or an artificial greenhouse in such a way that it is relatively at a higher temperature is known as the greenhouse effect. 15. Greenhouse gases: The gases which trap the solar rays and heat up the atmosphere are known as the greenhouse gases. A. CARBON DIOXIDEIntroductionCarbon dioxide is a gaseous compound made of one atom of carbon and two atoms of oxygen. Carbon dioxide occurs in a free or combined state in nature. In the free state, it is present in the atmosphere, about 0.03% by volume. In the atmosphere, carbon dioxide is released by the burning of fuels like diesel, petrol, kerosene, wood, coal, etc. and by the respiration of living beings. In the combined state, it is present in different types of carbonates, bicarbonates, etc. Since carbon dioxide is heavier than air, it is present at a lower level of the atmosphere and occurs in deep wells, mines, caves, etc. Carbon dioxide has a great role to play in the living world because it is used by green plants for photosynthesis to convert solar energy into chemical energy. This chemical energy is supplied to all animals for their existence. Carbon dioxide is produced by the reaction of limestone (CaCO3), dolomite (CaCO3.MgCO3) and magnesite (MgCO3) with dilute acids.p+= 8 p+= 6 p+= 8 n0= 8 n0= 6 n0= 8Molecular structure of CO2O C O

382 somE GasEs Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 383Laboratory Preparation of Carbon dioxidePrincipleWe can prepare carbon dioxide gas in the laboratory by reacting calcium carbonate with dilute hydrochloric acid. CaCO3 + 2HCl CaCl2 + H2O + CO2↑Apparatus requiredWoulfe’s bottle, thistle funnel, gas jar, delivery tube, corks, matchbox, moist blue litmus paperChemicals requiredi. Calcium carbonate (CaCO3)ii. Dilute hydrochloric acid (HCl)Dilute hydrochloric acidDelivery tubeCarbon dioxide Moist blue litmus paperGas jarThistle funnelCorkWoulfe's bottleLime stonepiecesLaboratory preparation of carbon dioxideProcedurei. We should keep some pieces of marble in a Woulfe’s bottle and arrange the apparatus as shown in the figure.ii. Then we have to pour some dilute hydrochloric acid through the thistle funnel until it covers the lower end of the thistle funnel and the marble pieces.iii. Chemical reaction takes place between the marble and hydrochloric acid, and it produces carbon dioxide.iv. The gas is passed through the delivery tube and collected in the gas jar by the upward displacement of air.FACTS WITH REASONSCarbon dioxide is collected in the gas jar by the upward displacement of air. Why?Carbon dioxide is collected in the gas jar by the upward displacement of air because it is heavier than air.The lower end of the thistle funnel should be dipped in the solution. Why?The lower end of the thistle funnel should be dipped in the solution otherwise carbon dioxide can escape.Carbon dioxide cannot be collected in the gas jar passing through water. Why?Carbon dioxide is soluble in water. So it cannot be collected in the gas jar by passing through water.

382 somE GasEs Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 383Test of carbon dioxidei. A burning match stick is kept near the mouth of the gas jar. If the burning match stick extinguishes, it proves that the gas jar is filled with carbon dioxide gas.ii. When carbon dioxide is passed through lime water, i.e. calcium hydroxide, it turns milky white due to the formation of insoluble calcium carbonate (CaCO3).CO2 + Ca(OH)2 CaCO3 + H2OWhen carbon dioxide is passed in lime water for a long time, the milky colour disappears slowly due to the formation of water soluble calcium bicarbonate [Ca(HCO3)2].CaCO3 + H2O + CO2 Ca(HCO3)2iii. Carbon dioxide is acidic in nature so it turns moist blue litmus paper red.Precautionsi. All apparatuses should be handled carefully.ii. The thistle funnel should be dipped into acidified water to block its mouth so that carbon dioxide will not escape from it. iii. The mouth of the delivery tube should not be dipped so that it can collect carbon dioxide.iv. Carbon dioxide must be diluted so that the reaction will be slow and safe. Properties of Carbon Dioxidea. Physical propertiesi. It is a colourless, odourless and tasteless gas.ii. It is soluble in water and produces carbonic acid when dissolved in water.iii. It is heavier than air.iv. It is an acidic gas. So, it turns moist blue litmus paper red.v. It is neither combustible nor a supporter of combustion.vi. When carbon dioxide is cooled to below -78°C, it is converted into solid form, which is known as dry ice. It is known as dry ice because it directly changes into carbon dioxide. No liquid is formed when it melts. b. Chemical propertiesi. Reaction with water : Carbon dioxide reacts with water and gives carbonic acid.CO2 + H2O H2CO3 (Carbonic acid)ii. Reaction with lime water : Carbon dioxide reacts with lime water and gives insoluble calcium carbonate (CaCO3), which makes the solution milky-white.CO2 + Ca(OH)2 CaCO3 + H2O

