Oasis School Science and Technology 8 New

Oasis School Science and Technology - 8 247 called elements. Iron (Fe), copper (Cu), gold (Au), hydrogen (H), sodium (Na), carbon (C) are some examples of elements. Elements are the pure substances made up of only one type of atoms. All the atoms of an element are identical but the atoms of different elements are different. Elements cannot be decomposed into any more simpler substances. Gold cannot be broken down into any simpler substance. So it is called an element. About 118 elements have been discovered by scientists so far. Among them, 92 elements are found in nature and remaining 26 elements have been prepared by scientists in laboratory. Atom Elements are made of tiny particles. These tiny particles are called atoms. An atom is the smallest particle of an element which can take part in a chemical reaction. All the atoms of an element are same but the atoms of different elements are different. For example, all the atoms of hydrogen are similar but the atoms of hydrogen differ from the atoms of silver or sodium or copper. The atoms of all 118 elements differ from each other. Atoms may or may not exist freely in nature. For example, atoms of helium, neon, argon, etc. can exist freely in nature but atoms of hydrogen, sodium, oxygen, etc. cannot exist freely in nature. Molecule A molecule is the smallest particle of an element or a compound which can exist freely. The molecule of an element consists of two or more atoms of the same kind. For example, a molecule of hydrogen element (H2 ) consists of two atoms of hydrogen. Similarly, a molecule of oxygen element (O2 ) consists of two atoms of oxygen. The molecule of a compound consists of two or more atoms of different elements. For example, a molecule of sodium chloride (NaCl) contains one atom of sodium (Na) and one atom of chlorine (Cl). Similarly, a molecule of water (H2 O) contains two atoms of hydrogen (H) and one atom of oxygen (O). Differences between atom and molecule Atom Molecule 1. An atom of an element is the smallest particle of that element. 1. A compound is made up of two or more types of atoms. 2. It can take part in chemical reaction 2. It is the result of chemical reaction. – + + – Shell or orbit Nucleus Proton Neutron Electron Fig. 9.1: A neutral atom Fact File Some atoms are always found in pair. They are called diatoms. Example. H₂ Na Cl A molecule of sodium chloride O H H A molecule of water Fig 9.2 H H A hydrogen molecule O O An oxygen molecule

248 Oasis School Science and Technology - 8 Structure of an Atom An atom is the smallest particle of a matter which may or may not exist freely in nature. An atom is made up of three fundamental particles, i.e. proton, neutron and electron. These particles are also called sub-atomic particles. Among three fundamental particles of an atom, protons and neutrons are located at the centre of the atom, i.e. nucleus whereas electrons revolve around the nucleus in elliptical orbits called shells. The structure of a carbon atom showing the position of sub-atomic particles is given in the fig 9.3. Reasonable Facts Atoms are electrically neutral. There are equal number of positively charged protons and negatively charged electrons in an atom. So, atoms are electrically neutral. Activity 1 Make a 3D model of an atom. Sub atomic particles of an atom Protons Protons are positively charged sub-atomic particles located in the nucleus of an atom. The mass of a proton is equal to the mass of a hydrogen atom. The mass of a hydrogen atom is regarded as 1 atomic mass unit (amu). A proton is denoted by the symbol p+ . The mass of a proton is 1837 times larger than that of an electron. Neutrons Neutrons are charge less sub-atomic particles located in the nucleus of an atom. The mass of a neutron is equal to the mass of a proton. A neutron is denoted by n0 . The mass of a neutron is 1837 times larger than that of an electron. Electrons Electrons are negatively charged sub-atomic particles that revolve around the nucleus in elliptical orbits called shells. The mass of an electron is very less as compared to that of a proton or a neutron. The mass of an electron is about 1 1837 th of a proton or a neutron. An electron is denoted by the symbol e– . – – – – – – + + + + + + Nucleus Proton (p+) Neutron (n0 ) Shell or orbit Electron (e– ) Fig. 9.3 Structure of a carbon atom

Oasis School Science and Technology - 8 249 Differences between proton and electron Proton Electron 1. Proton is the positively charged of an atom. 1. Electron is negatively charged subatomic particle. 2. It is located in the nucleus of an atom. 2. Electron revolves around the nucleus of an atom. Atomic Mass Unit The mass of an atom is extremely small. So it cannot be measured in kilogram (kg), gram (g), milligram (mg), etc. Therefore, the mass of an atom is expressed in amu, i.e. atomic mass unit. The mass of one hydrogen atom is taken as 1 amu. The mass of a proton is approximately equal to that of one hydrogen atom. So, the mass of a proton is about 1 amu. The mass of a proton is equal to that of a neutron whereas the mass of an electron is about 1 1837 th of that of a proton. About 6 × 1023 protons form 1 gram mass, i.e. 1 gram = 6 × 1023 amu. Electric Charge Out of three sub-atomic particles, proton and electron contain electric charge. Protons contain positive charge and electrons contain negative charge. But neutrons do not have any charge. They are charge-less sub-atomic particles. In SI system, electric charge is measured in coulomb. It is denoted by C or Coul. One coulomb charge contains about 6.25 × 1018 electrons. A neutral atom consists of equal number of protons and electrons. Therefore, an atom is electrically neutral due to the presence of equal number of protons and electrons having opposite charges. The comparative study of protons, neutrons and electrons is tabulated below: S.N Sub-atomic particles Symbol Location Mass Charge 1. Proton p+ Nucleus 1 amu + 2. Neutron n0 Nucleus 1 amu Nil (0) 3. Electron e– Shell or orbit 1 1837 amu – Atomic Number Atomic number of an atom is defined as the number of protons present in the nucleus of that atom. In a neutral atom, the number of electrons is equal to that of protons. So, atomic number can also be defined as the total number of electrons (e– ) present in a neutral atom. Fact File 1 amu is defined as 1 12 th th part of a carbon 12 isotope.

250 Oasis School Science and Technology - 8 Atomic number is denoted by 'Z'. Atomic Number (Z) = No. of p+ = No. of e– in a neutral atom For example, the atomic number of carbon atom is 6. It means that one atom of carbon has 6 protons and 6 electrons. Atomic Mass or Atomic Weight The atomic mass or atomic weight of an atom is the sum of total number of protons and neutrons. It is denoted by A. Since the mass of electrons is very less as compared to that of protons and neutrons, the mass of electrons is neglected while calculating the atomic mass. Atomic mass (A) = No. of p+ + No. of n0 Similarly, No. of n0 = Atomic weight (A) – No. of p+ No. of n0 = Atomic weight (A) – Atomic number (Z) ∴ Atomic number (Z) = No. of p+ Above relation is used to find out the number of protons, neutrons and electrons from the atomic number and atomic mass or atomic weight of an atom. Reasonable Facts Mass of electron is not added while calculating atomic mass of an atom. The mass of the electron is not added while calculating atomic mass of an atom because electrons are almost weightless. Worked out Numerical 1 The atomic number of a chlorine atom is 17 and its atomic weight is 35. Calculate the number of protons, neutrons and electrons present in that atom. Solution: Given , Atomic no. (Z) = 17 Atomic weight (A) = 35 Now, Atomic no. (Z) = No. of p+ = No. of e– in a neutral atom = 17 = 17 ∴ The no. of protons (p+ ) = 17 The no. of electrons (e– ) = 17 Again, Atomic weight (A) = No. of p+ + No. of n0

Oasis School Science and Technology - 8 251 or, 35 = 17 + No. of n0 or, No. of n0 = 35 – 17 = 18 ∴ The number of neutrons (n0 ) = 18 Electronic Configuration and Bohr and Bury's 2n² Rule Electrons revolve around the nucleus of an atom in elliptical orbits or shells. In 1921 AD, Bohr and Bury proposed a scheme to describe the arrangement of electrons in various orbits or energy levels around the nucleus of an atom. The systematic distribution of electrons in various orbits or energy levels around the nucleus of an atom is called electronic configuration. The shells or orbits are named as K, L, M, N, O, P and Q according to the distance from the nucleus of an atom as shown in the givenfigure.Electrons are distributed systematically in different shells or orbits. The number of electrons that can be accommodated by a certain shell or orbit can be determined by 2n² rule. The rule that determines the maximum number of electrons that can be accommodated in different shells of an atom is called 2n² rule. In 2n² rule, 'n' represents the number of shells or orbits. The total number of electrons accommodated by the first four shells, i.e. K, L, M and N is given below: Shell or Orbit No. of Shell No. of electrons accommodated by the shell K n = 1 2n² = 2 × 12 = 2 L n = 2 2n² = 2 × 22 = 8 M n = 3 2n² = 2 × 32 = 18 N n = 4 2n2 = 2 × 42 = 32 2n² rule is applicable to a few elements and shells K, L, M and N. But this rule is not applicable for the shells O, P and Q since the outermost shell cannot accommodate more than 8 electrons. The maximum number of electrons that can be accommodated by the shells O, P and Q is 32, 18 and 8 respectively. The energy level increases gradually from the first shell to the last shell of an atom. So, electrons are filled from the first shell, i.e. shell K to the higher shells, i.e. from lower energy level to the higher energy level. Fact File Even though many electrons can fit in the orbits they intend to stay in the group of 8. Fig.11.4 Electronic distribution in different shells Distribution of electrons Shells Nucleus

