Modern Concept Science and Technology 8 New

Modern Concept Science and Technology – 8 247 5. The switch is on and the bulb is glowing. The teacher said that in this case the circuit is closed. Elle is confused because teacher said that the circuit is closed but as she can observe the switch is on. Explain the technical terms to her in simple words so that she can understand. Most people find it confusing that the switch is on, bulb is glowing but they say the circuit is closed. Elle is also confused here. Let's explain the technical terms used in an electric circuit so that she can understand it. First of all electric circuit is a path in which electricity can flow. It contains a source which produces electricity. The electricity reaches the key. Key is device which can either connect or break the circuit. There are two ends of wires near the key. If the switch is on, the key can touch ends of both wires and current can flow. In this case, key is closed. The current will reach the bulb and go back to the source. Now the bulb will glow. It is called closed circuit. If the switch is off, key cannot connect to one of the wires. The current cannot flow. In this case key is open. Since current cannot flow, bulb won't glow. It is called open circuit. 6. Mr. Ram Bhakta is arguing with an electrician that he does not want to install separate fuses for each flat and rooms. He thinks it is waste of money. Let us explain it to him in such a way that he will understand that installing separate fuse for each flat and room is infact the wisest thing to do. Mr. Ram Bhakta is arguing with electrician that he does not want to install separate fuses flat wise. He thinks it is waste of money but he is wrong. It is actually an investment. We should let him know the advantages of using separate fuses flat wise. Some of them are listed below: i. Short circuit in one room will damage fuse of that room only. People of other rooms can still use electricity. ii. Fuses are cheapest technology to prevent fire from short circuit. iii. These are easy to install and replace. 7. If we are about to start domestic wiring, what precautions should we take? If we are starting domestic wiring we must be cautious. Some precautions are listed below: i. We should never do wiring in moist walls. ii. We should use properly insulated wires. iii. Appropriate wires should be used to operate particular appliances. iv. Turn off the MCB while wiring. 3 STEPS EXERCISE EXERCISE STEP1 1. Select the best answer from the given alternatives. a) How are loads connected in domestic circuit? i) series ii) parallel iii) mixed iv) none of the above

248 Electricity b) Which wire carries electric current to the load in a domestic circuit? i) live wire ii) neutral wire iii) earth wire iv) all of the above c) Which one of the following is not a part of domestic wiring? i) internet cable ii) live wire iii) neutral wire iv) earthing wire d) Which one of the following is a single use disposable circuit breaker? i) key ii) MCB iii) fuse iv) bulb e) Which of the following statement is true for an open circuit? i) load is working ii) current is flowing iii) current does not flow iv) brightness of bulb increases 2. Write True for the correct and False for the incorrect statements. a) A fuse is a safety device in an electric circuit. b) A fuse is made of copper and iron. c) The devices which produce electricity are called load of electricity. d) The lighting circuit is made using a live wire and a neutral wire only. e) A MCB allows current flow as long as the current in the circuit does not exceed the current rating of the fuse. 3. Fill in the blanks using a suitable word. a) Electricity is a form of energy which is produced due to the continuous flow of ........... through a conductor. b) The domestic wiring uses ............ connections of loads. c) Earthing wire conducts ............ current to the ground. d) A device which is used to open or close an electric circuit is called a ............ e) The electric circuit through which current can flow is called a ............ 4. Answer the following questions in very short. a) What is used as a safety device in an electric circuit? b) Which connection of loads is used in house wiring system? c) Which bulb is efficient to be used in home? d) What is reusable circuit breaker called? e) Which electrical wire of domestic circuit is at zero potential? STEP2 5. Give reasons a) Cell is called source of electricity. b) Loads are connected parallel in house wiring system. c) Fuse is compulsory in domestic wiring and electrical devices. d) Fuse and keys are installed in live wire. e) Colour code is important in house wiring. f) Bulb does not glow in open circuit. g) MCB is better than fuse.

Modern Concept Science and Technology – 8 249 6. Differentiate between. a) Source and load b) Fuse and MCB c) Open circuit and close circuit d) Corporation fuse and consumer's fuse e) Live wire and neutral wire 7. Answer the following in short. a) Define electricity. b) What are electric sources? Write with examples. c) What is a domestic circuit? d) Define live, neutral and earth wires used in a domestic circuit. e) Write the colour code of the different wires used in a domestic circuit. STEP3 8. Answer the following questions a) Explain domestic wiring with the help of a suitable diagram. b) What is the role of a fuse in an electric circuit? c) What is a fuse? Explain the working method of a fuse. d) What is a miniature circuit breaker (MCB)? Write advantages of MCB over a normal fuse. e) Write a short note on electrical components used in a domestic circuit. f) Let's suppose your electric heater has broken plug. You bought a new plug. How would you wire that plug to your electric heater? Write in brief. 9. Numerical a) The bulb with the power of 176 watts is used 8 hours daily, calculate the total bill for 30 days, if per unit costs Rs. 7. [Ans: Rs. 295.68] b) 10 tube lights of 20 W each are used in a house for 5 hours daily, calculate the bill for one month for the electricity at the rate of Rs. 7.30 per unit. [Ans: Rs. 4158] c) In a hostel, 20 LED bulbs of 15W are used 5 hours daily, 10 electric heaters each of 2 kW are used continuously for 180 minutes a day, 20 water heaters of 1500 W are used for 5 minutes every day, and 20 irons of 800 W are used for 30 minutes every week. What will be the consumption of electricity in a month? What change in the total consumption will be when all heaters of 2 kW are replaced by 1 kW heaters? If the cost of electricity is Rs. 12 per unit, how much money will be saved in a month? [Ans: 1952 units, 10,800 Rs.] Project Work Project Work 1. Make a model of a close circuit and explain their working method in your classroom. 2. Take a dry cell and cut it vertically with the help of a metal cutter. Observe the parts inside it and write their names. [Note: avoid any cut on your hand while handling metal cutter or take help of your parents]

250 Matter Key terms and terminologies of the unit 1. Matter : Anything that has mass and occupies volume is called matter. 2. Element : The pure form of a substance that cannot be further simplified into other simpler substances is called an element. 3. Atoms : Atoms are the smallest particles of elements that take part in chemical reactions without division. 4. Compounds : The pure substances which are formed by the combination of two or more elements in definite proportion by their weight are called compounds. 5. Molecule : The smallest particle of a compound or an element that can exist freely in nature is called molecule. 6. Sub-atomic particles : Each and every atom is made up of tiny particles (electrons, protons and neutrons) called subatomic particles or elementary particles. 7. Nucleus : The dense, solid and tiny core of an atom is called nucleus of an atom. 8. Orbits : The path through which electrons revolve around the nucleus is called an orbit or shell. Sequence of Curriculum Issued by CDC  Location of proton, neutron and electron in an atom  Atomic structure and electronic configuration based on shells  Introduction to valance orbit, valance electrons and valency  Valency of first 20 elements  Introduction of the periodic table  Modern periodic table  Periodic variation of elements (number of orbits, valency, atomic size and metallic properties)  Introduction to molecular formula, methods to write the molecular formula, molecular formula of a few compounds.  Introduction and calculation of atomic weight and molecular weight  Introduction to chemical equation, word equation and balanced formula equation UNIT Matter 9 Estimated teaching periods Theory Practical 12 3 Henry Moseley is popular for the chemical concept of the atomic number. He was born in England on 23rd November 1887 and died on 10th August 1915. He was a physicist and chemist. Mosley’s law advanced atomic physics, nuclear physics and quantum physics by providing experimental evidence in favour of Bohr’s theory. He was killed in world war I otherwise he was to receive Nobel Prize in Physics in 1916. Henry Gwyn Jeffreys Moseley About the Scientist

Modern Concept Science and Technology – 8 251 9. Protons : Protons are the positively charged sub-atomic particles of an atom. 10. Electrons : Electrons are the negatively charged sub-atomic particles of an atom. 11. Neutrons : Neutrons are the chargeless sub-atomic particles of an atom. 12. Atomic mass unit : Atomic mass unit is a unit that is used to express the weight of elementary particles like protons, neutrons and electrons. 13. Atomic number : Atomic number of an atom is the number of protons present in the nucleus of that atom. It is represented by ‘Z’. 14. Atomic mass : The sum of numbers of protons and neutrons present in the nucleus of an atom is called atomic mass. It is represented by ‘A’. 15. Electronic configuration : The systematic arrangement of electrons of an atom in its shells and subshells is called electronic configuration of an atom. 16. Symbol : The abbreviation of full name of an element is called a symbol. 17. Valance orbit : The outermost orbit of an atom is called valance orbit. 18. Valance electron : The electrons present in the outermost orbit of an atom is called valance electron. 19. Valency : The combining capacity of an atom or radical with other atom or radical to form a new molecule (compound) is called valency. 20. Duplet state : The state in which an atom contains 2 electrons in its outermost K- shell if atom has only one shell is called duplet state. 21. Octet state : The state in which an atom contains 8 electrons in its outermost shell is called octet state. 22. Noble elements : Elements that do not take part in chemical reaction are called noble elements. 23. Radical : Charged atom or group of atoms that behaves as a single unit during chemical reaction is called a radical. 24. Electropositive radical : Positively charged atoms or group of atoms are called electropositive radicals. 25. Electro negative radical : Negatively charged atoms or group of atoms are called electronegative radicals. 26. Molecular formula : The symbolic representation of a substance that shows the actual number and kinds of atoms present in it is called a molecular formula. 27. Molecular weight : The sum of mass of the atoms present in a molecule is called molecular weight (molecular mass). 28. Periodic table : The table in which elements are arranged on the basis of their similar and dissimilar physical and chemical properties is called periodic table. 29. Mendeleev’s periodic law : According to Mendeleev’s periodic law, the physical and chemical properties of elements are the periodic function of their atomic mass (weight). 30. Mendeleev’s periodic table : The table in which elements are arranged on the basis of increasing atomic weight is called Mendeleev’s periodic table. 31. Modern periodic law : According to modern periodic law the physical and chemical properties of elements are the periodic function of their atomic number. 32. Modern periodic table : The table in which elements are arranged on the basis of their increasing atomic number is called modern periodic table. 33. Periods : The horizontal rows of the periodic table are called periods. 34. Groups : The vertical columns of the periodic table are called groups.