384 somE GasEs Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 385When carbon dioxide is passed continuously into lime water for a long time, the milky colour disappears slowly due to the formation of water soluble calcium bicarbonate Ca(HCO3)2.CaCO3 + H2O + CO2 Ca(HCO3)2iii. Green plants convert solar energy into chemical energy by photosynthesis. In this process, carbon dioxide and water react together to give starch and oxygen.6CO2 + 6H2O sunlightchlorophyll C6H12O6 + 6O2iv. Carbon dioxide is neither combustible nor a supporter of combustion. But a burning magnesium strip burns in carbon dioxide with dazzling light. During this process, white powder (ash) of magnesium oxide and black particles of carbon are produced.2Mg + CO2 2MgO + Cv. Carbon monoxide is formed when carbon dioxide is heated with red hot coke at about 9000C.CO2 + C 900ºC 2COvi. Ammonia reacts with carbon dioxide at about 15000C and certain pressure (30 atm.) to form urea and water.2NH3 + CO2 1500ºCpressure (30atm.) NH2 – CO – NH2 + H2O (Urea)vii. Potassium hydroxide absorbs carbon dioxide.2KOH + CO2 K₂CO₃ + H₂OFACTS WITH REASONSThe level of water rises when we keep KOH in the gas jar containing CO2 in kept inverted in the water trough. Why?KOH absorbs carbon dioxide and creates vacuum in the gas jar. So water rises to fill the vacuum.Uses of Carbon dioxidei. Green plants use carbon dioxide to prepare food during photosynthesis.ii. It is used for making soft as well as hard drinks like soda water, coca cola, beer, etc.iii. It is used in the manufacture of fertilizers like urea and washing soda, i.e., sodium carbonate.iv. Solid carbon dioxide, i.e., dry ice is used as a refrigerant to preserve foods, fruits, meat, etc.v. It is used in fire extinguishers.vi. It is used in carbonation process to purify sugarcane juice.MEMORY TIPSCarbonic reacts with water to form carbonic acid but inside the leaf at chloroplast in the presence of sunlight, it makes glucose. MEMORY TIPSBoth ammonia and carbon dioxide solidify at -78°C.

384 somE GasEs Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 385Working Mechanism of a Fire ExtinguisherA fire extinguisher is a protective device which is used to extinguish fire. It has a red metallic vessel containing a sodium bicarbonate solution and a bottle of sulphuric acid.KnobNozzle for gasMetallic cylindersolutionConcentrated H2SO4Saturated sodium bicarbonateExternal and internal structure of a fire extinguisherWhen we need carbon dioxide to extinguish a fire, the vessel is inverted.Afterwards, the bottle of concentrated sulphuric acid strikes against the floor. The plug of the acid bottle falls down and acid comes in contact with the sodium bicarbonate solution. When those chemicals react together, they give carbon dioxide.2NaHCO3 + H2SO4 (conc.) Na2SO4 + 2H2O + 2CO2↑The gas produced in the above reaction comes out through the nozzle at high pressure and puts off the fire, just like putting a blanket over the surface of the flame.B. AMMONIAIntroductionAmmonia is a gaseous compound made up of one nitrogen atom and three hydrogen atoms. Pure ammonia was first prepared by heating the mixture of sal ammoniac (ammonium chloride) and lime (calcium hydroxide).Ammonia occurs in free as well as in combined states. In a free state, is present in air and soil whereas in a combined state, it is found in different ammonium salts like ammonium sulphate [(NH4)2SO4], ammonium phosphate [(NH4)3PO4], ammonium chloride [NH4Cl], etc. Ammonia is also synthesized in nature by nitrifying bacteria. p+ = 1n0 = 0p+ = 7n0 = 7p+ = 1n0 = 0 p+ = 1n0 = 0Molecular structure of ammonia (NH3)NH HH