252 Oasis School Science and Technology - 8 The atomic number, symbol, atomic weight, number of protons, neutrons and electrons, electronic configuration and valency of the first 20 elements are tabulated below: Atomic Number Name of Elements Symbol Number of Atomic Weight (p+ +n0 ) Electronic Configuration Valency P+ n0 e– K L M N 1. Hydrogen H 1 0 1 1+0 = 1 1 × × × 1 2. Helium He 2 2 2 2+2 = 4 2 × × × 0 3. Lithium Li 3 4 3 3+4 = 7 2 1 × × 1 4. Beryllium Be 4 5 4 4+5 = 9 2 2 × × 2 5. Boron B 5 6 5 5+6 = 11 2 3 × × 3 6. Carbon C 6 6 6 6+6 = 12 2 4 × × 4 7. Nitrogen N 7 7 7 7+7 = 14 2 5 × × 3 8. Oxygen O 8 8 8 8+8 = 16 2 6 × × 2 9. Fluorine F 9 10 9 9+10 = 19 2 7 × × 1 10. Neon Ne 10 10 10 10+10 = 20 2 8 × × 0 11. Sodium Na 11 12 11 11+12 = 23 2 8 1 × 1 12. Magnesium Mg 12 12 12 12+12=24 2 8 2 × 2 13. Aluminium Al 13 14 13 13+14 = 27 2 8 3 × 3 14. Silicon Si 14 14 14 14+14 = 28 2 8 4 × 4 15. Phosphorus P 15 16 15 15+16 = 31 2 8 5 × 3 16. Sulphur S 16 16 16 16+16 = 32 2 8 6 × 2 17. Chlorine Cl 17 18 17 17+18 = 35 2 8 7 × 1 18. Argon Ar 18 22 18 18+22 = 40 2 8 8 × 0 19. Potassium K 19 20 19 19+20 = 39 2 8 8 1 1 20. Calcium Ca 20 20 20 20+20 = 40 2 8 8 2 2 The atomic structure and electronic configuration of first twenty elements is given below: 1. Hydrogen (H) Shells K L M N No. of e– 1 × × × 2. Helium (He) Shell K L M N No. of e– 2 × × × Hydrogen 1p+ 0n0 e– K Helium 2p+ 2n0 e– K

Oasis School Science and Technology - 8 253 3. Lithium (Li) Shells K L M N No. of e– 2 1 × × 4. Beryllium (Be) Shells K L M N No. of e– 2 2 × × 5. Boron (B) Shells K L M N No. of e– 2 3 × × 6. Carbon (C) Shells K L M N No. of e– 2 4 × × 7. Nitrogen (N) Shells K L M N No. of e– 2 5 × × 8. Oxygen (O) Shells K L M N No. of e– 2 6 × × 9. Fluorine (F) Shells K L M N No. of e– 2 7 × × 10. Neon (Ne) Shells K L M N No. of e– 2 8 × × 3p+ 4n0 Lithium L K 5p+ 6n0 Boron K L 6p+ 6n0 Carbon K L 7p+ 7n0 Nitrogen K L 8p+ 8n0 Oxygen K L 9p+ 10n0 Fluorine K L 10p+ 10n0 Neon L K 4p+ 5n0 Beryllium K L

254 Oasis School Science and Technology - 8 11. Sodium (Na) Shells K L M N No. of e– 2 8 1 × 12. Magnesium (Mg) Shells K L M N No. of e– 2 8 2 × 13. Aluminium (Al) Shells K L M N No. of e– 2 8 3 × 14. Silicon (Si) Shells K L M N No. of e– 2 8 4 × 15. Phosphorus (P) Shells K L M N No. of e– 2 8 5 × 16. Sulphur (S) Shells K L M N No. of e– 2 8 6 × 17. Chlorine (Cl) Shells K L M N No. of e– 2 8 7 × 18. Argon (Ar) Shells K L M N No. of e– 2 8 8 × Sodium 11p+ 12n0 K L M Magnesium 12p K L M + 12n0 Aluminium K L M 13p+ 14n0 Silicon 14p K L M + 14n0 Phosphorus 15p+ 16n0 K L M Sulphur 16p+ 16n0 K L M Chlorine 17p+ 18n0 K L M Argon 18p K L M + 22n0

Oasis School Science and Technology - 8 255 19. Potassium (K) Shells K L M N No. of e– 2 8 8 1 20. Calcium (Ca) Shells K L M N No. of e– 2 8 8 2 Symbol A symbol is the abbreviation of the full name of an element which is represented by one or two English letters. It is used to make the study of elements easy and fast. If the symbol of an element has one letter, it is written in capital letter. If it has two letters, the first letter is written in capital and the second letter in small. Reasonable Facts Symbols are made for elements. Symbols are made for each elements so that it will be easy to write molecular formula and chemical equations. Some examples of elements which are represented by single letter are given below: Atomic number Elements Symbols Atomic number Elements Symbols 1 Hydrogen H 15 Phosphorus P 5 Boron B 16 Sulphur S 6 Carbon C 23 Vanadium V 7 Nitrogen N 53 Iodine I 8 Oxygen O 92 Uranium U 9 Fluorine F There are some cases where the names of elements start with the same letter. For example, hydrogen and helium begin with 'H'. Carbon, chlorine, chromium, cobalt, cadmium, and calcium start with 'C'. In such conditions, we take the first letter and another significant letter to symbolize these elements. Potassium K L M N 19p+ 20n0 Calcium K L M N 20p+ 20n0

256 Oasis School Science and Technology - 8 Atomic number Elements Symbols Atomic number Elements Symbols 2 Helium He 12 Magnesium Mg 3 Lithium Li 25 Manganese Mn 17 Chlorine Cl 30 Zinc Zn 20 Calcium Ca 40 Zirconium Zr 24 Chromium Cr 22 Titanium Ti 27 Cobalt Co 73 Tantalum Ta 48 Cadmium Cd There are certain elements whose names are written in English but symbols are used from the Latin or German name. English names Latin/German names Symbols Iron Ferrum (Latin name) Fe Copper Cuprum (Latin name) Cu Gold Aurum (Latin name) Au Silver Argentum (Latin name) Ag Mercury Hydrargyrum (Latin name) Hg Sodium Natrium (Latin name) Na Potassium Kalium (Latin name) K Tungesten Wolfan (German name) W Radicals Radicals are charged atoms or group of atoms having a common charge which act as a single unit during a chemical reaction. They have either positive charge or negative charge. Radicals are charged particles. So, they are highly reactive and least stable. Hence, they do not occur in free form and make different types of compounds. On the basis of electric charge, radicals are of two types: 1. Electropositive radicals or basic radicals The atoms or group of atoms which have positive charge in them are called electropositive radicals or basic radicals. Some examples of electropositive radicals with their valencies are given below:

Oasis School Science and Technology - 8 257 Radicals having valency 1 (Monovalent) Radicals having valency 2 (Bivalent) Radicals having valency 3 (Trivalent) Radical having valency 4 (Tetravalent) Hydrogen (H+ ) Beryllium (Be++) Boron (B+++) Stannic (Sn++++) Lithium (Li+ ) Magnesium (Mg++) Aluminium (Al+++) Plumbic (Pb++++) Sodium (Na+ ) Calcium (Ca++) Ferric (Fe+++) Silicon (Si++++) Potassium (K+ ) Strontium (Sr++) Auric (Au+++) Rubedium (Rb+ ) Barium (Ba++) Chromium (Cr+++) Caesium (Cs+ ) Cupric (Cu++) Manganic (Mn+++) Cuprous (Cu+ ) Mercuric (Hg++) Mercurous (Hg+ ) Stannous (Sn++) Ammonium (NH4 + ) Zinc (Zn++) Aurous (Au+ ) Nickel (Ni++) Manganous (Mn++) 2. Electronegative radicals or acidic radicals The atom or group of atoms which have negative charge in them are called electronegative radicals or acidic radicals. Some examples of electronegative radicals are given below: Radicals having valency 1 (Monovalent) Radicals having valency 2 (Bivalent) Radicals having valency 3 (Trivalent) Fluoride (F_ ) Oxide (O_ _ ) Nitride (N _ _ _ ) Chloride (Cl_ ) Sulphide (S_ _ ) Phosphate (PO4 _ _ _ ) Bromide (Br_ ) Sulphite (SO3 _ _ ) Phosphide (P _ _ _ ) Iodide (I – ) Carbonate (CO3 _ _ ) Phosphite (PO3 _ _ _ ) Nitrate (NO3 _ ) Sulphate (SO4 _ _ ) Nitrite (NO2 _ ) Zincate (ZnO2 _ _ ) Cyanide (CN_ ) Silicate (SiO3 – – ) Hydroxide (OH_ ) Peroxide (O2 _ _ )

258 Oasis School Science and Technology - 8 Chlorate (ClO3 _ ) Dichromate (Cr2 O7 _ _ ) Bisulphate (HSO4 _ ) Thiosulphate (S2 O3 _ _ ) Bicarbonate (HCO3 _ ) Metaluminate (AlO2 _ ) Valence Shell, Valence Electrons and Valency The outermost shell of an atom from where loss or gain of electrons takes place is called valence shell and the total number of electrons present in the valence shell (outer shell) are called valence electrons. For example, valence electrons in sodium, magnesium and chlorine are 1, 2 and 7 respectively. Valence electrons determine the valency of an atom. The combining capacity of an element or a radical with another element or radical to form a compound or molecule is called valency. In the past, valency was defined as the total number of hydrogen atoms which are combined with an element during chemical combination. For example: In HCl, the valency number of chlorine is 1. In H2 O, the valency number of oxygen is 2. In NH3 , the valency number of nitrogen is 3. In CH4 , the valency number of carbon is 4. But all compounds do not have hydrogen atoms. So, this concept has been modified and a new concept is put forth. According to the new concept, "The total number of electrons lost, gained or shared by an atom during chemical combination is called valency." Valency of sodium is 1 because it loses one electron and valency of oxygen is 2 because it gains two electrons. The valency of chlorine is 1 as it gains one electron from other elements. Similarly, the valency of carbon is 4 as it shares four electrons during chemical combination. Examples, 1. Find out the valency of aluminium in AlCl3 . In AlCl3 , three atoms of chlorine combine with one atom of aluminium. So, the valency of aluminium is 3. 2. Find out the valency of hydrogen and phosphate in H3 PO4 . In H3 PO4 , three atoms of hydrogen combine with one phosphate radical. So, valency of phosphate is 3 and that of hydrogen is 1. Duplet State and Octet State Helium is the first member of inert gases. It has only two electrons and according to the 2n² rule these two electrons are present in the first shell (K-shell). So, the arrangement of two electrons in the K-shell of an atom is called duplet state. Since helium atom has duplet state, it does not take part in chemical reaction and remains in atomic form.

