Page 1 :
Q. 1. Why should a magnesium ribbon be cleaned before burning in air ?, NCERT Book, Page 6, magnesium ribbon is cleaned to remove the protective layer of basic magnesium carbonate from, surface so that it may readily combine with the oxygen of air (on heating)., 92 Write the balanced equations for the following chemical reactions :, (i) Hydrogen, Chlorine, Hydrogen chloride, (ii) Barium, chloride, Aluminium, Barium, Aluminium, nchloride, sulphate, sulphate, Sodium hydroxide, (iii), Sodium, Water, Hydrogen, Ans. (i) H2, Cl, 2HCI, (ii) 3BACI, +, Al(SO,)3, 3BASO,, 2AICI,, (iii) 2Na, 2H,0, 2NAOH, H2, 0.3. Write balanced chemical equations with state symbols for the following reactions :, (i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulpha, and the solution of sodium chloride., (ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produ, sodium chloride solution and water., Ans. (i) BaCl, (aq), (ii) NaOH (aq), Na,SO, (aq), HCI (aq), BaSO, (s), NaCl (aq), 2NACI (aq), H,O (!), +, NCERT Book, Page 10, Q. 1. A solution of substance X is used for white washing., (i) Name the substance X and write its formula., (ii) Write the reaction of the substance X named in (i) above with water., Ans. See Sample Problem 1 on page 32 of this book., Q.2. Why is double the amount of a gas collected in one of the test-tubes in the electrolysis of water activity, Name this gas., Ans. See Sample problem 2 on page 33 of this book., NCERT Book, Page 13, Q.1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it ?, Ans. When an iron nail is dipped in copper sulphate solution, then the blue colour of copper sulphate solution, changes because iron displaces copper from copper sulphate solution to form a light green solution of iron, sulphate (For equation of this reaction, see Example 3 on page 33 of this book)., Q.2. Give an example of a double displacement reaction., Ans. When silver nitrate solution is added to sodium chloride solution, then a double displacement reaction, takes place in which a white precipitate of silver chloride is formed alongwith sodium nitrate solution (For, equation of this reaction, see Example 1 on page 35 of this book), 2.3. Identify the substances that are oxidised and the substances that are reduced in the following reactions, O, (g), H2 (g), 2Na,0 (s), Cu (s), (i) 4Na (s), +, (ii), CuO (s) +, H,O (1)

Page 2 :
322, SCIENCE FOR TENTH CLASS: CHEMISTRY, Ans. (i) Substance oxidised : Na, (ii) Substance oxidised : H2, Substance reduced, Substance reduced, CuO, NCERT Book, Pages 14, 15 and 16, Q.1. Which of the statements about the reaction below are incorrect ?, 2PBO (s), C (s), 2Pb (s), Co, (g), +], +], (a) Lead is getting reduced, (b) Carbon dioxide is getting oxidised, (c) Carbon is getting oxidised, (d) Lead oxide is getting reduced, (i) (a) and (b), (i) (a) and (c), (iii) (a), (b) and (c), (iv) all, Ans. () (a) and (b), Q.2. Fe;Oy + 2Al Al,O, + 2Fe, The above reaction is an example of a:, (b) double displacement reaction, (d) displacement reaction, (a) combination reaction, (c) decomposition reaction, Ans. (d) displacement reaction, Q.3. What happens when dilute hydrochloric acid is added to iron filings ? Tick the correct answer., (a) Hydrogen gas and iron chloride are produced, (b) Chlorine gas and iron hydroxide are produced, (c) No reaction takes place, (d) Iron salt and water are produced, Ans. (a) Hydrogen gas and iron chloride are produced, Q.4. What is a balanced chemical equation ? Why should chemical equations be balanced ?, Ans. A chemical