386 somE GasEs Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 387Laboratory Preparation of AmmoniaPrincipleWe can prepare ammonia gas in the laboratory by heating the mixture of ammonium chloride and calcium hydroxide. In the mixture for every 2 moles of ammonium chloride, there should be 1 mole of calcium hydroxide. It means the ratio of ammonium chloride to calcium hydroxide is 2:1. 2NH4Cl + Ca(OH)2 ∆ CaCl2 + 2H2O + 2NH3 ↑FACTS WITH REASONSAmmonia cannot be collected in a gas jar either by the downward displacement of water or upward displacement of air, why?Ammonia cannot be collected in a gas jar by the downward displacement of water because this gas is highly soluble in water. It forms ammonium hydroxide (NH4OH) when passed in water. Ammonia is lighter than air. So, it cannot be collected in the gas jar by the upward displacement of air.A moist red litmus paper is used to test ammonia gas, why?Ammonium hydroxide is an alkali. So, a wet red litmus paper is used to test if the gas is ammonia or not.Mixture of Ca(OH)2and NH4ClAmmonia gasLime towerCaOBurner StandStandLaboratory preparation of ammonia gasApparatus requiredHard glass test tube, delivery tube, gas jar, Bunsen burner, stands, red litmus paper, corkChemicals requiredi. Ammonium chloride [NH4Cl] ii. Calcium hydroxide [Ca(OH)2]Procedurei. Make a mixture of ammonium chloride and calcium hydroxide in the ratio 2:1.ii. Keep the mixture in a hard glass test tube, and arrange the apparatus as shown. Supply heat from the Bunsen burner.iii. After heating, ammonia is produced. It is collected in the gas jar by downward displacement of air.

386 somE GasEs Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 387Precautionsi. The apparatus should be made air-tight.ii. The hard glass test tube should be slightly slanted.iii. Ammonia is collected by downward displacement of air.Test of ammoniaAmmonia gas can be tested in the following ways:i. Litmus paper test: The red litmus paper can be used to test it because ammonia is basic. It changes red litmus into blue. First of all, we have to dip red litmus paper into water. Then we should put this wet litmus paper into the gas jar that contains this gas. If the moist litmus put into the gas jar changes into blue, we can assume that the gas is ammonia.ii. We can also test the presence of ammonia by using hydrochloric acid. First of all, we have to dip a glass rod in a concentrated hydrochloric acid. Then we should put that wet glass rod in the gas jar. If the white fumes are formed then we can assume that the gas in the jar is ammonia. The ammonia reacts with concentrated hydrochloric acid to form white fumes of ammonium chloride. NH3 + conc. HCl NH4Cl FACTS WITH REASONSWhy is the gas jar inverted during laboratory preparation of ammonia gas?The gas jar is inverted because ammonia is lighter than air. So, this gas is collected by the downward displacement of air.A hard glass test tube is slightly inclined during preparation of ammonia, why?A hard glass test tube is slightly inclined during preparation of ammonia, otherwise the steam produced during the reaction exerts pressure, which may crack the test tube. So, the hard glass test tube is slanted to prevent it from cracking.Why is lime tower (CaO) used during preparation of ammonia?Lime tower (CaO) is used to remove moisture and obtain dry and pure ammonia. Remember : Ammonia gas cannot be collected passing through concentrated sulphuric acid, calcium chloride and phosphorus pentachloride because these compounds react with ammonia gas.Properties of Ammoniaa. Physical propertiesi. Ammonia is soluble in water. It makes weak base ammonium hydroxide with water. ii. Air is made of nitrogen and oxygen. The molecular weight of nitrogen is 28 amu and oxygen is 32 amu. Whereas the molecular weight of ammonia is 17 amu only. Therefore, it is lighter than air. iii. Ammonia can be liquified at -33.4°C and solidified at -78°C.iv. Ammonia cannot be seen by eyes, it is colourless.v. Ammonia has a strong pungent smell.