Oasis School Science and Technology - 8 259 Reasonable Facts Helium is called duplet. Helium is called duplet because its valance orbit is naturally stable by two valance electrons. Except helium, the other five elements have eight electrons in their valence shell. It is also stable electronic configuration. The state of having eight electrons in valence shell (last shell) of an atom is called octet state. The presence of two electrons in helium (He) and eight electrons in Ne, Ar, Kr, Xe, and Rn is the main cause of stability of these elements. Hence, they have zero combining capacity (valency). The elements which have more or less than eight electrons in their last shell are chemically unstable and they always try to achieve this condition which is called octet rule. So, the tendency of elements by which they try to maintain eight electrons in their valence shell (last shell) either by transferring or sharing of electrons is called octet rule. Similarly, some elements like H, Li, Be, etc. try to maintain two electrons in the shell K (last shell) either by transferring or sharing of electrons which is called duplet rule. Reasonable Facts Neon atom can exist freely in nature. Neon has eight electrons in its outermost shell. So, neon atom can exist freely in nature. Differences between Duplet and Octet Duplet Octet 1. The state of an atom having two electrons in its valence shell to be stable is called duplet state. 1. The state of an atom having eight electrons in its valence shell is called octet state. 2. The state of helium atom is called duplet state. 2. The state of neon atom is called octet state. Molecular Formula The molecular formula of a molecule is the symbolic representation of the molecule of an element or a compound in molecular form. It represents the actual number of atoms of different elements in a molecule. For example, the molecular formula of sodium chloride is NaCl and that of water is H2 O. It means that one molecule of sodium chloride (NaCl) consists of one atom of sodium (Na) and one atom of chlorine (Cl). Similarly, one molecule of water (H2 O) consists of two atoms of hydrogen (2H) and one atom of oxygen. (O) In case of inert gases, i.e. He, Ne, Ar, Kr, Xe and Rn, the single atom represents atom as well as molecule because they are monoatomic molecules. Elements like hydrogen, nitrogen, oxygen, chlorine, bromine and iodine have two atoms in their molecule, viz. H2 , N2 , O2 , Cl2 , Br2 and I2 respectively. So, they are called diatomic molecules.

260 Oasis School Science and Technology - 8 We should follow the following steps to write the correct molecular formula of a molecule. i. Write the symbol of basic (positive) and acidic (negative) radicals side by side. ii. Write the valency of each radical on upper right corner of each. iii. Exchange the valency of these radicals. Take HCF if it is necessary. iv. Combine radicals with exchanged valency. Examples: Al 1 3 Cl AlCl3 Na 1 1 1 1 Cl NaCl Mg 1 2 3 2 1 1 Cl MgCl2 1. Sodium chloride 2. Magnesium chloride 3. Aluminium chloride C 1 4 1 4 1 2 Cl CCl4 Na 2 1 O Na2 O 2 1 1 2 4 4 (NH4 )2 SO4 4. Carbon tetrachloride 5. Sodium oxide 6. Ammonium sulphate Ca 2 2 2 2 CaSO4 Al 1 3 3 1 Al(NO3 )3 Ca2 1 2 2 1 Ca(HCO3 )2 7. Calcium sulphate 8. Aluminium nitrate 9. Calcium bicarbonate SO 2 4 NO 3 HCO 3 NH SO Information Obtained from Molecular Formula i. Molecular formula represents one molecule of a substance. ii. Itindicates total number of atoms ofthe same or different element/s in each molecule. iii. It indicates percentage composition of each element present in the compound. iv. The valency or combining capacity of each element can be found from the molecular formula. Example: In water molecule (H2 O), the valency of hydrogen is 1 and that of oxygen is 2. v. We can calculate molecular weight from the molecular formula. For example, The molecular weight of water (H2 O) = 2 × H + 1 × O = 2 ×1 + 1 × 16 = 18 amu.

Oasis School Science and Technology - 8 261 Molecular Weight The total weight of the molecule of a substance is called molecular weight. It is calculated by adding the atomic weights of all the atoms present in a molecule. So, the sum of atomic weights of the atoms present in a molecule is called molecular weight. One molecule of hydrogen (H2 ) contains two atoms of hydrogen. So the molecular weight of hydrogen molecule is 2 since atomic weight of hydrogen atom (H) is 1. The molecular weights of some common molecules are given below: S.N. Molecules Molecular Weights 1. Sodium chloride (NaCl) NaCl = 1 × Na + 1 × Cl = 1 × 23 + 1 × 35 = 58 amu 2. Water (H2 O) H2 O = 2 × H + 1 × O = 2 × 1 + 1 × 16 = 18 amu 3. Carbon diodixde (CO2 ) CO2 = 1 × C + 2 × O = 1 × 12 + 2 × 16 = 44 amu 4. Calcium carbonate (CaCO3 ) CaCO3 = 1 × Ca + 1 × C + 3 × O = 1 × 40 + 1 × 12 + 3 × 16 = 100 amu 5. Magnesium chloride (MgCl2 ) MgCl2 = 1 × Mg + 2 × Cl = 1 × 24 + 2 × 35 = 94 amu 6. Calcium chloride (CaCl2 ) CaCl2 = 1 × Ca + 2 × Cl = 1 × 40 + 2 × 35 = 110 amu 7. Ammonium sulphate [NH4 )2 SO4 ] (NH4 )2 SO4 = 2 × N + 8 × H + 1 × S + 4 × O = 2 × 14 + 8 × 1 + 1 × 32 + 4 × 16 = 132 amu Classification of Elements Scientists have discovered 118 elements so far. Among them, 92 elements are found in nature and remaining 26 elements are synthesized artificially in laboratories. It is difficult to study all these elements one by one. So scientists have arranged these elements into various categories like groups, periods, blocks, etc. on the basis of their similarities and differences.

262 Oasis School Science and Technology - 8 a. Mendeleev's Periodic Table Russian 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 above observation, Mendeleev formulated a 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 order of increasing atomic weights, the properties of the elements go on changing with atomic weights. But after a certain interval, they repeat the properties of previous elements and fall one below another in the same vertical column. With the help of above 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. A part of Mendeleev's periodic table b. Modern Periodic Table A group of chemists led by Henery Moseley studied the chemical and physical properties of all known elements. They found that properties of elements depended more correctly on atomic number rather than atomic weight. They said that atomic number is the fundamental property of the atom. Afterward, they proposed a new law which is called modern periodic law. Modern periodic law states that, "Physical and chemical properties of the elements are a periodic function of their atomic numbers." After the discovery of 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. The elements having gradual change in the properties were in the horizontal row called period. Hence, a more correct table than

Oasis School Science and Technology - 8 263 Mendeleev's table was obtained which is known as modern periodic table. So, "The table which is obtained after arranging elements on the basis of increasing atomic numbers is called modern periodic table." Differences between Modern Periodic table and Mendeleev's Periodic Table S.N. Modern Periodic Table S.N. Mendeleev's Periodic Table 1 It is based on increasing atomic numbers. 1 It is based on increasing atomic weights. 2 In this table, there are eighteen vertical columns. 2 In this table, there are eight vertical columns. 3 In this table, there is fixed position for hydrogen, noble gases, lanthanides and actinides. 3 In this table, there is no fixed position for hydrogen, noble gases, lanthanides and actinides. Periods In 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 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 period 1. First 2 Very short period 2. Second 8 Short period 3. Third 8 Short period 4. Fourth 18 Long period 5. Fifth 18 Long period 6. Sixth 32 Very long period 7. Seventh 32 Very long period Features of periods i. Horizontal rows of the periodic table are called periods. ii. In a periods the number of shells are same but number of electrons in each element is different. So, valency of the elements is also different. iii. In a period as we move from left to right, the size of elements decreases. iv. In a period as we move from left to right, the metallic character decreases and non-metallic character increases. v. In a period as we move from left to right, the physical and the chemical characters change. Groups In 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 another.

264 Oasis School Science and Technology - 8 In modern periodic table,representative elements are kept in groups 1(IA), 2(IIA), 13(IIIA), 14(IVA),15(VA), 16(VIA) and 17(VIIA). 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 or group VIIIA. Similarly, transitional elements are kept in groups 11(IB), 12(IIB), 3(IIIB), 4(IVB), 5(VB), 6(VIB), 7(VIIB), 8(VIIIB), 9(VIIIB) and 10(VIIIB). Features of group i. Vertical columns of the periodic table are called groups. ii. In a group, the number of shells increases continuously as we move from top to bottom but number of electrons in each element remains the same. So, valency of the elements is also same. iii. In a group as we move from top to bottom, the size of elements increases. iv. In a group as we move from top to bottom, the metallic character increases and non-metallic character decreases. v. In a group as we move from top to bottom, the physical and the chemical characters of the elements remain almost the same. Position of Elements in Modern Periodic Table 1. Position of hydrogen Hydrogen having the least atomic number, i.e., 1 is placed in group 1 (IA) of the modern periodic table. But it also shows the properties of group 17(VIIA) of the periodic table. 2. Position of metals Metals 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). 3. Position of non-metals Non-metals are placed on the left side of the modern periodic table. For example, halogens (F, Cl, Br, I and At) are placed in group 17(VIIA). 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 metalloids Metalloids (like Si, Ge, As, Te, etc.) are placed in between metals and non-metals. 5. Position of Lanthanides and Actinides Elements 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. 6. Position of inert gases Inert 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.

Oasis School Science and Technology - 8 265 Modern Periodic Table s-Block IA/1 IIA/2 IIIB/3 VB/5 VIIB/7 VIII/9 IB/11 IVB/4 1234567 VIB/6 VIII/8 VIII/10 IIB/12 IIIA/13 IVA/14 VA/15 VIA/16 VIIA/17 0/18 f-Block p-Block d-Block Lanthanide series Actinide series Alkali metals Alkaline earth metals Transition metals Basic metals Non metals Halogens Noble gases Actinides Lanthan- ides Metalloids

266 Oasis School Science and Technology - 8 Characteristics of Periods and Groups Atomic radius Atomic radius is the distance between the outermost shell and the centre of the nucleus of an atom. It determines the size of an atom. The atomic radius (atomic size) decreases on moving from left to right in a period of the Modern periodic table. Similarly, atomic radius of elements increases gradually on moving from top to bottom in a group of the Modern periodic table. Metallic character 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. Non-metallic character Non-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. Valence electrons The electrons present in the outermost shell (or valence shell) of an atom are called valence electrons. Number of valence electrons remains the same in a group, but it increases continuously in a period. Valency The 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. Number of shell The number of shells increases continuously in a group as we move top to bottom. But the number of shells remains the same in a period as we move left to right. Differences between Periods and Groups S.N. Periods S.N. Groups 1. The horizontal rows of periodic table are called periods. 1. The vertical columns of periodic table are called groups. 2. In periods, atomic size of elements decreases while moving from left to right. 2. In groups, 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.