252 Matter 35. Atomic radius : The distance between the outermost shell and centre of the nucleus of an atom is called atomic radius. 36. Metallic character : The tendency of an atom to lose electron is called metallic character. 37. Non-metallic character : The tendency of an atom to gain an electron is called non-metallic character. 38. Chemical reaction : The process which changes one or more substances into new substances of different chemical properties is known as chemical reaction (chemical change). 39. Reactant : The chemical substances which take part in a chemical reaction are called reactants. 40. Products : The chemical substances which are produced after the chemical reaction are called products. 41. Word equation : When a chemical change is expressed by naming the reactants and products in words is called word equation. 42. Formula equation : When a chemical change is expressed by writing the reactants and products in symbols is called formula equation. 43. Unbalanced chemical equation : The chemical equation in which the number of atoms of each element in reactants and products are unequal is called an unbalanced or a skeleton chemical equation. 44. Balanced chemical equation :The chemical equation in which the number of atoms of each element in reactants and products are equal is called a balanced chemical equation. 45. Irreversible reaction : The chemical reaction in which the products cannot be converted into reactants is called an irreversible reaction. 46. Reversible reaction : The chemical reaction in which the products can be converted into reactants is called a reversible reaction. 47. Endothermic reaction : The reactions that absorb heat from the surrounding are called endothermic reactions. 48. Exothermic reactions : The reactions that release heat into the surrounding are called exothermic reactions. Introduction Anything that has mass and occupies volume is called matter. Ice, brick, stone, sugar, alcohol, glycerine, gold, iron, soil, rock, smoke, cooking gas, water, air, etc. are some examples of matters. Solid, liquid and gas are three states of matter. Some matters are made up of only one kind of substance whereas others can be of different substances. The matters which are composed of the same kind of substances are called pure matters while the matters which are composed of different substances are called impure matters. Ice is a pure matter as it contains water only. Smoke is an impure matter as it contains many gaseous substances in it like oxygen, carbon dioxide, black soot. Mixtures are impure matters. Elements The pure form of a substance that cannot be further simplified into other simpler substances is called an element. Gold, silver, copper, oxygen, hydrogen, bromine, chlorine, etc. are some examples of elements. Can we prepare different substances from the foil of silver? No,

Modern Concept Science and Technology – 8 253 we cannot. Every time we break or try to simplify silver into a number of pieces, we get silver. Silver will never be decomposed to give other substances like iron, copper, gold, etc. So, silver is an element. Elements occur naturally but scientists have also synthesized some elements in laboratory. There are altogether 118 elements, in which 92 are natural and 26 are artificial. Atoms Atoms are the smallest particles of an element that take part in chemical reactions without division. Atoms of an element are similar in all features while atoms of different elements are different from each other. For example, all atoms of gold have the same features but atoms of gold and atoms of silver are different from each other. Therefore, there are altogether 118 different types of atoms of 118 different elements. Compounds The pure substances which are formed by the combination of two or more elements in a definite proportion by their weight are called compounds. Compounds are very useful substances in our daily life. Water, alcohol, carbon dioxide, sodium chloride, sugar, etc. are some examples of compounds. The earth contains millions of compounds. Molecules The smallest particles of a compound or an element that can exist freely in nature are called molecules. A molecule is formed when two or more atoms of the same or different elements combine with each other. For example, hydrogen (H) is an element but it exists as H2 which is a molecule of hydrogen. H2 is formed by combining two atoms of hydrogen. Gaseous elements exist in the form of a molecule. For example, O2 , N2 , Cl2 , etc. These gaseous molecules are formed by the combination of two similar atoms. Similarly, O3 is a molecule of ozone which is formed by the combination of three atoms of oxygen. H2 O is a molecule of water which is formed by the combination of two hydrogen atoms and one oxygen atom. Na Cl O H H H H O O Hydrogen molecule Oxygen molecule Sodium chloride molecule Water molecule FACTS WITH REASONS H2 is a molecule but H is an atom. Give reason. H2 is a molecule because it is a stable chemical compound that occurs freely in nature but H is an atom because it quickly undergoes chemical reaction to give a new product. Shell or orbit Nucleus Proton Neutron Electron Atom MEMORY TIPS 1. Sodium chloride (NaCl) is also known as table salt. 2. Water (H2 O) is the most abundant compound on the earth. 3. O represents an atom of oxygen, O2 represents molecule of oxygen and 2O represents 2 atoms of oxygen.

254 Matter 9.1 Atomic Structure Each and every atom is made up of tiny particles called fundamental particles or sub-atomic particles or elementary particles. They are protons, neutrons and electrons. These particles can neither be created nor be destroyed. Thus, an atom consists of sub-atomic particles. Protons Protons are positively charged sub-atomic particles of an atom. They are present along with neutrons in the nucleus of an atom. A proton is represented by p+ . The mass of a proton is equal to the mass of a hydrogen atom which is 1 amu (atomic mass unit). Neutrons Neutrons are chargeless sub-atomic particles of an atom. They are present along with the protons in the nucleus of an atom. A neutron is represented by n0 . The mass of a neutron is 1 amu. Its mass is equal to the mass of a proton, or 1837 times more than the mass of an electron. Electrons Electrons are negatively charged sub-atomic particles of an atom. They revolve round the nucleus in an elliptical orbit. While revolving, they do not colloid with each other. An electron is represented by the symbol e- . The mass of an electron is 1 1837 amu. Atomic mass unit We cannot see the sub-atomic particles with our naked eyes. They are extremely small and their weight is negligible. Due to their importance in energy change, we need to know their weight. They cannot be weighed using physical balance and their mass cannot be expressed in kilogram, grams and milligrams. Atomic mass unit is a unit that is used to express the weight of elementary particles like protons, neutrons, electrons, etc. From various mathematical calculations it is known that, 6.023 × 1023 amu is equal to 1 gram. The mass of one proton is equivalent to 1 amu. Therefore, we need 6.023 × 1023 protons to make 1 gram of protons. Also, mass of electron is equal to 1 1837 mass of protons. Hence we can say that, the mass of 1proton is equal to the mass of 1 neutron and it is also equal to the mass of 1837 electrons. That is: the mass of 1p+ = the mass of 1n° = the mass of 1837 e– Electric charge We know that protons and electrons have electrical charge on them. Protons are positively charged and electrons are negatively charged. In SI unit, their charge is measured by a unit – – – – – – + + + + + + Nucleus Proton (p+) Neutron (n0 ) Shell or orbit Electron (e– ) Structure of atom MEMORY TIPS A positively charged hydrogen ion (H+ ) is also called proton because it has no electron no neutron but a proton only. MEMORY TIPS 1 amu is equal to 1/12 part of the carbon-12 atom isotope.

Modern Concept Science and Technology – 8 255 known as coulomb (C). Mathematically, 1 coulomb = 6.25 × 1018 electrons It means that one coulomb charge is equivalent to the charge of 6.25 × 1018 electrons. An atom has equal number of protons and electrons. Equal number of positively charged protons and negatively charged electrons cancel each other. Therefore, an atom is electrically neutral in nature. 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 – FACTS WITH REASONS An atom is electrically neutral in nature, why? Atoms are electrically neutral in nature because in an atom the number of positively charged protons is exactly equal to the number of negatively charged electrons. Atomic number All 118 elements have definite number of protons in their nucleus. The number of protons gives atomic number. Thus, atomic number of an atom is the number of protons present in the nucleus of that atom. It is represented by ‘Z’. Since, an atom has equal number of protons and electrons, so it can be represented as; Atomic number (Z) = No. of protons (p+) = No. of electrons (e-) Atomic mass or atomic weight We know that protons, neutrons and electrons have some mass. The mass of electron is negligible in comparison to the mass of a proton and neutron. Therefore, atomic mass (weight) is the sum of numbers of protons and neutrons present in the nucleus of an atom. It is represented by ‘A’. Atomic mass is calculated by: Atomic mass (A) = number of protons (p+ ) + number of neutrons (n°) A = p+ + n° On the basis of above formula, the number of neutrons of an atom can be calculated as: Number of neutrons (n°) = A – p+ Number of neutrons (n°) = A – Z A Z X Chemical symbol for the element Mass number Atomic number = number of protons Li 7 3