388 somE GasEs Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 389ACTIVITY 1To demonstrate that ammonia is highly soluble in water and basic in natureA round bottom flask containing ammonia is taken, and a delivery tube is fitted on it as shown in the diagram. The other end of the delivery tube is kept within the water containing some drops of phenolphthalein. The delivery tube carries a jet inside the round bottom flask. When ammonia cools, it creates low pressure inside the flask, so some water drops rise up in the flask and form ammonium hydroxide. As a result, more vacuum is created inside the flask. Hence, water rushes up with high pressure to fill up the vacuum, which looks like a fountain. Afterwards the entire flask appears pink. This experiment proves that ammonia is highly soluble in water and basic in nature.Note : If we take kerosene in place of water, it does not form ammonium hydroxide and does not give a pink color.NH3Round bottom flaskWaterStandb. Chemical propertiesi. Ammonia dissolves in water and forms an alkali, i.e., ammonium hydrxide. NH3 + H2O 1500 0CPressure NH4OHii. Ammonia is a basic gas, so it reacts with acid to give salt.NH3 + HCl NH4Cl2NH3 + H2SO4 (NH4)2SO4NH3 + HNO3 NH4NO3iii. Ammonium hydroxide, i.e.,ammonia solution, reacts with acid to give salt and water.2NH4OH + H2SO4 (NH4)2 SO4 + 2H2ONH4OH + HCl NH4Cl + H2ONH4OH + HNO3 NH4NO3 + H2Oiv. Ammonia and carbon dioxide react together at about 1500 °C and under certain pressure (30 atm.) to give urea. It is a very useful chemical fertilizer rich in nitrogen.2NH3 + CO2 NH2 - CO - NH2 + H2O (urea)v. Ammonia reacts with conc. hydrochloric acid and forms solid particles of ammonium chloride.NH3 + conc. HCl NH4Clvi. Ammonia burns with oxygen and forms nitrogen and water. It is exothermic reaction because it releases heat into the surrounding.4NH3 + 3O2 2N2 + 6H2O

388 somE GasEs Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 389vii. Ammonia reacts with oxygen to produce water and nitrogen normally, but it produces water and nitric oxide if it is passed through an 800°C hot platinum gauge. 4 NH3 + 5O2 800°C + Pt 6H2O + 2NOviii. Ammonia gas reacts with molten sodium metal to give sodamide and hydrogen gas.2NH3 + 2Na 2NaNH2 + H2Uses of Ammoniai. Ammonia is used in refrigerators as a cooling agent instead of CFCs. ii. Ammonia is used along with carbon dioxide to make urea fertilizer. It is also used to make other fertilizers such as ammonium sulphate, ammonium phosphate and ammonium nitrate. iii. Ammonia is used to make ammonium salts which are used for medicine. iv. Ammonia is used to make ammonium hydroxide which is used as a cleaning agents. v. It is also used to make blueprints of maps of lands, buildings etc. vi. Ammonia is used for the industrial preparation of plastic, explosives, nitric acids and washing soda. GreenhouseA greenhouse can either be an artificial (greenhouse made up of transparent glass or plastic) or a natural greenhouse i.e. the earth having dense atmosphere. In the artificial greenhouse, summer plants can be grown even in winter. This is possible as the glass or plastic that covers the greenhouse traps the heat from solar rays maintaining a higher temperature inside. Similarly, the earth also traps the heat of the sun in the same way as an artificial greenhouse. Therefore, it is called a natural greenhouse. Artificial GreenhouseAn artificial greenhouse is a structure covered with polyethylene (plastic) or glass that traps solar rays inside.Due to the trapping of the solar rays, it is hotter inside a greenhose than outside. This enables us to grow summer plants or off season plants even in winter season. A greenhouse is called a greenhouse because we mostly grow green plants inside it to maintain their greenery.FACTS WITH REASONSIt is warmer inside the artificial greenhouse than its surroundings. It is warmer inside the artificial greenhouse than its surroundings because every ray of sunlight that enters through the glass walls and roof of the greenhouse is trapped inside it due to a decrease in wavelength.MEMORY TIPSSome natural greenhouse gases are carbon dioxide, methane, chlorofluorocarbons, nitrous oxides, ozone etc. They trap solar energy and keep the earth warm. Artificial Greenhouse