Oasis School Science and Technology - 8 267 Chemical Reaction When hydrogen gas (H2 ) burns in oxygen (O2 ), it forms water (H2 O). Here, hydrogen combines with oxygen and forms water. When calcium carbonate (CaCO3 ) is heated, it decomposes or breaks down into calcium oxide (CaO) and carbon dioxide (CO2 ). Similarly, when zinc (Zn) reacts with sulphuric acid (H2 SO4 ), it displaces hydrogen from sulphuric acid and forms zinc sulphate (ZnSO4 ) and hydrogen (H2 ) gas. So, the combination, decomposition or displacement that occurs in the molecules of matter during a chemical change is called chemical reaction. Examples: 1. When hydrogen gas burns in oxygen gas, it forms water, i.e. Hydrogen + Oxygen burn Water H2 + O2 burn H2 O 2. When calcium carbonate is heated, it forms calcium oxide and carbon dioxide, i.e. Calcium carbonate heat Calcium oxide + Carbon dioxide CaCO3 ∆ CaO + CO2 3. When zinc reacts with sulphuric acid, it forms zinc sulphate and hydrogen gas, i.e. Zinc + Sulphuric acid Zinc sulphate + Hydrogen Zn + H2 SO4 ZnSO4 + H2 A chemical reaction is expressed in word equation and chemical equation or formula equation. Word Equation The chemical reaction expressed by writing the full names of reactants and products is called a word equation. Examples: 1. Hydrogen + Oxygen 2. 3. 4. Sodium + Chlorine Calcium carbonate Zinc + Hydrochloric acid Water Sodium chloride Calcium oxide + Carbon dioxide Zinc chloride + Hydrogen Burn Heat Contact Reasonable Facts Word equations are less preferred. Word equations are less preferred because it cannot tell the number of atoms of elements, cannot be balanced and inconvenient to write.

268 Oasis School Science and Technology - 8 Chemical Equation The chemical reaction expressed by writing symbols and molecular formulae of reactants and products is called chemical equation. A chemical equation is more informative than a word equation. Examples: 2H2 + O2 1. 2. 3. 4. 2Na + Cl2 CaCO3 Zn + 2HCl 2H2 O 2NaCl CaO + CO2 ZnCl2 +H2 ∆ ∆ heat Reactants and Products The chemical substances which take part in a chemical reaction are called reactants. The chemical substances which are produced after chemical reaction are called products. Reactants are written on the left side of the arrow whereas products are written on the right side of the arrow while writing a chemical equation. For example, Differences between Reactants and Products Reactants Products 1. The chemical substances which take paret in a chemical reaction are called reactants. 1. The chemical substances which are produced after chemical reaction are called products. 2. Amount of reactants keep decreasing during the chemical reaction. 2. Amount of products keep increasing during the chemical reaction. Unbalanced or Skeleton Chemical Equation The chemical equation in which the total number of atoms of each element in reactants and products are not equal is called unbalanced or skeleton chemical equation. For example: In above equation, the number of oxygen atoms in reactant and product sides is not equal so it is called unbalanced chemical equation. Some more examples of unbalanced chemical equations are as follows: HCl + NaOH Reactants Products NaCl + H2 O H2 + O2 H2 O

Oasis School Science and Technology - 8 269 Mg + HCl KOH + H2 SO4 H2 O2 MnO2 HCl + Ca(OH)2 MgCl2 + H2 K2 SO4 + H2 O H2 O + O2 CaCl2 + H2 O Balanced Chemical Equation The chemical equation written by balancing the total number of atoms of each element in reactants and products is called balanced chemical equation. In this chemical equation, the number of atoms of each element is equal in reactants and products. It gives more information than the unbalanced chemical equation. Above unbalanced chemical equations can be balanced as follows: 2H2 + O2 Mg + 2HCl 2KOH + H2 SO4 2H2 O 2 2HCl + Ca(OH)2 2H2 O MgCl2 + H2 K2 SO4 + 2H2 O 2H2 O + O2 CaCl2 + 2H2 O ∆ MnO2 In the above chemical equations, the number of atoms of the same element in reactant and product sides are equal. So, they are called balanced chemical equations. Reasonable Facts Formula equation is more meaningful than a word equation. Word equation represents the names of reactants and products in the form word whereas formula equation represents the names of reactants atoms and molecules present in reactants and products. So, formula equation is more meaningful than a word equation. Methods of Writing Balanced Chemical Equation Following points should be remembered while balancing the chemical equation: i. First of all, the chemical change is written correctly in the form of word equation. For example: Hydrogen + Oxygen Water ii. The word equation is written correctly in the form of formula equation or chemical equation. For example: H2 + O2 H2 O iii. The number of atoms of each element are balanced by using suitable coefficient without changing the molecular formulae of reactants and products.

270 Oasis School Science and Technology - 8 For example: 2H2 + O2 2H2 O iv. The number of atoms in the biggest molecule should be balanced before balancing the number of hydrogen and oxygen atoms. This method of balancing chemical equation is called hit and trial method. Some More Examples of Balanced Chemical Equation i. Word equation : Sodium + Chlorine Sodium chloride Unbalanced formula equation : Na + Cl2 NaCl Balanced formula equation: 2Na + Cl2 2NaCl ii. Word equation : Potassium + Oxygen Potassium oxide Unbalanced formula equation : K + O2 K2 O Balanced formula equation: 2K + O2 2KO iii. Word equation : Magnesium + Oxygen Magnesium oxide Unbalanced formula equation : Mg + O2 MgO Balanced formula equation: 2 Mg + O2 2MgO iv. Word equation : Zinc + Hydrochloric acid Zinc chloride + Hydrogen Unbalanced formula equation: Zn + HCl ZnCl2 + H2 Balanced formula equation: Zn + 2HCl ZnCl2 + H2 v. Word equation: Sulphuric acid + Sodium hydroxide Sodium sulphate + Water Unbalanced formula equation: H2 SO4 +NaOH Na2 SO4 + H2 O Balanced formula equation: H2 SO4 + 2NaOH Na2 SO4 + 2H2 O Reversible Reaction The chemical reaction in which the products can recombine to give back the reactants is called reversible reaction. For example, H2 + I2 heat 2HI When hydrogen (H2 ) and iodine (I2 ) are heated, hydrogen iodide (HI) is formed. When hydrogen iodide is heated in a closed vessel, it also forms hydrogen and iodine. Therefore, the given reaction is a reversible reaction. Reversible reactions are written by giving a double-way arrow between reactants and products as follows: H2 + I2 2HI N2 + 3H2 2NH3 2H2 + O2 2H2 O

Oasis School Science and Technology - 8 271 Irreversible Reaction The chemical reaction in which the products cannot recombine to give back reactants is called irreversible reaction. A single way arrow is used to denote an irreversible reaction. Examples: CaCO3 ∆ CaO + CO2 ↑ 2Na + 2H2 O 2NaOH + H2 ↑ 2KClO3 ∆ 2KCl + 3O2 ↑ Activity 2 • Bring a chart paper and draw the atomic models of the first twenty elements. • Put suitable colours in the pictures and submit to your science teacher. Reasonable Thinking Skill R T S 1. Elements of zero valency are stable. Elements of zero valency are stable because they already satisfied either by the octet or duplet rule. They don’t need to either lose or gain electrons so they remain stable. 2. Ammonium is called an electropositive radical. Ammonium is called an electropositive radical because it contains positive charge. 3. Valency of sodium is 1. The sodium atom has one valance electron. It can lose this electron during the chemical reaction. Since it can lose one electron during a chemical reaction its valency is one. 4. Reactivity of metal increases down the group. Metals are supposed to lose electrons. Down the group, metallic size increases due to the addition of an extra shell. As a result, valance electrons are far from the nucleus, held loosely and are lost easily. So, the reactivity of metal increases down the group. 5. Discuss the importance of molecular formulas. The molecular formula is very important because: i. It is easy to write chemical equations. ii. It is easy to know the involved elements. iii. We can know the number of atoms of involved elements. iv. We can calculate molecular weight. v. It helps to find valency. 6. Identify and convert the given virtual word equation into a balanced chemical equation. ABC3 → AO + BC2 Calcium carbonate → Calcium oxide + Carbon dioxide CaCO3 → CaO + CO2 7. The size of an atom decreases across the period. The size of the atom decreases across the period because there is increase in electrostatic force in the atom due to increase in nuclear charge. The increase in nuclear charge and electrostatic force shrink the atom.