256 Matter From the above expression, it is clear that, once we know the atomic number and atomic mass of an atom, we can find its protons, neutrons and electrons. EXAMPLE : 1 Calculate the mass of an oxygen atom. (Hint: oxygen atoms have 8 protons and 8 neutrons). Solution: We have, No. of protons (p+ ) = 8 No. of neutrons (n0 ) = 8 Therefore, atomic mass = no. of p+ + No. of n° = 8 + 8 = 16 EXAMPLE : 2 Calculate the number of neutrons in a sodium atom whose atomic number is 11 and atomic mass is 23. Solution: We have, Atomic number = 11 Atomic mass = 23 We know, atomic number (Z) = no. of protons = 11 Atomic mass = no. of p+ + No. of n° or, 23 = 11 + n° or, 23 – 11 = n° Therefore, n° = 12 Electronic Configuration of an Atom Systematic arrangement of electrons of an atom in its shells and sub-shells is called electronic configuration. Electrons are arranged outside the nucleus in different elliptical orbits but for our convenience we draw a circular orbit to arrange electrons. The force of attraction between a nucleus and electrons holds the electrons in their orbit. The number of electrons that an orbit can hold increases as their distance from the nucleus increases. The orbits are named using English alphabets like K, L, M, N, etc. The distance between the orbit and nucleus increases as it moves from the first orbit (K) to other orbits like L, M, N, etc. Bohr and Bury’s 2n2 rule for electronic configuration The maximum number of electrons that can be arranged in a shell of an atom is given by a formula known as 2n2 rule. ‘n’ in the formula denotes the shell number. Generally, the outermost shell of an atom holds maximum 8 electrons. So, this rule is applicable in special cases only. Arrangement of the electrons in different shells according to 2n2 rule is shown below: Distribution of electrons Shells Nucleus Electronic configuration of argon

Modern Concept Science and Technology – 8 257 Shell Symbol n 2n2 No. of electrons First shell K n = 1 2.12 2 Second shell L n = 2 2.22 8 Third shell M n = 3 2.32 18 Fourth shell N n = 4 2.42 32 2n2 rule is not applicable for all 118 elements. This rule can be applied in the shells K, L, M and N only. It cannot be applied in the shells O, P and Q. This is because in these shells we can arrange 32, 18 and 8 electrons respectively. The name of elements, symbol, atomic number, atomic weight, number of protons, neutrons and electrons and electronic configuration of first 20 elements from hydrogen to calcium is given in the following table: 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

258 Matter The atomic structure and electronic configuration of first twenty elements is given below: 1. Hydrogen (H) 2. Helium (He) Shell K L M N No. of e– 2 × × × Shell K L M N No. of e– 1 × × × 1p+ 0n0 K Hydrogen 2p+ 2n0 K Helium 3. Lithium (Li) 4. Beryllium (Be) Shell K L M N No. of e– 2 2 × × Shell K L M N No. of e– 2 1 × × 3p+ 4n0 K L Lithium 4p+ 5n0 K L Beryllium 5. Boron (B) 6. Carbon (C) Shell K L M N No. of e– 2 4 × × Shell K L M N No. of e– 2 3 × × 5p+ 6n0 K L Boron 6p+ 6n0 K L Carbon 7. Nitrogen (N) 8. Oxygen (O) Shell K L M N No. of e– 2 6 × × Shell K L M N No. of e– 2 5 × × 7p+ 7n0 K L Nitrogen 8p+ 8n0 K L Oxygen

Modern Concept Science and Technology – 8 259 9. Fluorine (F) 10. Neon (Ne) Shell K L M N No. of e– 2 8 × × Shell K L M N No. of e– 2 7 × × 10p+ 10n0 L K Neon 9p+ 10n0 K L Fluorine 11. Sodium (Na) 12. Magnesium (Mg) Shell K L M N No. of e– 2 8 2 × Shell K L M N No. of e– 2 8 1 × 11p+ 12n0 K L M Sodium 12p K L M + 12n0 Magnesium 13. Aluminium (Al) 14. Silicon (Si) Shell K L M N No. of e– 2 8 4 × Shell K L M N No. of e– 2 8 3 × K L M 13p+ 14n0 Aluminium 14p K L M + 14n0 Silicon 15. Phosphorus (P) 16. Sulphur (S) Shell K L M N No. of e– 2 8 6 × Shell K L M N No. of e– 2 8 5 × 16p+ 16n0 K L M Sulphur 15p+ 16n0 K L M Phosphorus

260 Matter 17. Chlorine (Cl) 18. Argon (Ar) Shell K L M N No. of e– 2 8 8 × Shell K L M N No. of e– 2 8 7 × 17p+ 18n0 K L M Chlorine 18p K L M + 22n0 Argon 19. Potassium (K) 20. Calcium (Ca) Shell K L M N No. of e– 2 8 8 2 Shell K L M N No. of e– 2 8 8 1 K L M N 19p+ 20n0 Potassium K L M N 20p+ 20n0 Calcium ACTIVITY 1 Collect a thin metallic wire and 18 spherical candy of three different colours (6 of one colour). Cut the wire into two pieces of different lengths. Poke 2 identical candy by the smallest wire. Bend the wire to make a circle and keep the candy apart from each other. Similarly, poke other 4 candy of the same colour by another wire. Bend the wire to make a circle again and keep the candy apart from each other. Take a cardboard and fix the larger circular wire on the board. Then, fix the smaller circular wire inside the larger wire. At last, fix 6 candy of one colour at one side of the centre and the remaining 6 candy at other side of the centre. CONCLUSION : In this way, a model of carbon atom becomes ready. Symbol In chemistry we use one or two letters to represent the name of an element. This representation is called a symbol.Thus, the abbreviation of full name of an element is called a symbol. A symbol also represents the atom of that element. It is tedious and time consuming to write the

Modern Concept Science and Technology – 8 261 exact names of the elements in chemistry. Therefore, symbols are used as the short, quick and convenient way to write the name of elements. Name of many elements have English words while the name of few other elements come from Latin words. Thus, the symbols of elements are derived either from first or first and second or first and third letter of their English or Latin names. The symbol always have first letter as capital letter and if it has second letter it is always small. Therefore, the symbol of an element is either one or two English alphabet which is shorter than the name of element. Some elements are represented by single capital letter. For example,the symbol of Hydrogen is “H”. Similarly, the symbol of Carbon is “C”. Some other examples are given in the table below: Elements Symbol Elements Symbol Elements Symbol Hydrogen H Nitrogen N Phosphorous P Boron B Oxygen O Sulphur S Carbon C Fluorine F Iodine I The symbol ‘H’ is already used for Hydrogen. Therefore, the symbol of Helium is “He”. Here Hydrogen and Helium are English words. Similarly, Chlorine and Chromium both have “Ch” as their first and second letter. The symbol of both elements cannot be the same. So, they have symbol “Cl” for Chlorine and “Cr” for Chromium. Such kind of some elements are given in the table below: Elements Symbol Elements Symbol Elements Symbol Helium He Chlorine Cl Manganese Mn Lithium Li Chromium Cr Zinc Zn Berylium Be Magnesium Mg Zirconium Zr On the basis of above discussion, the symbol of Sodium should be “S” or “So” but it is actually “Na”. Similarly, the symbol of Potassium is “K”. It is because symbol of Sodium and Potassium are derived from their Latin words “Natrium” and “Kalium” respectively. Such kinds of other examples with their Latin words are given in the table. English Names Latin Names Symbol Sodium Natrium Na Potassium Kalium K Iron Ferrum Fe Gold Aurum Au Copper Cuprum Cu Silver Argentum Ag 9.2 Valency The combining capacity of an atom or radical with other atom or radical to form a new molecule (compound) is called valency. In other words, valency of an atom is the number of electrons gained, lost or shared during chemical combination. The valency of an atom is determined by the number of electrons in the outermost shell (valence shell). Thus, the number of electrons which are present in the valence shell of an atom are called valence electrons. Most of the

262 Matter atoms of different elements tend to gain or lose or share valence electrons in order to attain stable electronic configuration. To be in stable electronic configuration, an element should have eight electrons in its outermost shell (except K shell as outermost shell). Therefore, an atom has tendency to be stable by fulfilling eight electrons in its outermost shell (two electrons if K shell is the outermost shell). For this, they take part in chemical combination with other atoms or radicals and become stable. FACTS WITH REASONS Elements with 0 valency are stable elements, why? Elements with 0 valency are stable elements because they neither gain nor lose electrons in normal conditions. Duplet and Octet State The state in which an atom contains 2 electrons in its outermost K shell is called a duplet state. A helium atom is in duplet state. The rule of an atom to be stable by making two electrons in its outermost shell (K-shell) either by gaining or losing or sharing of electrons is called duplet rule. Similarly, The state in which an atom contains 8 electrons in its outermost shell is called an octet state. For example, the atoms of elements: neon, argon, krypton, xenon and radon are in octet state. The tendency of an atom to make 8 electrons in its outermost shell either by gaining or losing or sharing of electrons is called octet rule. A sodium atom has one electron in its outermost shell. So, it has a tendency to lose one electron to get an octet state. Similarly, a chlorine atom has seven electrons in its outermost shell. So, it is easy for a chlorine atom to accept one electron from other atoms to make eight electrons in its outermost shell. Therefore, to form sodium chloride (NaCl), a sodium atom loses one electron and chlorine atom gains one electron. This makes eight electrons in the last shell of each atom. That is, there are 8 electrons in each L-shell of the sodium atom and M-shell of the chlorine. Due to this reason, both sodium and chlorine have valency of 1. Sodium Chlorine Sodium chloride Atoms of the elements having one, two, and three electrons in their outermost shell usually lose electrons to gain an octet state. So, their valency is 1, 2, and 3 respectively. Generally, the atoms having four electrons in their outermost shell share their electrons to other atoms to gain an octet state. So their valency is 4. Similarly, the atoms having five, six, and seven electrons accept electrons from the atoms of other elements. So, their valency is 3, 2, and 1 respectively. FACTS WITH REASONS The valency of both oxygen and magnesium is 2, why? Valency of both oxygen and magnesium is 2 because an oxygen atom satisfies the octet rule by gaining two electrons and a magnesium atom satisfies the octet rule by losing two electrons.