390 somE GasEs Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 391Phenomenon of heat trappingWhen the solar rays enter the glass or plastic, most of the short wavelength having higher energy are allowed to enter inside. These rays strike the ground and plants and are absorbed. But when they are emitted, the rays have a long wavelengths having low energy. These emitted rays cannot penetrate the glass or plastic and thus gets trapped inside.Use and importance of artificial greenhousei. A greenhouse is used to grow the plants that need higher temperature.ii. It helps to grow fruits, vegetables and flowers commercially in a large quantity even in the off season.iii. It protects the plants from excess frost and dew formation.iv. It helps to protect the plants from air pollution.v. It prevents the breakage of plant stems during heavy rainfall.vi. It also protects the smaller plants from the winds. The main function of an artificial greenhouse is to preserve the heat of the sun inside its glass or plastic structure. FACTS WITH REASONSThere is no greenhouse effect on the moon. There is no greenhouse effect on the moon because there is no atmosphere and no greenhouse gases. Due to the absence of greenhouse gases, the heat of sunlight is not captured. Hence, there is no greenhouse effect on the moon.Natural greenhouse:The earth acts as a natural greenhouse. The atmosphere of the earth contains greenhouse gases. The ozone layer of the atmosphere acts as a roof. The sunlight that enters the atmosphere is mostly absorbed by greenhouse gases present in the atmosphere. Some greenhouse gases in the atmosphere are carbon dioxide, water vapour and methane. They trap heat energy during the day and release it at night. Hence the temperature of the earth remains warm even at night. The reflected light from the earth cannot leave the earth because the ozone layer blocks it. Therefore the earth acts exactly like a greenhouse. So, it is called a natural greenhouse. The Greenhouse EffectThe earth receives two kinds of radiation from the sun: shortwave radiation (UV rays and visible light mostly) and long wave radiation (Infrared rays mostly). The short wave radiation Natural greenhouse