272 Oasis School Science and Technology - 8 8. Differentiate between atom and radical. The differences between atoms and radicals are: SN Atoms SN Radicals 1 An atom is the smallest particle of an element that may or may not take part in a chemical reaction. 1 The atom or group of atoms which has a charge is called a radical. 2 An atom is electrically neutral. 2 A radical is either positively or negatively charged. Exercises 1. Multiple choice questions a. What is the smallest particle of an element called? i. molecule ii. matter iii. atom iv. compound b. What are the subatomic particles that revolve around the nucleus called? i. protons ii. electrons iii. neutrons iv. quarks c. How many electrons can accommodate in L orbit at maximum? i. 2 ii. 4 iii. 6 iv. 8 d. What is the molecular formula of Aluminium oxide? i. AlO3 iii. Al3 O2 iv. Al3 O v. Al2 O3 e. How many electrons are present in 1-coulomb charge? i. 6.25 × 1018 ii. 62.5 × 1018 iii. 65.2 × 1018 iv. 625 × 1018 2. Define the following terms with required examples. a. Matter b. Compound c. Elements d. Atom e. Molecule f. Proton g. Neutron h. Electron i. Atomic number j. Atomic mass k. Electronic configuration l. Symbol m. Radical n. Eectropositive radical o. Electronegative radical p. Valance shell

Oasis School Science and Technology - 8 273 q. Valance electron r. Valency s. Duplet rule t. Octet rule u. Molecular formula v. Mendeleev’s periodic law w. Modern periodic law x. Period y. Group z. Chemical reaction 3. Very short questions a. What is the smallest particle of a compound called? b. What is the mass of a single proton equivalent to? c. What is the sum of the number of protons and neutrons present in the nucleus of an atom called? d. What is the table in which elements are arranged based on properties called? e. What is the imaginary path in which an electron revolves around the nucleus called? 4. Give reason a. Atoms are electrically neutral. b. Molecules are stable. c. Mass of an electron is neglected while calculating atomic mass. d. Molecular formulas are important. e. Elements having similar physical and chemical properties are found after a certain interval of elements. f. Atomic radius increases down the group but decreases from right to left in a period. g. Metallic character of metals increases down the group. h. Metallic character of metals decreases from right to left in the period. i. Chemical reaction must be balanced. j. Valency of calcium is 2 but helium is zero even though both have 2 valance electrons. 5. Differentiate between : a. Atom and molecule b. Proton and neutron c. Electropositive radical and electronegative radical d. Period and group e. Reversible and irreversible reaction f. Word equation and chemical equation

274 Oasis School Science and Technology - 8 6. Short questions a. Write the positions of each subatomic particle of an atom. b. Write down the atomic mass of oxygen, sodium, aluminium and argon. c. Calculate the maximum number of electrons found in shell N by using the rule. d. Write down the valency of oxygen, magnesium and calcium. e. Calculate the molecular weight of the following: i. NaCl ii. CaCO3 iii. Na2 SO4 iv. CaCl2 v. NH4 OH vi. MgSO4 f. Describe reversible reaction with an example. g. Write down the molecular formula of each of the given compounds. i. Sodium chloride ii. Calcium oxide iii. Magnesium chloride iv. Calcium carbonate v. Water vi. Potassium sulphate 7. Long questions a. Describe the structure of an atom with a neat diagram. b. Draw the atomic structure of nitrogen, aluminium, chlorine and calcium. c. The atomic number of a chlorine atom is 11 and its atomic weight is 23. Calculate the number of protons, neutrons and electrons present in that atom. d. Write the importance of molecular formula. e. Write characteristics of periods of the modern periodic table. f. Write characteristics of groups of the modern periodic table. g. Complete the given word equations and express them in the form of balanced formula equations. i. Hydrogen + Oxygen ∆ .................. ii. Calcium carbonate ∆ .................. + .................. iii. Water electrolysis .................. + .................. iv. Zinc + .................. Zinc sulphate + .................. v. Sodium + ................ Sodium hydroxide + ..................

Oasis School Science and Technology - 8 275 10 MATERIALS USED IN DAILY LIFE UNIT

276 Oasis School Science and Technology - 8 UNIT 10.1 ACID, BASE AND SALT Estimated teaching periods Theory 6 Practical 2  Physical properties of acid, base and salt  Chemical properties of acid, base and salt, simple chemical equations  Uses of acid, base and salt  Introduction and causes of acid rain  Impact of acid rain on the environment The Sequence of Curriculum Issued by CDC Antoine Lavoisier is popular for the chemical revolution. He was born in France on 26th August 1743 and died on 8th May 1794. He recognized oxygen and hydrogen. He reformed chemical nomenclature. He noticed that matter can change form or shape but the mass remains constant, known as the ‘Law of conservation of mass. He was one of the first persons to propose the scientific concept of acid and base. He is also popular for calorimetry, carbon cycle, redox reaction and combustion. About the Scientist Antoine Lavoisier Key terms and terminologies 1. Acid : The chemical species that release hydrogen ions when dissolved in water are called acids. 2. Organic acid : Acids that come from living things and have carbon atoms in their molecules are called organic acids. 3. Mineral acids : The acids which are obtained from minerals present on the earth's crust are called mineral acids. 4. Strong acid : Strong acids are those, that when placed in an aqueous solution, almost entirely ionize to produce hydrogen ions. 5. Weak acids : Weak acids are those, that do not completely ionize to give hydrogen ions in an aqueous solution. 6. Bases : Bases are metallic oxides and hydroxides that have a bitter taste and produce hydroxyl ions when dissolved in water. 7. Alkali : Those metallic oxides which are soluble in water to give hydroxides are called alkali. 8. Strong base : Strong bases are those bases that undergo nearly complete ionization to produce a high concentration of hydroxyl ions in an aqueous solution.

Oasis School Science and Technology - 8 277 9. Weak bases : Weak bases are those bases that do not undergo nearly complete ionization in an aqueous solution and produce very low concentrations of hydroxyl ions. 10. Salt : A salt is a chemical compound which is formed by partial or complete substitution of hydrogen ion by a metal or ammonium radical from an acid. 11. Neutral salts : Neutral salts are made when strong acids and strong bases react or weak acids and weak bases react. 12. Acidic salts : The salts which are prepared by the reaction of strong acids with weak bases are called acidic salts. 13. Basic salts : Basic salts are those salts formed by the reaction of weak acids and strong bases. 14. Indicators : Indicators are chemical compounds that are used to determine whether a substance is acidic or basic. 15. Acid rain : The rain containing a small quantity of acids formed from gases like oxides of sulphur, nitrogen, etc. present in the atmosphere is called acid rain. Introduction In our daily lives, we use different kinds of chemicals that come from nature or are made by people. Food, vegetables, fruits, medicines, drinks, flavourings, and so on are all examples. The smells and tastes of these things are all different. Some fruits and vegetables, like lemon, pummelo, apple, grapes, orange, etc., taste sour, while others, like caustic soda, bitter-guard, caustic potash, alum, potash, etc., taste bitter. Most of the time, chemicals that have a sour taste are called acids, and chemicals that have a bitter taste are called bases. Salt is the name for the third group of things that taste salty. Acid, bases, and salt are all things that are very useful. They are used in foods that we eat every day. Some of them are also used to make drugs and medicines. In this unit, we will discuss about acid, base, and salt. Acid Taste some seasonal fruits and vegetables like apple, mango, orange, grape, lemon, tomato, etc. How do you feel different tastes? Some fruits and vegetables taste sour. This is because acids are found in them. So, chemicals that taste sour are called acids. When an acid is dissolved in water, it gives off hydrogen ions. Acids can be found in nature or they can be made in a lab using a chemical process. Therefore, acids are chemical substances that are either found in nature or made by people. When dissolved in water, they taste sour and release hydrogen ions. Acids are classified into different groups on the basis of their features. i. Organic acids ii. Inorganic acids or mineral acids iii. Strong acids iv. Weak acids Fact File The term acid has been derived from "Acidus" meaning sour.

278 Oasis School Science and Technology - 8 v. Oxy acids vi. Hydra acids vii. Mono basic acids vii. Dibasic acids ix. Tri-basic acids Reasonable Facts Acid is called proton donor. Acid is called proton donor because when they dissolve in water they give hydrogen ion which is just a proton. Naturally occurring acids (Organic acids) We can get different kinds of acids from different living things, like citric acid from grapes, oranges, lemons, etc., oxalic acid from tomatoes, lactic acid from sour milk, amino acid from protein, carbonic acid from soft drinks, etc. They are are found in nature, and their molecules also have carbon atoms. Because of this, they are also called organic acids. Acids that come from living things and have carbon atoms in their molecules are called organic acids. Some organic acids and their sources are listed below: Natural Source Acid Present Apples Malice acid Grapes, oranges, lemons Citric acid Vinegar Acetic acid Aerated drinks Carbonic acid Protein Amino acid Sour milk Lactic acid Tea Tannic acid Tomatoes Oxalic acid Activity 1 Visit the kitchen and find any three foods that are acidic and alkaline. Mineral acids (Inorganic acids) The acids which are obtained from minerals present on the earth's crust are called mineral acids. Fact File Organic acids are acids that are derived from living organisms. Organic acids are naturally occurring acids that are weak in nature.

Oasis School Science and Technology - 8 279 Many acids are derived from minerals found in the earth's crust. For example, sulphuric acid (H2 SO4 ), nitric acid (HNO3 ), hydrochloride acid (HCI), etc. These acids have no carbon atoms in their molecular structure. They can also be prepared in a laboratory or in the industry. In nature, they are both strong and weak. Reasonable Facts Ant bites hurts for a long time. Ant bites hurts for a long time because they inject formic acid into our body. Differences between organic and mineral acids S.N. Organic Acids S.N. Mineral Acids 1. 2. 3. Organic acids are obtained from the living organisms(plants and animals) They are weak in nature. They are poor conductors of electricity. 1. 2. 3. Mineral acids are obtained from minerals present on the earth's crust. They are strong as well as weak in nature. They are poor as well as good conductor of electricity. Classification of acids or the basis of strength On the basis of strength, acids are classified into two groups. They are: strong acid and weak acid. a. Strong acids There are many acids such as hydrochloric acid, sulphuric acid, nitric acid, etc; which undergo almost complete ionization in an aqueous solution to give hydrogen ions. They are called strong acids. Strong acids are good conductors of electricity. Strong acids are those acids, which undergo complete ionization in an aqueous solution. HCl H+ +ClH2 SO4 2H+ + SO4 -- HNO3 H+ + NO3 Activity 2 Put dilute hydrochloric acid in a glass test tube under the supervision of teacher. Put some granulated zinc in it. We can now see air bubbles. What it could be? Discuss with your teacher. b. Weak acids Weak acids include citric acid, acetic acid, carbonic acid, tartaric acid, etc. because they produce extremely low hydrogen ions in an aqueous solution. These are poor Fact File Sulphuric acid is called king of acid.