Modern Concept Science and Technology – 8 263 According to duplet rule, a helium atom has two electrons in its K-shell. It is in a duplet state. Which makes a helium atom stable. It does not take part in chemical combination with other atoms. Elements like neon, argon and krypton, etc. are stable as their atoms already contain eight electrons in the outermost shell. Therefore, they do not take part in chemical combination with other elements. Elements that do not take part in chemical reaction are called noble elements. Their valency is 0. FACTS WITH REASONS Neon does not take part in chemical reaction, why? Neon does not take part in chemical reaction because its atom is in stable state by octet rule. It does not lose, gain or share electrons with other atoms. Argon is also called a noble gas, why? Argon is called a noble gas because its atom has 8 electrons in the outermost shell. Which is an octet state and makes argon stable. So, argon does not take part in chemical reaction and called a noble gas. Radicals When sodium chloride is kept in water, it ionizes into sodium ion (Na+ ) and chloride ion (Cl- ). These species which have charge are called radicals. Thus, the charged atoms or groups of atoms that behave as a single unit during chemical reactions are called radicals. They are also known as ions. They bear charge on them based on their ability to gain or loss electrons. They are either positively charged or negatively charged. Due to the presence of charge in them they are unstable and react with other opposite charged particles to form stable compounds. Based on their charges, radicals are of two types. They are electropositive radicals (basic radicals) and electronegative radicals (acidic radicals). Electropositive radicals Positively charged atoms or group of atoms are called electropositive radicals. They are also known as basic radicals or cations. Examples of electropositive radials with their valency are given in the table below: 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++)

264 Matter Mercurous (Hg+ ) Stannous (Sn++) Ammonium (NH4 + ) Zinc (Zn++) Aurous (Au+ ) Nickel (Ni++) Manganous (Mn++) FACTS WITH REASONS Ammonium is an electropositive radical, why? Ammonium is an electropositive radical because it is a positive charged group of atoms (NH4 +) which behaves as a single unit during chemical reaction. Electronegative radicals Negatively charged atoms or group of atoms are called electronegative radicals. They are also known as acid radicals or anions. Examples of electronegative radicals with their valency are provided in the table 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_ _ ) Phosphide (P _ _ _ ) Bromide (Br_ ) Sulphite (SO3 _ _ ) Phosphite (PO3 _ _ _ ) Iodide (I – ) Sulphate (SO4 _ _ ) Phosphate (PO4 _ _ _ ) Nitrite (NO2 _ ) Carbonate (CO3 _ _ ) Nitrate (NO3 _ ) Zincate (ZnO2 _ _ ) Cyanide (CN_ ) Silicate (SiO3 – – ) Hydroxide (OH_ ) Peroxide (O2 _ _ ) Chlorate (ClO3 _ ) Dichromate (Cr2 O7 _ _ ) Bisulphate (HSO4 _ ) Thiosulphate (S2 O3 _ _ ) Bicarbonate (HCO3 _ ) Metaluminate (AlO2 _ ) FACTS WITH REASONS Nitrate is an electronegative radical, why? Nitrate is an electronegative radical because it is a negatively charged group of atoms (NO3 – ) which behaves as a single unit during chemical reaction. 9.3 Classification of Elements Periodic table 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.

Modern Concept Science and Technology – 8 265 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 an 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 Group I Group II Group III Group IV Group V Group VI Group VII Group VIII Period 1 H Period 2 Li Be B C N O F Period 3 Na Mg Al Si P S Cl Period 4 K Cu Ca Zn 1* 2* Ti 3* V As Cr Se Mn Br Fe Co Ni Period 5 Rb Ag Sr Cd Y In Zr Sn Nb Sb Mo Te 4* I Ru Rh Pd Period 6 Cs Au Ba Hg La Th Hf Pb Ta Bi W Po Re At Os Ir Pt 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, “The physical and chemical properties of 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 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 number is called modern periodic table.”

266 Matter 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

Modern Concept Science and Technology – 8 267 Differences between Modern periodic table and Mendeleev’s periodic table SN Modern periodic table SN 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. 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. 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

268 Matter 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 nonmetallic 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. In modern periodic table, representative elements are kept in groups IA(1), IIA(2), IIIA(13), IVA(14), VA(15), VIA(16) and VIIA(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 or group VIIIA(18). Similarly, transitional elements are kept in groups IB(11), IIB(12), IIIB(3), IVB(4), VB(5), VIB(6), VIIB(7) and VIII(8, 9 and10). The group VIII is further divided into three vertical columns. 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 nonmetallic 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. Differences between periods and groups SN Periods SN Groups 1 The horizontal rows of a periodic table are called periods. 1 The vertical columns of a 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.

Modern Concept Science and Technology – 8 269 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. Characteristics of Periods and Groups 1. Atomic Radius Atomic 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 99 Na Mg Al Si P S Cl Atomic size decreases Similarly, 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 group Li 123 Li The smallest atom Na 157 Na K 202 K Rb 216 Rb Cs 235 Cs Fr 256 Fr The biggest atom 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. 2. 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. Period Group Number of valence electrons remains the same in a group Number of valence electrons increases from left to right in a period

270 Matter 3. 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. Period Group Valency of elements remains the same in a group Valency increases from 1 to 4 and then decreases to zero 4. 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. Period Group Metallic character increases from top to bottom in a group Metallic character decreases from left to right in a period 5. 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. Period Group Non-metallic character decreases on moving from top to bottom in a group Non-metallic character increases on moving from left to right in a period 6. Number of shells The number of shell remains same in every element of a same period. The number of shells increases from top to bottom in every elements in a group. 9.4 Molecular Formula The symbolic representation of a substance that shows the actual number and kinds of atoms present in it is called a molecular formula. H2 O is a molecular formula of water. It shows that two atoms of hydrogen and one atom of oxygen are present in it. 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 , F2 , Cl2 , Br2 and I2 respectively. So, they are called diatomic molecules.

Modern Concept Science and Technology – 8 271 Molecular formulae of some other molecules are shown in the table below: 1. Ammonia NH3 2. Carbon dioxide CO2 3. Sodium chloride NaCl 4. Glucose C6 H12O6 5. Calcium carbonate CaCO3 6. Calcium chloride CaCl2 7. Sulphuric acid H2 SO4 8. Nitric acid HNO3 9. Magnesium sulphate MgSO4 10. Caustic soda NaOH Differences between symbol and molecular formula. SN Symbol SN Molecular formula 1 The abbreviation of the full name of an element is called a symbol. 1 The symbolic representation of a substance that shows the actual number and kinds of atoms present in it is called a molecular formula. 2 It represents one atom of an element. Examples: H, Na, K, etc. 2 It represents one molecule of an element or a compound. Examples: H2 , NaCl, H2 SO4 , etc. Way of writing a molecular formula 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 their upper right corner. iii) Exchange the valency of these radicals. Take HCF if it is necessary. iv) Combine radicals with their exchanged valency. Examples:

272 Matter Information obtained from a molecular formula i) Molecular formula represents one molecule of a substance. ii) It indicates total number of atoms of the 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. vi) 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. 9.5 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 a 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

Modern Concept Science and Technology – 8 273 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 9.6 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: i) When hydrogen gas burns in oxygen gas, it forms water, i.e. Hydrogen + Oxygen Water H2 + O2 H2 O ii) When calcium carbonate is heated, it forms calcium oxide and carbon dioxide, i.e. Calcium carbonate Calcium oxide + Carbon dioxide CaCO3 CaO + CO2

274 Matter iii) 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: Hydrogen + Oxygen Water Sodium + Chlorine Sodium chlorine Calcium carbonate Calcium oxide + Carbon dioxide Zinc + Hydrochloric acid Zinc chloride + Hydrogen Chemical Equation The chemical reaction expressed by writing symbols and molecular formulae of reactants and products is called a chemical equation. A chemical equation is more informative than a word equation. Examples: 1. 2H2 + O2 ∆ 2H2 O 2. 2Na + Cl2 2NaCl 3. CaCO3 ∆ CaO + CO2 4. Zn + 2HCl ZnCl2 + H2 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. Examples: NaOH + HCl H2 O + NaCl Reactants Products 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. Examples: H2 + O2 H2 O