390 somE GasEs Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 391can pass easily into the earth’s atmosphere while the long wave radiation is absorbed. When these solar rays are absorbed by the earth’s surface, it starts to reflect infrared rays (heat carrying rays). They are immediately re-absorbed by the atmosphere. After absorption, the atmosphere also gives heat to all directions. This atmospheric heat is again absorbed by the ground. In this way, there is a continuous exchange of heat between the atmosphere and the earth’s surface due to which the temperature of the earth is maintained warmer. This phenomenon is known as the greenhouse effect.FACTS WITH REASONSEarth does not freeze entirely even during the winters. Earth does not freeze entirely even during the winters because of the greenhouse effect. The carbon dioxide and other greenhouse gases present in the atmosphere absorb heat from sunlight during the day. It helps to prevent temperature falls during winter too. The phenomenon of trapping of solar heat inside the earth or an artificial greenhouse in such a way that it is relatively at a higher temperature is known as the greenhouse effect.The greenhouse effect is mostly advantageous to us as it maintains a constant surface temperature on the earth and prevents it from chilling. But if there is excessive greenhouse effect, it can cause the overall temperature of the earth to rise significantly, which can then be disadvantageous to us. Hence, greenhouse effect is both beneficial as well as harmful to us. FACTS WITH REASONSRefrigerators that use CFCs are banned. Refrigerators that use CFCs are banned because CFCs can destroy the ozone layer. If the ozone layer depletes more solar radiation will come into Earth. The atmosphere traps more heat energy and the temperature of the earth increases faster. It will result in climate change, global warming, natural disasters, loss of biodiversity etc. Among several gases of the atmosphere, ozone (O3), carbon dioxide (CO2), water vapour (H2O), methane (CH4), Nitrous oxide (N2O), chlorofluorocarbon (CFC), etc. have higher ability of absorbing solar rays compared to other gases. These gases which trap the solar rays and heat up the atmosphere are known as the greenhouse gases. Some of the greenhouse gases and their sources are given below:i. Carbon dioxide (CO2): burning of fossil fuels, coal, petrochemicals, wood, respiration etc.ii. Water vapour (H2O): evaporation from seas, oceans, rivers, etc., burning of hydrocarbons etc.iii. Methane (CH4): decomposition of organic substances like dung, faeces, waste vegetables, dead organisms, etc.iv. Chlorofluorocarbon (CFC): aerosols and coolantsv. Nitrous oxide (N2O): fertilizers, waste water management, burning of fossil fuels, livestock manure, etc.Adverse impacts of greenhouse effect i. It causes global warming. Global warming will cause natural disasters such as the rise of sea levels, irregular water cycles, tsunamis, cyclones, desertification etc. ii. Bacterial and parasitic diseases will spread and cause pandemics.

392 somE GasEs Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 393iii. Increased amount of greenhouse gases causes air pollution. It can cause respiratory diseases, dry eyes etc. iv. Biodiversity will be lost due to increases in temperature, irregular rainfall, disturbances in the water cycle, changes in climate and extreme weather. Some species will be endangered, some migrate and some go extinct. v. Agricultural production decreases due to unsuitable temperatures, lack of water, spread of pests, climate change, lack of bees and extreme weather conditions.FACTS WITH REASONSAn increase in the concentration of carbon dioxide gas in the atmosphere is bad. Increases in the concentration of carbon dioxide gas in the atmosphere are bad because it is a greenhouse gas that can trap heat from sunlight, cause global warming and change climate which can start natural disasters and cause loss of biodiversity.Ways of controlling greenhouse effect The greenhouse effect is caused by greenhouse gases present in the atmosphere. Therefore, the best methods to reduce greenhouse effects are to reduce the production of greenhouse and to remove greenhouse gases from the atmosphere. We can decrease the production of greenhouse gases by using clean energy, alternative sources of energy, focusing on solar energy etc. instead of fossil fuel. We can go electric. We can use electric machines, electric bikes, electric buses etc. instead of fossil-fuel-powered machines, stoves and vehicles. We can also decrease carbon production by minimizing forest fires. Plants can naturally absorb carbon dioxide from the atmosphere. So, afforestation, discouraging deforestation and reducing forest fires also help to reduce carbon dioxide from the air. Hence, there will be fewer greenhouse gases in the air and the greenhouse effect will be less and limited only.Acid RainThe rain fall whose pH value is 3 to 5 is known as acid ran. Rain water is lightly acidic due to the presence of CO2, NO and SO2. The pH value of common monsoon rain is 6. But, occasionally its acidity may increase due to the mixing of different acids like Hydrochloric acid (HCl), Sulphuric acid (H2SO4), Nitric acid (HNO3), Carbonic acid (H2CO3), etc. These acids are formed when industrial gases like CO2, SO2, NOx and Cl2 react with water vapour of the atmosphere. After their formation, these acids mix with rain and fall down as acid rain. The chemical reactions involving their formation are given below:Carbonic acid rainCO2 + H2O H2CO3Sulphuric acid rain2SO2 +O2 2SO3SO3 + H2O H2SO4