280 Oasis School Science and Technology - 8 electrical conductors because the concentration of hydrogen ions is too low. Generally, all organic acids and certain inorganic acids are weak in nature. Weak acids are those acids which do not completely ionize to give hydrogen ions in an aqueous solution. Differences between strong and weak acids Strong acids Strong acid Weak acids 1. Strong acid undergo almost complete ionization to give high concentration of hydrogen ions. 1. Weak acids undergo very less ionization, so give less concentration of hydrogen ions. 2. Mineral acids are mostly strong acids. 2. All organic acids and some mineral acids are weak acids. 3. They are more corrosive in nature. 3. They are less corrosive in nature. 4. They are good conductor of electricity. 4. They are poor conductor of electricity. Physical Properties of Acids i. Acids are generally sour in taste, but tasting all acids is dangerous because some acids are corrosive in nature. ii. They have a corrosive nature. Strong acids are more corrosive and can burn our skin, tongue, and other organs. iii. When acids are dissolved in water, they produce hydrogen ions. HCl → H+ + Cl⁻ H2 SO4 → 2H⁺ + SO4 — iv. They change the color of the indicators. For example, blue litmus paper turns red, methyl orange turns red, and phenolphthalein turns colorless. v. Acid ionizes into opposite ions and conduct electricity. Chemical Properties of Acids i. They neutralize the base and give salt and water. For example: HCl + NaOH → NaCl + H2 O ii. Dilute acids react with metals to give hydrogen gas. For example: Zn + HCl → ZnCl2 → H₂ Fact File Hydrochloric acid is present in the stomachs of animals because there is no carbon atom in this acid. It is an example of inorganic acid. H₂CO₃ contains carbon atom, but it is derived from minerals and has electrovalent bonding, it is classified as an inorganic acid.

Oasis School Science and Technology - 8 281 iii. When acids react with carbonates and bicarbonates, they give carbon dioxide gas. For example: NaHCO3 + HCl → NaCl + H2 O + CO2 CaCO3 + 2HCl → CaCl2 + H2 O + CO2 iv. When acids react with sulphides, they give hydrogen sulphide gas. For example; ZnS + 2HCI → ZnCl2 + H2 S Uses of acids i. Sulphuric acid (H2 SO4 ) is used to make various chemical fertilizers such as ammonium sulphate, superphosphate, etc. ii. Sulphuric acid is used in a variety of chemical reactions as a dehydrating agent. iii. Batteries contain sulphuric acid. iv. Nitric acid(HNO₃)is usedto make various chemical fertilizers such as ammonium nitrate, calcium nitrate, etc. v. Nitric acid is used in the production of explosives such as TNT and nitroglycerine. vi. Boric acid is used to clean the eyes. vii. Acetic acid is used to flavor food. viii. Soft drinks contain carbonic acid. ix. Tanning and printing industries use hydrochloric acid. x. Citric acid is used in medicine as a vitamin C source. Reasonable Facts Egg shell gets dissolved in vinegar. Egg shell gets dissolved in vinegar because egg shell contains calcium which gets dissolved in vinegar which is acid. Base Do you know what Eno is? What is its function? We take it to reduce acidity. It contains sodium bicarbonate. Similarly, when we go to the doctor, he or she may give us a bottle of white liquid known as an antacid (it is also known as milk of magnesia). It is used to treat excessive stomach acidity. This antacid contains magnesium hydroxide Mg(OH)2 as a base. Ammonia is also used to clean common household glasses. To counteract the acidic nature of the soil, farmers use a chemical fertilizer containing calcium oxide. Sodium hydroxide is used in the manufacturing of soap, detergent, dyes, papers, medicines, rayons, and other products. All of the chemicals listed above are either metallic oxides or hydroxides. Fact File The full form of TNT is trinitrotoluene; it is very strong an explosive substance.

282 Oasis School Science and Technology - 8 Bases are metallic oxides and hydroxides that have a bitter taste and produce hydroxy ions when dissolved in water. Bases include sodium oxide (Na2 O), magnesium oxide (MgO), calcium oxide (CaO), potassium oxide (K2 O), copper oxide (CuO), aluminum oxide (Al₂O₃), zinc oxide (ZnO), ferric oxide (Fe₂O₃), mercuric oxide (HgO), etc. Alkali Put CaO, MgO, CuO, K₂O, HgO, Fe₂O₃, Al₂O₃, and other metallic oxides in water one at a time. Except for CuO, all of the above metallic oxides are soluble in water and form their corresponding hydroxides. These water-soluble metallic oxides are called alkali (i.e. bases). CaO + H2 O → Ca(OH)2 MgO + H2 O → Mg(OH)2 K2 O + H2O → 2KOH Al2 O3 + 3H2 O → 2Al(OH)3 Na2 O + H2 O → 2NaOH Reasonable Facts We should not put strong alkali in hand. We should not put strong alkali in hand because they can burn our hands. They can even burn through gloves. Those metallic oxides which are soluble in water to give hydroxides are called alkali. By dissolving metallic oxides in water, Ca(OH)2 , Mg(OH)2 , KOH, Al(OH)3 , NaOH, and other compounds are formed. In an aqueous solution, the alkali listed above dissociate to form hydroxyl ions (OH⁻). Ca(OH)2 → Cat++ + 2OH Mg(OH)2 → Mg++ + 2OH KOH → K+ + OH- Al(OH)3 → A1+++ + 3OH– NaOH → Na+ + OH– According to the previous discussion, there are numerous metallic oxides, but only a few of them are soluble in water to form hydroxides. Alkali are metallic oxides that dissolve in water to give metal hydroxides. As a result, all alkali are bases, but all bases are not alkali. Differences between bases and alkalis S.B. Bases S.N. Alkalis 1. All metallic oxides are called bases. 1. Only water-soluble metallic oxides or hydroxides are called alkalis. 2. They may be water soluble or insoluble in nature Example: Na2 O, K2 O, CaO, MgO, Al2 O3 , CuO, etc. 2. They are water soluble in nature. Examples: NaOH, KOH, Mg(OH)2 , Ca(OH)2 , etc.

Oasis School Science and Technology - 8 283 Types of bases On the basis of strength, there are two types of bases. They are: a. Strong bases Strong bases are those bases that undergo almost complete ionization to produce a high concentration of hydroxyl ions in an aqueous solution, such as sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide Ca(OH)2 , and others. These bases, like strong acids, are good conductors of electricity. Strong bases are those bases that undergo nearly complete ionization to produce a high concentration of hydroxyl ions in an aqueous solution. b. Weak bases Some bases, such as copper hydroxide Cu(OH)2 , ferric hydroxide Fe(OH)2 , ammonium hydroxide (NH4 OH), and others, do not undergo nearly complete ionization in an acid. As a result, they are referred to as weak bases because they produce a very low concentration of hydroxyl ion in solution. Weak bases are those bases that do not undergo nearly complete ionization in an aqueous solution and produce very low concentrations of hydroxyl ions. Physical properties of bases i. Strong bases are bitter in taste and slippery or soapy to touch. ii. They turn litmus paper red into blue, methyl orange into yellow, and phenolphthalein into pink. iii. Some strong bases, such as sodium hydroxide and potassium hydroxide, are corrosive in nature, just like acids. iv. Bases, like acids, undergo ionization to produce opposite ions and thus conduct electricity. Chemical properties of the bases i. Bases and acids combine to form salt and water (i.e. neutralization reaction). Base + Acid → Salt + Water NaOH + HCI → NaCl + H2 O 2KOH + H2 SO4 → K2 SO4 + 2H2 O ii. Alkalis give carbonates when they absorb carbon dioxide from air. Alkalis + Carbon dioxide → Carbonate + Water 2KOH + CO2 → K2 CO3 + H2 O Ca(OH)2 + CO2 → CaCO3 + H2 O iii. Alkalis react with ammonium salts to give ammonia gas. Alkalis + Ammonium salts → Salt + Water + Ammonia Fact File Because ethanol (C₂H₅OH) does not ionize into ions, it is not considered a base.

284 Oasis School Science and Technology - 8 NaOH + NH4 CI → NaCl + H2 O + NH3 iv. Heavy metal salts react with alkalis to produce insoluble metal hydroxides. Heavy metal salt + Alkalis → Hydroxide + Salt CuSO4 + 2NaOH → Cu(OH)2 + Na2 SO4 Uses of bases i. Aluminum hydroxide Al(OH)3 , magnesium hydroxide Mg(OH)2 , and other alkalis are used to treat stomach hyperacidity. ii. Potassium hydroxide (KOH) is used in batteries as well as the production of soap and detergent. iii. Sodium hydroxide (NaOH) is used in the production of soap, detergent, paper, rayon, and other products. iv. Calcium hydroxide Ca(OH)2 is used to make bleaching powder, reduce water hardness, neutralize soil acidity, and so on. v. Lime water Ca(OH)2 is used as a laboratory reagent. vi. As a laboratory reagent, ammonium hydroxide (NH4 OH) is used. Salt Let's observe the following chemical reactions; H2 SO4 + 2NaOH → Na2 SO4 + 2H2 O H2 SO4 + NaOH → NaHSO4 + H2 O HCl + NaOH → NaCl + H2 O HCl + NH4 OH → NH4 Cl+ H2 O In the firstreaction, sodium metalreplaces both hydrogen atoms of an acid, whereas in the second reaction, sodium metal replaces only one hydrogen atom of sulphuric acid. There is only one hydrogen atom in the third reaction, which is also replaced by sodium metal. The ammonium radical replaces the hydrogen of acid in the fourth chemical reaction. In the preceding reactions, the hydrogen of acids is partially or completely replaced to produce salt and water. Thus, A salt is a chemical compound which is formed by partial or complete substitution of hydrogen ion by a metal or ammonium radicals from an acid. We commonly believe that salts are neutral substances, but this is not always the case. Some may be acidic in nature, while others may be basic. Because only one hydrogen atom of sulphuric acid is replaced by sodium metal in the above second reaction, the resulting salt is acidic in nature. The reaction of an acid with a base to produce salts is known as the neutralization reaction. Acid + Base → Salt + Water HCl + NaOH → NaCl + H₂O H2 SO4 + 2KOH → K2 SO4 + 2H2 O