Modern Concept Science and Technology – 8 275 In above equation, the number of oxygen atoms in reactant and product sides is not equal, so it is called an unbalanced chemical equation. Some more examples of unbalanced chemical equations are as follows: Mg + HCl MgCl2 + H2 KOH + H2 SO4 K2 SO4 + H2 O H2 O2 MnO2 H2 O + O2 HCl + Ca(OH)2 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: Examples: 2H2 + O2 ∆ 2H2 O Mg + 2HCl MgCl2 + H2 2KOH + H2 SO4 K2 SO4 + 2H2 O 2H2 O2 MnO2 2H2 O + O2 2HCl + Ca(OH)2 CaCl2 + 2H2 O In the above chemical equations, the numbers of atoms of the same element in reactant and product sides are equal. So, they are called balanced chemical equations. 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 a 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. 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 a hit and trial method. Hydrogen peroxide Water Oxygen

276 Matter 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: 4K + O2 2K2 O 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 Information obtained from a balanced chemical equation Following pieces of information can be obtained from a balanced chemical equation. i) The names of reactants and products ii) The symbols and molecular formulae of reactants and products iii) The total number of atoms or molecules of reactants and products iv) The ratio of molecular weight of reactant and product molecules v) The type of chemical reaction Limitation of a balanced chemical equation A balanced chemical equation cannot provide information about i) The physical state of reactants and products ii) Concentration of reactants iii) Conditions required for the reaction like heat, light, pressure, catalyst, etc. iv) The duration of a chemical reaction Modification of Chemical Equation To make the chemical reaction more informative following modifications are done. i) The physical state of reactants and products are denoted by ‘s’ for solid, ‘l’ for liquid, ‘g’ for gas and ‘aq’ for aqueous solution. ii) Concentration of reactants are denoted by ‘dil.’ for dilute and ‘conc.’ for concentrated solution. iii) The conditions like temperature, pressure, light, catalyst, etc. are written above or below the arrow.

Modern Concept Science and Technology – 8 277 iv) A double-way arrow ( ) is used for reversible reaction and a single way arrow (→) is used for the irreversible reaction. For example, 2Na(s) + 2H2 O (l) 2NaOH (aq) + H2 ↑ N2 (g) + 3H2 (g) 2NH3 ↑ 2HgO (s) 2Hg (l) + O2 ↑ Reversible Reaction The chemical reaction in which the products can recombine to give back the reactants is called a reversible reaction. For example, H2 + I2 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 Irreversible Reaction The chemical reaction in which the products cannot recombine to give back reactants is called an 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 ↑ HOT SKILL HIGHER ORDER THINKING SKILL 1. The size of an atom keeps increasing down the group but keeps decreasing from left to right in the period. Why? The size of an atom keeps increasing down the group due to addition of extra orbit. The size of an atom keeps decreasing from left to right in the period due to increasing nuclear charge that pulls the electrons inward strongly. 2. What are the terms used for vertical columns and horizontal rows in periodic tables? Vertical columns of the periodic table are termed as groups and horizontal rows of the periodic table are termed as periods. 2H2 O 2H2 O2

278 Matter 3. Write down the charge, mass and position of different sub-atomic particles. Sub-atomic particles Charge Mass Position Proton Positive 1 amu Nucleus Electron Negative 1 1837 amu Shells Neutron Chargeless 1 amu Nucleus 4. Atoms are electrically neutral, why? Atoms are electrically neutral because they consist of equal number of positively charged protons and negatively charged electrons which cancel each other to make atoms neutral. 5. Chemical equation is more informative than word equation, why? Chemical equation is more informative than word equation because from the chemical equation, we can know the symbol of elements and radicals with their valency. It makes easy to balance the chemical equation which is not possible in word equation. 6. Write down any two differences between elements and compounds. Differences between elements and compounds are: S.N. Elements S.N. Compounds 1 The simplest and pure form of a substance which cannot be broken into simpler substance are called elements. 1 The pure chemical substances which are obtained by the chemical combination of two or more than two elements are called compounds. 2 There are 118 elements. Examples: Hydrogen, Oxygen, Magnesium. 2 There are millions of compounds. Examples: Ammonia, Nitrous oxide. 7. Why is it necessary to arrange elements in the periodic table? . It is necessary to arrange elements in the periodic table because: i) Periodic table makes study of elements easier and systematic. ii) It helps to predict the properties of new elements. iii) It explains the gradual change in properties of elements. iv) With the help of a periodic table, we can compare the properties of different elements.

Modern Concept Science and Technology – 8 279 3 STEPS EXERCISE EXERCISE STEP1 1. Select the best answer from the given alternatives. a) What is the atomic number of neon? i) 9 ii) 10 iii) 11 iv) 12 b) Which is a chargeless particle of an atom? i) Proton ii) Neutron iii) Electron iv) Positron c) What is H2 ? i) Molecule ii) Atom iii) Element iv) Radical d) What is the molecular formula of hydrogen bromide? i) HBr2 ii) HB iii) HBr iv) HBm e) What is the molecular mass of sodium chloride? i) 58 amu ii) 56 amu iii) 60 amu iv) 100 amu f) On what basis does Mendeleeev’s periodic table made? i) Atomic number ii) Molecular mass iii) Atomic mass iv) Valency g) What is the molecular formula of calcium carbonate? i) CaCO2 ii) CaCO3 iii) CaCO iv) CaCO4 2. Write True for the correct and False for the incorrect statements. a) Compounds are impure substances. b) The valency of Aluminium is 3. c) Mendeleev’s periodic table is based on increasing atomic number. d) An atom is electrically neutral. e) The size of atoms decreases downwards in the groups. f) Changing the magnesium ribbon into magnesium oxide is a chemical change. g) The molecular weight of calcium carbonate is 100. h) The valency of helium is 2. i) Precipitated products are symbolized by downward arrow. 3. Fill in the blanks with appropriate words. a) O2 represents one ……………… of oxygen. b) Atomic number is the number of ……………… present in an atom. c) The electrons present in the outermost shell of an atom are called ………… electrons. d) The sum of mass of each atom present in a molecule is called .................... e) The valency of argon is………………

280 Matter f) Modern periodic table has……………periods. g) The substances which are formed as a result of chemical reaction are called ………… h) Horizontal rows of a periodic table are called ………… 4. Answer the following questions in one word. a) How many short periods are there in a modern periodic table? b) How many elements are there in very short periods of modern periodic table? c) Who introduced modern periodic table? d) What is the valency of inert gases? e) What is the name of tiny, rigid and solid centre of an atom? STEP2 5. Give reasons. a) Argon is also called a noble gas. b) The valency of both oxygen and magnesium is 2. c) H2 is a molecule but H is an atom. d) Neon does not take part in a chemical reaction. e) Ammonium is an electropositive radical. f) Nitrate is an electronegative radical. 6. Write any two differences between: a) Atoms and molecules b) Elements and compounds c) Mendeleev’s periodic table and modern periodic table d) Periods and groups e) Molecular mass and atomic mass f) Electropositive radicals and electronegative radicals g) Protons and electrons h) Word equation and formula equation i) Balanced equation and skeleton equation 7. Answer the following in short. a) Define atomic number and atomic mass. b) Which can exist freely in nature H or H2 ? c) What is electronic configuration? d) Show electronic configuration of calcium and chlorine. e) What is 2n2 rule? How many electrons does M shell accommodate? f) What are noble elements? g) What is a periodic table? How many elements are discovered so far? h) What is atomic number of the following elements? potassium, chlorine, magnesium, sulphur, aluminium and nitrogen i) What is a chemical reaction?

Modern Concept Science and Technology – 8 281 j) Write down the molecular formula and name of the elements present in following compounds: i) Sulphuric acid ii) Calcium carbonate iii) Nitric acid iv) Ammonia v) Magnesium sulphate STEP3 8. Answer the following questions. a) Explain octet and duplet rule with an example of each. b) Define radicals and write down their types with any four examples of each. c) What is Mendeleev’s periodic law? List out the merits and demerits of Mendeleev’s periodic table. d) What is modern periodic law? List out the characteristics of modern periodic table. e) Write down the molecular formula of the following molecules: hydrochloric acid, water, calcium chloride, ammonia and nitric acid f) Calculate the molecular mass of the following molecules. CH4 , NaCl, CaO, Ca(OH)2 , MgO and CO2 g) Write down the pieces of information that are not provided by a balanced chemical equation. h) List out any three major modifications in chemical equation that provide complete information. i) Define reversible and irreversible chemical equation with an example of each. j) Complete and balance the following chemical equations. i) KClO3 ……………. + 3O2 ii) NaOH + …………… NaCl + …………… iii) Zn + HCl ……………+ H2 iv) KClO3 KCl + …………… v) ……………+ Ca(OH)2 CaSO4 + H2 O k. Draw the electronic configuration of the following atoms: i) Helium ii) Carbon iii) Neon iv) Sodium v) Phosphorous vi) Calcium

282 Materials Used in Dailly Life Materials Used in Dailly Life Unit 10 Introduction We use various materials in our daily life. They make our life easier. Toothpaste, soap and detergent are cleaning agents. Some of them are acid or base and some of them are salt. Acid and base are corrosive. They can dissolve metals and burn our skin. Sour fruits and vegetables contain organic acid in them. Cabbage, grapefruit and watermelon contain bases. Acids, bases and salts are very useful chemicals for us. Acid is used in industries, bleaching and cleaning. Bases are used to make soap, paper and textiles. Salts are used as diet, fertilizer and medicine. The rainfall which contains low concentration of acid in it is called acid rain. Acid rain is formed when rainwater dissolves industrial gases. It pollutes land and water. Water is a universal solvent. It dissolves various minerals. The presence of minerals causes the hardness of the water. Chlorides, sulphates and bicarbonates of magnesium and calcium cause the hardness of the water. Their absence makes water soft. Soft water is widely used for washing clothes and dishes. Whereas hard water is good for drinking and bathing. We use metals and their alloys also in our daily life. Alloys are used because they can be moulded at a lower temperature than metals and they have certain special properties too. Brass, bronze and steel are common alloys. Brass and bronze are commonly used for making decorative parts, door knobs, handles, utensils, statues, musical instruments and water pipes. Steel is used for construction, railings, utensils and bathroom equipment.