392 somE GasEs Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 393Effects of acid raini. Acid rain causes respiratory problems, skin and eye irritation in humans. ii. It increases the acidity of soil and decreases the fertility.iii. It kills aquatic and amphibian animals. iv. It affects the aquatic ecosystems.v. Acid rain falling on plant leaves, fruits and flowers or absorbed by soil decreases the productivity. vi. It corrodes the statues, monuments, shrines, metallic architectures and marble works.CaCO3 + dil. H2SO4 CaSO4 + CO2 + H2OWays of controlling acid raini. Reduce carbon emission: The production of carbon dioxide can be decreased by using renewable sources of energy such as hydroelectricity, solar energy etc. ii. Remove industries, factories and old vehicles from human settlement: Industries, factories and old vehicles produce carbon dioxide, carbon monoxide, sulphur dioxide and oxides of nitrogen. These gases make acid rain in cities. Therefore industries, factories and old vehicles should be removed from human settlements. iii. Afforestation: Plants absorb carbon dioxide from the air. If we plant sufficient plants, it can reduce carbon dioxide from the air and reduce the chances of acid rain.HOT SKILL HIGHER ORDER THINKING SKILL1. Air bubbles can be seen around the eggshells if it is placed in vinegar for a few hours. Eggshells are made of calcium carbonate and vinegar is an acid. So, when eggshells are kept in vinegar for a few hours, they react to form carbon dioxides which can be seen as air bubbles around the eggs. 2. We should not collect ammonia gas by downward displacement of water. We should not collect ammonia gas by downward displacement of water because the ammonia gas will start to dissolve in water to form ammonium hydroxide and no ammonia gas will be collected in the gas jar. 3. What happens when ammonia is passed through hot molten sodium metal?If ammonia gas is passed through hot molten sodium metal it produces hydrogen gas and sodamide. 2NH3 + 2Na ∆ 2NaNH2 + H24. Answer the following questions after observing the given figure of laboratory preparation of ammonia. i) Write the principle of laboratory preparation of ammonia. Ammonia can be prepared in the laboratory by heating the mixture of ammonium chloride and calcium hydroxide in a ratio of 2:1. 2NH4Cl + Ca(OH)2→ CaCl2 + 2H2O + 2NH3

394 somE GasEs Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 395ii) Write the name of the solids present in the hard glass test tube. The solids present in that hard glass test tube are ammonium chloride and calcium hydroxide. iii) How is the gas tested? This gas is tested by using moist red litmus paper which turns blue on contact with ammonia.It is also tested by introducing hydrochloric acid which turns into a white cloud of ammonium chloride. iv) Why is the gas collected by the downward displacement of air?This gas (ammonia) is collected by the downward displacement of air because it is lighter than the air. 5. Why is it dangerous and suffocating to go inside deep wells?It is dangerous and suffocating to go inside the deep wells because there could be a higher concentration of carbon dioxide. Since carbon dioxide gas is denser than air, it is collected in deep wells, caves and mines. 6. What happens when ammonia is mixed in water? Write its use. When ammonia is mixed with water it reacts to produce ammonium hydroxide. It is also called liquor ammonia. NH3 + H2O → NH4OHLiquor ammonia is used as a cleaning agent and emulsifier. 7. What is formed when carbon dioxide is passed through clean lime water for a short duration?When carbon dioxide is passed through clean lime water for a short duration calcium carbonate will be formed. Since calcium carbonate is insoluble in water, the lime water turns milky. Word equation :Calcium hydroxide (lime water) + Carbon dioxide → Calcium carbonate + Water Balanced formula equation: Ca(OH)2 + CO2→ CaCO3 + H2O 8. Differentiate between lab preparation of ammonia and lab preparation of carbon dioxide. SN Lab preparation of ammonia SN Lab preparation of carbon dioxide 1. Lime tower is necessary. 1. Lime tower is not necessary. 2. A mixture of ammonium chloride 2 parts and calcium hydroxide 1 part is heated in a hard glass test tube. 2. Dilute hydrochloric acid is passed into calcium carbonate to react inside a Woulfe’s bottle. 3. The mixture is heated. 3. Heating is not necessary.4. Ammonia is collected by the downward displacement of air.4. Carbon dioxide is collected by the upward displacement of air.9. Material A and B are produced by the thermal decomposition of substance C. Material A is solid and material B is gas. Material A is used to change hard water into soft water. Material B is a greenhouse gas. Answer the following questions based on this information.