Oasis School Science and Technology - 8 285 Classification of salts We have already talked about how acids and bases react to make salts. All the salts are not neutral. Some salts have an acidic taste, some have a basic taste, and some are neutral. What kind of salt it is depends on what kinds of acids and bases are used to make it. Here are some examples of the different kinds of salts. Salts Parent acid Parent base Nature NaCl Strong Strong Neutral NH4 Cl Strong Weak Acidic CH3 COONa Weak Strong Basic CH3 COONH4 Weak Weak Neutral K2 SO4 Strong Strong Neutral Na2 CO3 Weak Strong Basic Depending upon pH value, salts can be classified into neutral, acidic and basic salts. i. Neutral salts Neutral salts are like sodium chloride (NaCl), sodium sulphate (Na2 SO4 ), potassium chloride (KCI), etc., which are made when a strong acid reacts with a strong base. Neutral salts are those salts which are formed when strong acids and strong bases react or weak acid and weak base react. Reasonable Facts Table salt is called a neutral salt. Table salt is called a neutral salt because it is formed from the reaction of strong acid and strong base. It does not contain any replaceable hydrogen atoms. ii. Acidic salts Ammonium chloride (NH4 Cl), aluminium sulphate Al2 (SO4 )3 , ammonium nitrate NH4 NO3 , and other acidic salts are formed when strong acids react with weak bases. Those salts which are prepared by the reaction of strong acids with weak bases are called acidic salts. iii. Basic salts Sodium acetate (CH3 COONa), sodium carbonate (Na2 CO3 ), and other basic salts are formed when strong bases react with weak acids. Basic salts are those salts which are formed by the reaction of weak acids and strong bases. Fact File Sodium chloride is called table salt and it is used as regular diet in our food.

286 Oasis School Science and Technology - 8 Physical properties of salts i. The majority of salts are solid in nature. ii. Salts are usually soluble in water, but some salts do not dissolve in water, such as calcium carbonate, calcium sulphate, magnesium sulphate, silver chloride, lead chloride, and so on. iii. Salt solutions conduct electricity as they ionize into opposing ions (i.e. cation and anion). iv. Salt solutions can be neutral, acidic, or alkaline depending on their nature. v. Some salts are amorphous in nature, whereas others are crystalline. vi. Some salts are white or colorless, while others are colorful. Chemical properties of salt i. Potassium chlorate decomposes into potassium chloride and oxygen. 2KClO3 → 2KCl + 3O2 ii. Some salts can exchange radicals during chemical reaction. NaNO3 + AgCl → NaCl + AgNO3 iii. Salts can ionize in solution state. NaCl → Na+ + Cliv. Ammonium salts can react with base to produce ammonia. NH4 Cl + NaOH → NaCl + H2 O + NH3 v. Salts can take part in reduction reaction, where highly reactive metal can displace less reactive metal of the salt. Fe + CuSO4 → FeSO4 + Cu Uses of Salt i. Common salt (NaCl) is used as a preservative in our food. ii. Baking powder is made from baking soda (NaHCO3 ). iii. Washing soda (Na2 CO3 .10H2 O) is used in the production of glasses, soap, detergent, and various industrial chemicals. iv. Potash alum (K2 SO4 .Al2 (SO4 ) 3 .24H2 O) is used in water purification. v. Iron sulphate (FeSO4 .7H2 O) is a key component of pharmaceuticals and bluish ink. Copper sulphate (CuSO4 .5H2 O) is used as a fungicide. vi. Ammonium nitrate (NH4 NO2 ) is used in the production of fertilizers and explosives. vii. Calcium sulphate (CaSO4 .2H2 O) is used in cement and the production of plaster of Paris. viii. Silver bromide (AgBr) is used in photography. ix. Silver nitrate (AgNO3 ) is used to treat infections and various chemical reactions.

Oasis School Science and Technology - 8 287 Indicators We have already discussed that some acids are extremely strong and highly corrosive. We can not taste them. In such cases, to find whether a given substance is acidic or basic we use a chemical called indicators. Indicators are the chemical compounds that are used to determine whether a substance is acidic or basic. Indicators include litmus paper, methyl orange, phenolphthalein, red cabbage juice, etc. As they come into contact with chemical compounds, they alter their own color. Indicators have no chemical activity. They do not interact with the compounds whose nature will be determined. They are derived from the parts of plants such as flowers, leaves, roots, bark, etc. The table given below describes how the color alters with various indications. S.N. Indicators Colour in acid Colour in basic solution Colour in salt solution 1. Red litmus paper No change in colour Changes into blue colour No change in colour 2. Blue litmus paper Changes into red colour No change in colour No change in colour 3. Methyl orange Changes into red colour Changes into yellow colour No change in colour 4. Phenolphthalein No change in colour Changes into pink colour No change in colour Activity 3 Collect some vegetables and use litmus papers to find if they are either acidic or alkaline or neutral. Acid Rain Various gases like carbon dioxide, sulphur dioxide, sulphur trioxide, nitric oxide, nitrogen dioxide, nitrogen pentoxide form acids when they react with water of the atmosphere. These gases are either produced naturally during volcano, leaching, organic decomposition or by humans from industries, factories, kitchen and vehicles. When those acids get mixed with rain and fall down in the form of rain, it is called acid rain. The rain containing small amount of acids formed from gases like oxides of sulphur nitrogen, etc. present in atmosphere is called acid rain. The process of formation of acids in atmosphere is given below: Formation of Sulphuric acid (H2 SO4 ) 2SO2 + O2 2SO3 Fact File Acid rain is common over industrial cities.

288 Oasis School Science and Technology - 8 SO3 + H2 O H2 SO4 (Sulphuric acid) Formation of Carbonic acid (H2 CO3 ) H2 O + CO2 H2 CO3 (Carbonic acid) The acids like H2 SO4 , HNO3 , H2 CO3 , etc. get mixed with rain and fall down on the earth in the form of acid rain. Effects of acid rain i. Acid rain corrodes historical monuments like statues, sculptures of metals, marble, etc. ii. Acid rain increases the acidity of soil and reduces the production of crop. iii. Acid rain increases the acidity of water and may kill the aquatic animals and plants. iv. Acid rain causes different types of skin and respiratory diseases in human beings and other animals. Control measures of acid rain i. The emission of industrial gases should be minimized. ii. The use of alternative energy should be increased. iii. Industrial gases like SO2 and CO2 should be reused before emission from industries. iv. The use of clean fuels should be increased. v. Pollution control devices should be developed and installed in industries. Reasonable Facts Acid rain is harmful for cultural heritage. Acid rain is harmful for cultural heritage because most cultural heritage are stone monuments left by the ancestors which are continuously eroded by acid rain. Reasonable Thinking Skill R T S 1. Hydrogen chloride is acid. Hydrogen chloride is called acid because it can ionize in water to produce hydrogen ions. 2. HCOOH is a weak acid. HCOOH is called a weak acid because it can only ionize partially and give a very low concentration of hydrogen ions in the solution. Fig. acid rain can corrode stone monuments

Oasis School Science and Technology - 8 289 3. Differentiate between acid and base. The differences between acid and base are: SN Acid SN base 1 The chemical species that give hydrogen ions when put in water is called an acid. 1 The chemical species that give hydroxyl ion when put in water is called a base. 2 The acid is sour and usually turns blue litmus into red. 2 The base is bitter and usually turns red litmus into blue. 4. Write the use of aluminium hydroxide. Aluminium hydroxide is the base. It is usually used to neutralize hyper-acidity in our stomach. It is also used to treat hyper-acidity in agricultural fields. 5. What is potassium chlorate used for? Potassium chlorate is used to produce oxygen gas in the science laboratory. 6. What happens if a blue litmus paper is dropped into lemon water juice? Why? What is blue litmus paper? When a blue litmus paper is dropped into lemon water juice it turns red. The blue litmus turns red when put into lemon water solution because lemon water is acidic. Acid changes blue litmus into red. Blue litmus paper is an indicator. It can identify if a given solution is either acidic or alkaline. 7. First rainfall after a few months in the city is very harmful to the environment and health. First rainfall after a few months in the city is very harmful to the environment and health because over a few months, the smoke from industries and vehicles is collected over the sky. During rainfall, this smoke dissolves in water and forms respective acids. Carbon dioxide + Water → Carbonic acid CO2 + H2 O → H2 CO3 This acid mixes with rain water and falls down. This water is called acid rain. Acid rain is harmful to human health and the environment. Because: i. It makes land and water acidic. ii. It is allergic to the skin. iii. It pollutes water. iv. It erodes stone buildings. 8. Lemon is mixed in warm water and used to clean kitchen tiles and marbles. How does it work? Lemon is mixed in warm water and used to clean kitchen tiles and marbles. It works because acid is corrosive. It can kill germs and remove stains.

290 Oasis School Science and Technology - 8 Exercises 1. Choose the best answer from the given alternatives. a. What is the chemical species that produces hydrogen ions in the solution state called? i. acid ii. base iii. salt iv. water b. Which one of the following is a weak acid? i. HCl ii. H2 CO3 iii. H2 SO4 iv. HNO3 c. What chemical species is sodium bicarbonate? i. acid ii. base iii. acidic salt iv. neutral substance d. What is a litmus paper? i. acidic substance ii. basic substance iii. indicator iv. neutral substance e. What is the cause of acid rain? i. water pollution ii. air pollution iii. land pollution iv. sound pollution 2. Define the following terms with required examples. a. Acid b. Organic acid c. Mineral acid d. Strong acid e. Weak acid f. Bases g. Alkali h. Strong base i. Weak base j. Salt k. Neutral salt l. Acidic salt m. Basic salt n. Indicators o. Acid rain 3. Very short questions a. What are proton donors called? b. What is the source of citric acid? c. Give any two examples of strong acids and weak acids. d. What is nitric acid used for? e. What are highly water-soluble bases called? f. Which base is used while manufacturing soap? g. What kind of salt is sodium carbonate? h. What will happen if blue litmus is put in a solution of sodium bicarbonate? 4. Give a reasons. a. Acid is sour.