Modern Concept Science and Technology – 8 283 Key terms and terminologies of the unit 1. Acids : Acids are those chemical substances which give hydrogen ions when dissolved in water. 2. Strong acids : Acids which undergo almost complete dissociation in aqueous solution and produce high concentration of hydrogen ions are called strong acids. 3. Weak acids : Acids which undergo partial dissociation in aqueous solution and produce low concentration of hydrogen ions are called weak acids. 4. Organic acids : Acids which are obtained from living organisms and contain carbon atoms are called organic acids. 5. Inorganic acids : Acids which are obtained from minerals and do not have carbon atom are called inorganic acids or mineral acids. 6. Bases : The chemical species that give hydroxyl ions in solution state are called bases. Usally metal oxides and metal hydroxides are bases. 7. Alkalis : The bases that dissolve readily in water and produce hydroxyl (OH- ) ions are called alkalis. 8. Strong bases : Bases (alkalis) which give more amounts of hydroxyl ions in aqueous solution are called strong bases. 9. Weak bases : Bases (alkalis) which give very less amount of hydroxyl ions in aqueous solution are called weak bases. Sequence of Curriculum Issued by CDC  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 UNIT Acid, Base and Salt 10.1 Estimated teaching periods Theory Practical 6 2 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 people to propose the scientific concept of acid and base. He is also popular for calorimetry, carbon cycle, redox reaction and combustion. Antoine Lavoisier About the Scientist

284 Acid, Base and Salt 10. Salt : Salt is a chemical substance which is formed by partial or complete replacement of hydrogen ion of an acid molecule by a metal or ammonium radical. 11. Indicators : Indicators are those chemical substances which are used to indicate whether the substance is acidic or basic or neutral in nature. 12. Ordinary indicators : Ordinary indicators are obtained from the parts of plants and indicate only whether the substance is acid, base or salt. 13. Universal indicators: A universal indicator is a special kind of indicator which is obtained from many simple indicators and is used to measure the strength of acidity or alkalinity. 14. Acid rain : The rain which is acidic in nature and contains various acids in it is called acid rain. Introduction We eat different types of foods, fruits and vegetables. Among them, some have sour taste like orange, lemon, grape, apple; some have bitter taste like bitter gourd, edible soda; and some have salty taste like table salt. In our surroundings, there are several kinds of compounds which cannot be tasted due to their corrosive and poisonous nature. So, to study all those edible and non-edible compounds, they are divided into three groups. They are acids, bases and salts. 10.1.1 Acid The word acid is derived from Latin word “acidus” which means sour in taste. Most of the acids, which are edible or non-edible, are sour in taste. But it is dangerous to touch or taste acids in laboratory. According to Swedish chemist Arrhenius, “Acids give hydrogen ions when dissolved in water and conduct electricity.” So, “acids are those chemical substances which give hydrogen ions when dissolved in water.” HCl H+ + Cl– HNO3 H+ + NO3 – H2 SO4 2H+ + SO4 – – Classification of Acids Classification of acids on the basis of strength a) Strong acids: Acids which undergo almost complete dissociation in aqueous solution and produce high concentration of hydrogen ions are called strong acids. Due to more hydrogen ion concentration, they are good conductor of electricity and have low pH value. Examples: Hydrochloric acid (HCl), Sulphuric acid (H2 SO4 ), Nitric acid (HNO3 ), etc. b) Weak acids: Acids which undergo a partial dissociation in aqueous solution and produce low concentration of hydrogen ions are called weak acids. Due to low concentration of hydrogen MEMORY TIPS 1. Milk also contains acid called lactic acid which helps to form yoghurt. 2. Proteins are made up of amino acids. 3. Vitamin C is also an acid called ascorbic acid. MEMORY TIPS When the degree of dissociation is more than 30%, it is considered a strong acid. A strong acid such as HCl dissociates completely into its ions. A weak acid such as H2 CO3 does not dissociate completely.

Modern Concept Science and Technology – 8 285 ions, they do not conduct electricity easily and have a high pH value. Examples: Acetic acid (CH3 COOH), Carbonic acid (H2 CO3 ), Formic acid (HCOOH), Ascorbic acid (C6 H8 O6 ), etc. Classification of acids on the basis of chemical nature a) Organic acids: Acids which are obtained from living organisms and contain carbon atom are called organic acids. Examples: Acetic acid (CH3 COOH), Formic acid (HCOOH), Maleic acid (C4 H4 O4 ), Ascorbic acid (C6 H8 O6 ), Citric acid (C6 H8 O7 ), etc. These are weak acids and produce less concentration of hydrogen ions in aqueous solution. b) Inorganic acids (Mineral acids): Acids which are obtained from minerals and do not contain carbon atom are called inorganic acids or mineral acids. Inorganic acids may be strong or weak. Inorganic acids are commonly used in laboratories. Examples: Hydrochloric acid (HCl), Nitric acid (HNO3 ), Sulphuric acid (H2 SO4 ), Carbonic acid (H2 CO3 ), etc. FACTS WITH REASONS Why is hydrogen sulphate called an acid? Hydrogen sulphate is called an acid because it gives hydrogen ion when dissolved in water. Properties of Acids a) Physical properties of acids i) Acids have a sour taste due to the presence of hydrogen ions (H+ ). Most of the fruits are sour in taste. However, it is dangerous to touch and taste acids in laboratory as they burn our skin, tongue, etc. ii) Acids change blue litmus paper into red and methyl orange into red. iii) Strong acids are corrosive in nature. b) Chemical properties of acids i) Acids react with bases/alkalis and produce salt and water. Acid + Base Salt + Water HCl + NaOH NaCl + H2 O H2 SO4 + MgO MgSO4 + H2 O ii) Dilute acids react with active metals and produce salt and hydrogen gas. Dilute acid + Metal Salt + Hydrogen 2HCl + Mg MgCl2 + H2 ↑ 2HCl + Ca CaCl2 + H2 ↑ H2 SO4 + Zn ZnSO4 + H2 ↑ iii) Acids react with carbonates and bicarbonates and produce salt, water and carbon dioxide. Carbonate + Acid Salt + Water + Carbon dioxide MgCO3 + H2 SO4 MgSO4 + H2 O + CO2 ↑

286 Acid, Base and Salt CaCO3 + 2HCl CaCl2 + H2 O + CO2 ↑ Na2 CO3 + 2HCl 2NaCl + H2 O + CO2 ↑ Bicarbonate + Acid Salt + Water + Carbon dioxide NaHCO3 + HCl NaCl + H2 O + CO2 ↑ Ca(HCO3 )2 + 2HCl CaCl2 + H2 O + CO2 ↑ iv) Acids dissolve in water and give hydrogen ions. HCl H+ + Cl– H2 SO4 2H+ + SO4 – – HNO3 H+ + NO3 – FACTS WITH REASONS HCl is a strong acid but CH3 COOH is a weak acid, why? HCl is a strong acid because it undergoes almost complete ionization to give a large amount of hydrogen ion. But CH3 COOH is a weak acid because it does not ionize completely in water. Uses of acids i) Sulphuric acid is used in industries for making drugs, detergents and chemical fertilizers. It is widely used in laboratories and industries. Therefore, sulphuric acid is also called the ‘king of chemicals’. ii) Hydrochloric acid is used in laboratories and in tanning and printing industries. iii) Nitric acid is used for making explosives, plastics and dyes. iv) Carbolic acid (Phenol) is used to kill germs. v) Boric acid is used for washing eyes and wounds. vi) Acetic acid (vinegar) is used for preserving and flavouring foods. vii) Citric acid is used in medicines, as a source of vitamin C and flavouring drinks. viii) Carbonic acid is used in soft drinks and soda water. ix) Oxalic acid is used to remove ink-stain. x) Tartaric acid is added to baking soda (NaHCO3 ) for making baking powder. It neutralises the bitterness produced by the baking soda. Some acids of our daily use with their sources are as follows: S.N. Name of acids Source 1. Citric acid Lemon, tomato 2. Lactic acid Milk, curd 3. Ascorbic acid Sour fruits 4. Tartaric acid Grapes 5. Oxalic acid Chariamilo 6. Formic acid Red ant