394 somE GasEs Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 395i. Write the molecular formula of substance C. This substance C is calcium carbonate and its molecular formula is CaCO3. ii. What happens if the concentration of material B increases in the atmosphere? If the concentration of material B increases in the atmosphere, the greenhouse effect occurs faster. Global warming occur, climate will change and frequent natural disasters will occur. iii. Show the chemical change explained in the question in a balanced chemical equation.Chemical change: Substance C has broken into A and B due to heat (thermal decomposition).Chemical reaction: Calcium carbonate decomposes into calcium oxide and carbon dioxide on heating. Word equation: Calcium carbonate heat Calcium oxide + Carbon dioxide Balanced equation: CaCO3∆ CaO + CO210. Earth acts as a natural greenhouse. How? How has it helped to sustain life on earth? Explain. Earth has an atmosphere. It contains greenhouse gases such as carbon dioxide, methane and water vapour naturally. These greenhouse gases trap solar energy and keep the earth warm. Radiations reflected by the earth cannot leave the earth’s atmosphere due to the ozone layer. As a result, the earth acts like a natural greenhouse.Earth acting as a natural greenhouse helps to support life in the following ways:i. It helps to keep the earth warm. ii. It prevents the earth from freezing. iii. It helps to maintain weather and climate. 4 STEPS EXERCISE EXERCISESTEP11. Multiple Choice Questions a. During the laboratory preparation of carbon dioxide gas, a test tube filled with limewater was used to collect carbon dioxide. The white precipitate was formed on the test tube. Write its name. i. calcium carbonate ii. calcium bicarbonate iii. milk iv. milk of magnesia b. Hard glass test tube is slanted during laboratory preparation of ammonia. Why? i. to balance the centre of gravity so that the hard glass test tube won’t fall.ii. to maintain uniform heating of reactantsiii. to avoid falling of condensed water droplets on the reactants iv. to align the hard glass test tube with the delivery tube.c. Why does phenolphthalein solution turn pink, when ammonia is passed through it?i. ammonia is base, so it can turn phenolphthalein pink.

396 somE GasEs Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 397ii. ammonia is acidic, so it can turn phenolphthalein pink.iii. ammonia is pink, so it changes everything into pinkiv. ammonia dissolves in water to produce ammonium hydroxide, which gives pink colour to phenolphthalein. d. How is carbon dioxide manufactured?i. dissolving limestone in acid ii. heating limestone in the furnace iii. burning coal in excessive oxygeniv. burning coal in minimum oxygen e. Which one of the following is ammonia gas? Reacts with oxygen to release heat (exothermic)Reacts with oxygen by absorbing heat (endothermic)Increase the temperature of air in the atmosphere A BDecrease the temperature of air in the fridge C D i. A ii. Biii. C iv. D f. Which of the following set of characteristics represents carbon dioxide?i. heavier than air, solidify at– 78°C, acidicii. lighter than air, solidify at– 78°C, basic iii. used to make a blueprint, absorbs heat, makes urea iv. make urea, coolant in the fridge, solidify at –78°C g. Which gas is formed when the outer shell of the egg is mixed with dilute hydrochloric acid?i. ammonia ii. methane iii. carbon dioxide iv. chlorofluorocarbonsh. Why is the gas jar erect during the laboratory preparation of carbon dioxide?i. carbon dioxide is heavier than airii. carbon dioxide is lighter than air iii. carbon dioxide floats in the airiv. carbon dioxide is acidici. Which set of gases is a greenhouse gas?i. nitrogen, ammonia, carbon dioxide, water vapourii. ammonia, carbon dioxide, water vapour, methane iii. carbon dioxide, methane, water vapour, nitrous oxide iv. methane, water vapour, nitrous oxide, ammonia