Oasis School Science and Technology - 8 291 b. Acid is used to clean toilets, tiles and kitchens. c. We should wear gloves while using acid. d. Acid is stored in glass bottles. e. Pickled radish last longer. f. Acetic acid is called weak organic acid. g. Aluminium hydroxide is used to treat hyperacidity. h. All alkalis are bases but all bases are not alkalis. i. Ammonium hydroxide is called weak alkali. j. Sodium chloride is called neutral salt but sodium carbonate is called a basic salt. k. Acid rain can decrease agricultural production. l. Stones temples in cities are eroded. 5. Differentiate between : a. Acid and base b. Strong acid and weak acid c. Alkali and base 6. Short questions a. Write any three physical and four chemical properties of acid. b. Write any three physical and four chemical properties of the base. c. Write any four physical and four chemical properties of the salt. d. What are the harmful effects of acid rain on the environment? e. What can be done to prevent acid rain? 7. Long questions a. What happens if we drop a red litmus paper and a blue litmus paper together in lemon water, sodium carbonate solution and table salt solution? b. How is acid rain formed? Describe with the help of at least one suitable reaction. c. If you become a Mayor of your city, what will you do to reduce the impact of acid rain? d. Write any four uses of acid, base and salt. e. Fill out the table with appropriate responses. S.N. Indicators Colour in acid Colour inbasic solution Colour in salt solution 1. Red litmus paper 2. Blue litmus paper 3. Methyl orange 4. Phenolphthalein

292 Oasis School Science and Technology - 8 Key terms and terminologies 1. Soft water : The water that does not contain chloride, sulphate and bicarbonate salts of calcium and magnesium is called soft water. 2. Hard water : The water that contains the chloride, sulphate and bicarbonate salts of calcium and magnesium is called hard water. 3. Temporary hard water : The hard water containing bicarbonate salts of calcium and magnesium is called temporary hard water. 4. Permanent hard water : The hard water containing chloride and sulphate salts of calcium and magnesium is called permanent hard water. 5. Alloy : The homogeneous mixture of two or more metals or metals and non-metals is called an alloy. 6. Amalgam : The alloy which is formed by mixing metal and mercury is called amalgam. 7. Brass : Brass is an alloy of copper and zinc. 8. Bronze : Bronze is an alloy of copper and tin 9. Steel : Steel is an alloy of iron and carbon. UNIT 10.2 WATER Estimated teaching periods Theory 5 Practical 2  Introduction to hard water and soft water  Causes of hardness of water  Methods of removal of hardness of water by boiling or using washing soda  Advantages and disadvantages of hard water and soft water  Introduction to alloy  Introduction of stell, brass and bronze and their application in daily life The Sequence of Curriculum Issued by CDC Harry Brearley is well known for the discovery of steel. He was born in England on 18th February and died on 14th July 1948. He was a metallurgist and an inventor. After he discovered rustless steel. He improvised and made various forms of the alloy of iron. Harry went to Woodside Board School. A couple of years later he started as a bottle washer in the chemical laboratory at Firths. He began to study metallurgy and learnt so quickly that he was able to set up a new laboratory at Kayser Ellison's. About the Scientist Harry Brearley

Oasis School Science and Technology - 8 293 Introduction Water is one of the most important and abundant substances needed for living beings. It covers approximately 70% of the earth's surface. All living beings need water to survive. Not only is it present around us but is also inside our body. Water makes up about 70% of human body. We can survive for a month without food but cannot live more than a week without water. Therefore, we should drink water every day. Water is essential for conducting various chemical reactions that occur inside the body. It is used to digest food, absorb nutrients and transport them to various body parts, and remove waste products from the body. Similarly, water is required to transport raw materials from root to leaves for photosynthesis. The prepared food is also transported to different parts of plants from leaves with the help of water. About 97% of the water present on the earth is found only in the oceans. It consists of a lot of salt dissolved. So it is unfit for drinking. The dead sea contains about 33.7% of salt. So it is too salty and hence organisms cannot survive in dead sea. But living beings float on dead sea due to high density of sea water. In this unit, we will study the properties and uses of water and methods of removing hardness of water. Physical Properties of Water i. Pure water is colourless, odourless and tasteless. ii. It exists in solid, liquid and gaseous states. iii. Pure water is transparent. iv. It is a universal solvent. v. Pure water is a bad conductor of electricity. vi. Its boiling point is 100 0 C and freezing point is 0 0 C. Types of Water On the basis of salts of calcium and magnesium, water is classified into two types, viz. soft water and hard water. Soap is used to wash clothes. Whenever we get enough amount of foam or lather by applying a little soap, we say that the water is soft. But water from some sources like rivers, ponds, lakes, instead of producing foam, forms sticky particles. Such water is called hard water. Hardness of water is due to the presence of soluble salts of calcium and magnesium, viz. chloride, sulphate and bicarbonate salts of calcium and magnesium. Soft water The water that does not contain chloride, sulphate and bicarbonate salts of calcium and magnesium is called soft water. It produces lather with soap easily. Rain water and distilled water are some examples of soft water.

294 Oasis School Science and Technology - 8 Reasonable Facts People prefer soft water to wash clothes. People prefer soft water to wash clothes because soap can produce more lather and remove stain from clothes in soft water. Hard water The water that contains the chloride, sulphate and bicarbonate salts of calcium and magnesium is called hard water. It does not produce lather with soap easily. So hard water leads to wastage of soap. The water of wells, rivers, oceans, etc. is hard water. On the basis of salts dissolved, hardness of water is of two types: i. Temporary hardness ii. Permanent hardness i. Temporary hardness The hard water containing bicarbonate salts of calcium and magnesium is called temporary hard water. Such type of water is tasty while drinking and makes our bones healthy. This type of hardness can be removed easily. Removal of temporary hardness of water Temporary hardness of water can be removed easily by boiling the water or treating with lime-water. a. By boiling water When the temporary hard water is boiled, the soluble bicarbonate salts of calcium and magnesium get converted into water insoluble bicarbonate salts and settle down at the bottom of the container. These insoluble salts can be filtered away. Calcium bicarbonate heat Calcium carbonate + Water + Carbon dioxide Ca(HCO3 )2 ∆ CaCO3 + H2 O + CO2 ↑ Magnesium bicarbonate heat Magnesium carbonate + Water + Carbon dioxide Mg(HCO3 )2 ∆ MgCO3 + H2 O + CO2 ↑ Activity 1 • Observe various sources of water in your locality. • What are the uses of these water sources? Prepare a short report and submit to your science teacher.

Oasis School Science and Technology - 8 295 b. By treating with lime-water (Clark's method) Temporary hardness of water can also be removed by adding lime water or slaked lime into the hard water. Calcium hydroxide, i.e. lime water reacts with bicarbonate of calcium and magnesium and forms insoluble calcium carbonate which makes the water soft. This method is called Clark's method. Calcium bicarbonate + Calcium hydroxide Calcium carbonate + Water Ca(HCO3 ) 2 + Ca(OH)2 2CaCO3 + 2H2 O Magnesium bicarbonate + Calcium hydroxide Calcium carbonate + Water + Magnesium hydroxide Mg(HCO3 )2 + 2Ca(OH)2 2CaCO3 + 2 H2 O + Mg(OH)2 Activity 2 Put hard water in a plastic bottle. Also put a hardwater in another bottle but some lime in it for few minutes. Then try to dissolve some soap in both of them. Which one will produce more lather and why? ii. Permanent hardness The hard water containing chloride and sulphate salts of calcium and magnesium is called permanent hard water. Removal of permanent hardness of water Permanent hardness of water can be removed by treating the hard water with washing soda and permutit process. a. By treating hard water with washing soda When permanent hard water is treated with washing soda or sodium carbonate, the salts present in it change into carbonates and water becomes soft. The chemical reactions involved in this process are given below: Calcium chloride + Sodium carbonate Calcium carbonate + Sodium chloride CaCl2 + Na2 CO3 CaCO3 + 2NaCl Calcium sulphate + Sodium carbonate Calcium carbonate + Sodium sulphate CaSO4 + Na2 CO3 CaCO3 + Na2 SO4 Magnesium chloride + Sodium carbonate Magnesium carbonate + Sodium chloride MgCl2 + Na2 CO3 MgCO3 + 2NaCl Magnesium sulphate + Sodium carbonate Magnesium carbonate + Sodium sulphate MgSO4 + Na2 CO3 MgCO3 + Na2 SO4

296 Oasis School Science and Technology - 8 Activity 3 To demonstrate that permanent hardness of water can be removed by adding washing soda • Take a beaker and put some water into it. Add some calcium chloride or magnesium chloride into it and stir. • Now add 2-3 drops of shampoo or solution of soap into the beaker and stir it with a glass rod. The water does not produce lather with soap as it is a hard water. • Now, add some sodium carbonate into the beaker and stir the mixture with a glass rod. • Again, put a few drops of solution of soap and stir. Observe whether the water forms lather with soap or not. • After adding sodium carbonate or washing soda, the water produces lather with soap. This activity proves that permanent hardness of water can be removed by using washing soda. b. By permutit process In permutit process, the permanent hard water is passed through sodium aluminosilicate (Na2 Al2 SiO8 ) or sodium zeolite (Na2 –Z). When hard water passes through the zeolite or permutit, the calcium and magnesium ions of hard water get replaced by sodium ions of permutit. As a result, water becomes soft. The chemical reactions involved in permutit method are given below: Sodium zeolite + Calcium chloride Calcium zeolite + Sodium chloride Na2 –Z + CaCl2 Ca – Z + 2NaCl Sodium zeolite + Magnesium chloride Magnesium zeolite + Sodium chloride Na2 – Z + MgCl2 Mg–Z + 2NaCl Sodium zeolite + Calcium sulphate Calcium zeolite + Sodium sulphate Na2 – Z + CaSO4 Ca–Z + Na2 SO4 Sodium zeolite + Magnesium sulphate Magnesium zeolite + Sodium sulphate Na2 –Z + MgSO4 Mg–Z + Na2 SO4 Hard water Sodium chloride Zeolite Soft water Column Finegravel Fig. Permutit process to remove hardness of water