Modern Concept Science and Technology – 8 287 10.1.2 Base Metallic oxides and hydroxides are called bases. Most bases or metal oxides dissolve in water and give hydroxyl ions. The bases that dissolve in water and produce hydroxyl (OH- ) ions are called alkalis. e.g. NaOH, KOH, Ca(OH)2 , Mg(OH)2 , etc. Some bases like PbO, BaO, HgO, CuO, etc. do not dissolve in water. So these compounds are bases but not alkalis. Therefore, all alkalis are bases but all bases are not alkalis. FACTS WITH REASONS Ferrous oxide is a base but not an alkali, why? Ferrous oxide is a base because it is a metal oxide but it is not an alkali as it does not dissolve in water to give hydroxide. NaOH is an alkali, why? NaOH is an alkali because it dissolves in water and produces hydroxyl ion. All alkalis are bases but all bases are not alkalis, why? All alkalis are bases because all alkalis are metal oxides or metal hydroxides which dissolve in water. All bases are not alkalis because all bases do not dissolve in water. Differences between bases and alkalis SN Bases SN Alkalis 1 Bases are chemical species that produce hydroxyl ion in solution state. 1 Only water soluble metallic oxides or bases are called alkalis. 2 All bases do not give hydroxyl ions when dissolved in water. 2 All alkalis give hydroxyl ions when dissolved in water. 3 All bases are not alkalis. Examples: MgO, Na2 O, HgO, Fe2 O3 , etc. 3 All alkalis are bases. Examples: NaOH, KOH, Mg(OH)2 , NH4 OH, etc. Strong bases and weak bases Bases (alkalis) which give more amount of hydroxyl ions (OH⁻) in aqueous solution are called strong bases. e.g. Sodium hydroxide (NaOH), Potassium hydroxide (KOH), Calcium hydroxide [Ca(OH)2 ], etc. They undergo almost complete dissociation and have a high pH value. Bases (alkalis) which give very less amount of hydroxyl ions (OH⁻) in aqueous solution are called weak bases. e.g. Ferric hydroxide [Fe(OH)3 ], Copric hydroxide [Cu(OH)2 ], etc. They have very less degree of ionization and have a low pH value. Properties of bases or alkalis a) Physical properties of bases or alkalis i) Bases are soapy in touch and bitter in taste. ii) Bases turn red litmus paper into blue, methyl orange into yellow and phenolphthalein into pink. iii) Strong bases or alkalis like NaOH, KOH , etc. dissolve in oil and grease. iv) Strong bases or alkalis burn our skin.

288 Acid, Base and Salt b) Chemical properties of bases or alkalis i) Bases or alkalis react with acids to form salt and water. Base + Acid Salt + Water 2KOH + H2 SO4 K2 SO4 + H2 O NaOH + HCl NaCl + H2 O Mg(OH)2 + H2 SO4 MgSO4 + 2H2 O ii) Bases or alkalis react with carbon dioxide and form corresponding carbonate and water. Alkali + Carbon dioxide Carbonate + Water 2NaOH + CO2 Na2 CO3 + H2 O 2KOH + CO2 K2 CO3 + H2 O Ca(OH)2 + CO2 CaCO3 + H2 O iii) Alkalis react with ammonium salts and form salt, water and ammonia gas. Alkali + Ammonium salt Salt + Water + Ammonia NaOH + NH4 Cl NaCl + H2 O + NH3 ↑ Ca(OH)2 + 2NH4 Cl CaCl2 + 2H2 O + 2NH3 ↑ Mg(OH)2 + (NH4 ) 2 CO3 MgCO3 + 2H2 O + 2NH3 ↑ Uses of Bases i) Sodium hydroxide (NaOH) is used to make soaps, detergents, papers, etc. and purification of petroleum products. ii) Calcium hydroxide [Ca(OH)2 ] or slaked lime is used for making mortar and bleaching powder, to reduce hardness of water and to neutralize acidity of soil. iii) Potassium hydroxide (KOH) is used in alkaline batteries and to make soft soap. iv) Aluminium hydroxide [Al(OH)3 ] and magnesium hydroxide [Mg(OH)2 ] are used to reduce hyperacidity of stomach. v) Ammonium hydroxide (NH4 OH) is used to remove grease and stains from clothes and to make fertilizers. vi) Calcium oxide (CaO) or quick lime is used for softening hard water, purification of sugar and production of cement. 10.1.3 Salt Salt is a chemical substance which is formed by partial or complete replacement of hydrogen ion of an acid molecule by a metal or ammonium radical. In general, salts are neutral compounds but some may be acidic or basic in nature. The process by which acid and base react together to give salt and water is called neutralization reaction. In this reaction, H+ ions of the acid are completely replaced by a metal. Fig: table salt

Modern Concept Science and Technology – 8 289 Acid + Base Salt + Water HCl + NaOH NaCl + H2 O H2 SO4 + 2KOH K2 SO4 + 2H2 O Classification of Salt There are three types of salts. They are: Neutral salt: Neutral salts are those salts which are formed when strong acids and strong bases react, or weak acid and weak base react. Example: Sodium sulphate (Na2 SO4 ), Sodium chloride (NaCl) Acidic salt: Those salts which are prepared by the reaction of strong acids with weak bases are called acidic salts. Example: Ammonium chloride Basic salt: Basic salts are those salts which are formed by the reaction of weak acids and strong bases. Example: Sodium carbonate (Na2 CO3 ) Properties of salts i) Generally, salts are neutral but some may be acidic or basic in nature. ii) Most of the salts are water soluble but chloride salts of silver and lead and sulphate salts of lead and barium are insoluble. iii) Salts of metals like Na, K, Mg, Ca, Al and Ba are white or colourless whereas salts of Cu, Co, Mn, Ni, Fe and Cr are colourful. iv) Some salts are salty in taste but most salts are bitter. v) Salts conduct electricity in molten or solution state. vi) Some salts are amorphous whereas some are crystalline. vii) Acidic salt dissolves in water to form acidic solution. Basic salt dissolves in water to from basic solution. Neutral salt do not change pH of solution. Chemical properties of salt i. Calcium carbonate produces carbon dioxide and lime when heated. CaCO₃ CaO + CO₂ ii. Potassium chlorate produces oxygen and potassium chloride on heating. 2KClO₃ 2 KCl + 3O₂ iii. Electrolysis of sodium chloride produces sodium hydroxide, chlorine and hydrogen. NaCl (electrolysis) NaOH + Cl₂ + H₂ iv. Silver nitrate reacts with sodium chloride to produce silver chloride and sodium nitrate. AgNO₃ + NaCl AgCl + NaNO₃ v. Salts can take part in reduction reaction, where highly reactive metal can displace less reactive metal from the salt solution. Fe + CuSO4 FeSO4 + Cu Uses of salts i) Table salt (NaCl) is used in our foods and also as a preservative. ii) Sodium carbonate is used for manufacture of soaps, detergents and glasses. It is also used to reduce hardness of water.

290 Acid, Base and Salt iii) Calcium sulphate is used in medical field for plastering of fractured bones and for mixing in cement. iv) Sodium bicarbonate is used as baking powder, for reducing hyperacidity and in fire extinguisher. v) Copper sulphate is used for making fungicides and in copper plating. vi) Ammonium chloride is used in dry cells as an electrolyte. vii) Ammonium sulphate and potassium nitrate is used as chemical fertilizer. viii) Ferrous sulphate is used as a medicine to treat iron deficiency in anaemia patients. 10.1.4 Indicators Indicators are those chemical substances which are used to indicate whether the given substance is acidic or basic or neutral in nature. Indicators, when treated with acidic or basic solution, change their own colour. They remain chemically unchanged but indicate the end point of the reaction. Litmus paper, methyl orange and phenolphthalein are some common indicators. They are obtained from different parts of plants like roots, flowers, leaves, etc. These are collected, crushed and mixed with organic solvent to obtain indicators. The colour change of different indicators with acidic, basic and salt solution is given in the table: S.N. Indicators Colour in acid solution Colour in basic solution Colour in neutral salt solution 1) Red litmus paper No change in colour Changes into blue No change in colour 2) Blue litmus paper Changes into red No change in colour No change in colour 3) Methyl orange Changes into red Changes into yellow No change in colour 4) Phenolphthalein No change in colour Changes into pink No change in colour 5) Red cabbage juice Changes into red Changes into green Changes into rose red ACTIVITY 1 OBJECTIVE : To prepare a simple indicator (litmus paper) from the petals of flowers. 1. Collect petals of some flowers and crush them along with sand to get a fine paste. 2. Mix the paste with alcohol and filter the mixture to obtain a clear solution. 3. Collect the solution in a beaker and immerse some strips of paper into the solution. 4. Dry these strips of paper in the sun. In this way, litmus paper can be prepared. Now, use these litmus papers to test an acid, a base and a salt. 10.1.5 Acid Rain The mixing of small amount of acids in rain water due to the chemical reactions caused by industrial gases in the atmosphere is called acid rain. Simply, acid rain is the rain or any other form of precipitation that is acidic in nature. It produces harmful effects in the environment and affects the natural resources. MEMORY TIPS 1. Normal rain has a pH of 6.5. Acid rain has a pH of 5.7 or less. 2. Acid fog is ten times more acidic than acid rain.

Modern Concept Science and Technology – 8 291 Cause of acid rain Air pollution is the major cause of acid rain. These gases are either produced naturally during volcano, leaching, organic decomposition or by humans from industries, factories, kitchen and vehicles. When industrial gases like carbon dioxide, sulphur dioxide, nitrous oxide, etc. are released into the atmosphere, they undergo several chemical reactions and combine with rain water. Then, they fall upon the animals, plants, soil etc. and affect them. The acids that mix with rain are: hydrochloric acid (HCl), sulphuric acid (H2 SO4 ), nitric acid (HNO3 ), etc. The different types of natural events like volcano, forest fire, decomposition of various dead bodies also release above gases. The chemical reactions below show the formation of these acids from industrial gases. H2 O + CO2 H2 CO3 (Carbonic Acid) 2SO2 + O2 2SO3 SO3 + H2 O H2 SO4 (Sulphuric acid) NO + O3 NO2 + O2 4NO2 + O2 2N2 O5 N2 O5 + H2 O 2HNO3 (Nitric acid) Effects of acid rain i) Acid rain corrodes the skin of animals and human beings and causes skin diseases. ii) It affects the growth and development of plants by making the soil acidic. iii) It increases the acidity of water and affects aquatic plants and animals. iv) It corrodes historical and architectural monuments like buildings, statues, arts, etc. v) It increases the acidity of land, kills microorganisms and decreases the fertility of the soil. vi) It damages the crops and decreases productivity. Control measures of acid rain i) Reducing the use of fossil fuels and promoting alternative and clean sources of energy. ii) Adopting techniques of reuse of emitted gases from industries. ii) Development of technology that controls production of SO2 and CO2 . ACTIVITY 2 Discuss the major environmental problems occurring in your neighbourhood including its causes, effects and control or preventive measures.

292 Acid, Base and Salt HOT SKILL HIGHER ORDER THINKING SKILL 1. Acid is used to clean tiles and marbles in house. Tiles and marbles contains metallic minerals. When they are dirty acid is used to clean them. Acid can dissolves metals from the surface of tiles and marbles and remove dirt in the process. 2. Write any two differences between acid and base. Differences between acid and base are: S.N. Acid S.N. Base 1 Acids are those chemical substances which give hydrogen ions when dissolved in water. 1 The chemical species that produce hydroxyl ions in solution state are called bases. 2 Acids have pH value less than seven. 2 Bases have pH value more than seven. 3. Why are citrus and amala sour in taste? Citrus contains citric acid and amala contains ascorbic acid. So, they have a sour taste. 4. We should handle acids very carefully. Strong acids are corrosive in nature. They can burn skin, clothes, etc. So, it is necessary to handle them carefully. 5. Nettle stings and ant bite is painful. Stinging hairs of a Nettle and saliva of an ant contain formic acid. As this acid enters into our skin, we have a burning sensation which is painful. 6. We use aluminium hydroxide and magnesium hydroxide to reduce acidity in our stomach, why? Aluminium hydroxide and magnesium hydroxide are alkalis. They neutralize the excess amount of hydrochloric acid present in our stomach. So, acidity can be controlled by using these alkalis. 7. Krishna mandir is a temple at Patan durbar square. It is one of the finest stone monuments. It is slowly eroded each year. What might be the reasons? Clarify. Krishna mandir is one of the finest stone monument of Nepal. It is in patan durbar square which is a busy city area. It is slowly being eroded. The reason for erosion is acid rain. Kathmandu valley is a heavily pouplated area. Lots of LPG gas is consumed each day for domestic use. Thousands of vehicles run every day. There are numerous brick industries with in the valley. Multiple industries are in operation. So huge quantity of carbon dioxide gas and other industrial gases are collected in the sky of Kathmandu valley. These gases mix with dust and water vapour to make photo chemical smog. During rainfall, these gases dissolve with water and change into respective acids. The acidulated water rains down from sky. It is called acid rain. Acid rain is corrosive. They can dissolve metals from the stone. So each drop of acid rain that falls on stone monuments will erode them. As a result Krishna mandir, a stone built temple, has been eroded since decades.

Modern Concept Science and Technology – 8 293 3 STEPS EXERCISE EXERCISE STEP1 1. Select the best answer from the given alternatives. a) What will be the colour of methyl orange in acidic solution? i) red ii) colourless iii) yellow iv) orange b) Which of them is a mineral acid? i) acetic acid ii) citric acid iii) hydrogen chloride iv) malic acid c) Which is the strongest base? i) copper oxide ii) calcium hydroxide iii) sodium hydroxide iv) aluminium oxide d) Which salt is used as a fungicide and pesticide? i) NaCl ii) CuSO4 iii) NH4 NO3 iv) KNO3 e) Which colour do red litmus paper show for base? i) red ii) colourless iii) blue iv) green 2. Write True for the correct and False for the incorrect statements. a) Citric acid is an organic acid. b) Sodium hydroxide is a strong base. c) Potassium hydroxide is a weak base. d) Potassium nitrate is used in fertilizers. e) Industrial gases are major cause of acid rain. 3. Fill in the blanks with appropriate words. a) Acid give …………. ions in a solution. b) Alkalis are ………… soluble bases. c) The chemical substances to test if a given substance is either acidic or alkaline is called ............ d) Carbon dioxide dissolves in rain water to form …………… acid. e) The colour of phenolphthalein in basic medium is…………… 4. Answer the following questions in one word. a) Which chemical species are called proton donor? b) What is the taste of acid and base? c) What is the rain water which is acidic called? d) Which salt is called table salt? Write moleclar formula. e) What is the mixing of small amount of acid in the rainfall called?

294 Acid, Base and Salt STEP2 5. Write any two differences between: a) Acid and base b) Organic acids and inorganic acids c) Strong acids and weak acids d) Base and alkali e) Sodium hydroxide and hydrochloric acid 6. Give reasons. a) Hydrogen sulphate is an acid. b) NaOH is an alkali. c) Ferrous oxide is a base but not an alkali. d) HCl is a strong acid but CH3 COOH is a weak acid. e) All alkalis are bases but all bases are not alkalis. 7. Answer the following questions a) What is an acid? Name three organic acids and three inorganic acids with their molecular formula. b) What is a bases? Name three bases and three alkalis with their molecular formula. c) Define alkali. d) Name a strong acid present in human stomach. e) What is salt? List the names of any five salts with chemical formula. f) Define indicator. g) Define acid rain. STEP3 8. Answer the following questions. a) List three physical properties of acids and bases each. b) Mention three important chemical properties of acids and bases each. c) Write any four uses of acids, base and salt. d) Name a strong acid present in human stomach. e) How is acid rain formed? f) Sky above the major industrial area is always covered with photochemical smog which forms acid rain in monsoon. What kind of harmful effects are to be expected in such areas? g) Enlist the causes of acid rain. What should we do minimize the acid rain? h) “A” denotes a salt solution, “B” demotes an acid solution and “C” denotes an alkali solution. On the basis of this information, complete the table given below: Indicators Substance A Substance B Substance C Blue litmus paper Red litmus paper Phenolphthalein Methyl orange

Modern Concept Science and Technology – 8 295 Key terms and terminologies of the unit 1. Water : Water is one of the most important and abundant substances needed by living beings. It covers approximately 70% of the earth's surface. 2. Solvent property of water : Water is a universal solvent. It dissolves various organic and inorganic substances as well as different types of acids, bases salts, etc. 3. Soft water : The water that does not contain chloride, sulphate and bicarbonate salts of calcium and magnesium is called soft water. 4. Hard water : The water that contains the chloride, sulphate and bicarbonate salts of calcium and magnesium is called hard water. 5. Temporary hard water : The hard water containing bicarbonate salts of calcium and magnesium is called temporary hard water. 6. Permanent hard water : The hard water containing chloride and sulphate salts of calcium and magnesium is called permanent hard water. 7. Permutit process : The process of removal of permanent hardness of water by using sodium zeolite is called permutit process. 8. Alloy : The homogeneous mixture of two or more metals or metals and non-metals is called alloy. 9. Amalgam : The alloy which is formed by mixing a metal and mercury is called amalgam. 10. Brass : Brass is an alloy of copper and zinc. 11. Bronze : Bronze is an alloy of copper and tin. 12. Steel : Steel is an alloy of iron and carbon. Sequence of Curriculum Issued by CDC  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 UNIT Water 10.2 Estimated teaching periods Theory Practical 5 2 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. Harry Brearley About the Scientist

296 Water Introduction Water is one of the most important and abundant substances needed by 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 weak 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 sources of water, properties and uses of water and methods of removing hardness of water. 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 scum. Such water is called hard water. 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. Advantages of soft water i. It is used for washing clothes because it can produce huge quantity of lather. ii. It is used for washing dishes. iii. It is used for cleaning floors too. iv. It is also used for making carbonated drinks and beverages. Disadvantages of soft water i. Softwater corrodes metal pipes. ii. Softwater is not tasty to drink. iii. It contains too much sodium than hardwater.