equation having an equal number of atoms of different elements in the reactants and products, is called a balanced chemical equation. Here is an example. Zinc reacts with dilute sulphuric acid to form, zinc sulphate and hydrogen :, ZNSO,, Zinc sulphate, H,SO,, H2, Hydrogen, Zn, Zinc, Sulphuric acid, Now, this equation has an equal number of Zn atoms (1 each), H atoms (2 each), S atoms (1 each), and O, atoms (4 each) in reactants and products, so it is a balanced chemical equation., The chemical equations are balanced to satisfy the law of conservation of mass which says that 'matter, (or atoms) can neither be created nor destroyed in a chemical reaction'. So, the total mass of all the elements, present in the products of a chemical reaction should be equal to the total mass of all the elements present, in the reactants. In other words, the number of atoms of each element in the products must be equal to the, number of atoms of these elements in the reactants,, 0.5. Translate the following statements into chemical equations and then balance them:, (a) Hydrogen gas combines with nitrogen to form ammonia., (b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide, (c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of, barium sulphate., () Potassium metal reacts with water to give potassium hydroxide and hydrogen gas., Ans. (a) 3H2, N2, 2NH,, (b) 2H,S, 302, 2H,0, 2S02, (c) 3BaCl,, Al,(SO)s, 2AICI,, 3BASO,, 2H,0, 2КОН, H2, (d) 2K

Page 3 :
reaction two or more substances combine to form a single substance whereas in a decomposition reaction, a, single substance splits up to form two or more simpler substances., Ans. The decomposition reactions are called the opposite of combination reactions because in a combination, Q.7. Write the balanced chemical equations for the following reactions:, (a) When hydrogen burns in oxygen, it forms water :, 323, quations:, (a) HNO,, +., Ca(OH)2, CatNO,)2, H,SO,, H,O, H20, NANO,, (b) NaOH, (c) NaCl, (d) BaCl, Na,SO,, ABCI, BaSO,, AGNO,, H,SO,, Ca(OH);, H,SO,, AgNO,, H,SO,, Ans. (a) 2HNO,, (b) 2NAOH, HCI, Ca(NO,)2, Na,SO,, 2H,0, 21,0, (c) NaCl, AgCl, NaNO,, (d) BaCly, BaSO,, +., 21HCI, (e) Calcium hydroxide + Carbon dioxide, +Calcium carbonate, (b) Zinc, + Silver nitrate, Water, Zinc nitrate, (c) Aluminium, (d) Barium chloride, Silver, + Copper chloride, + Potassium sulphate, CO,, 2AGNO,, 3CuCl, KSO,, Aluminium chloride +, Copper, - Barium sulphate, +], Potassium chloride, CACO,, Zn(NO,)2, 2AICI,, BaSO4, 08 Write the balanced chemical equations for the following and identify the type of reaction in each case:, Ans. (a) Ca(OH)2, +., H,O, (b), Zn, 2Ag, +., (c), 2A1, 3Cu, (4) BaCl, 2KCI, (a) Potassium, bromide (aq), Barium, Potassium, iodide (aq), Barium, bromide (s), iodide (aq), (b) Zinc carbonate (s), Zinc oxide (s), Carbon dioxide (g), (c) Hydrogen (g), (d) Magnesium (s), Chlorine (g), Hydrochloric, acid (aq), Hydrogen chloride (g), Magnesium, chloride (aq), Hydrogen (g), 2KI (aq), This is a double displacement reaction (which is also a precipitation reaction)., Ans. (a) 2KBr (aq), Bal2 (aq), BaBr, (s), (b) ZNCO, (s) ZnO (s), This is a decomposition reaction., (c) H2 (g), This is a combination reaction., CO2 (g), +, Cl, (g), 2HCI (g), → MgClz (aq), H, (g), (d) Mg (s), This is a displacement reaction., + 2HCI (aq), s. What does one mean by exothermic and endothermic reactions ? Give examples., Ans. See, page 10 of this book., Kespiration is considered an exothermic reaction because energy is released in this process. During, piration, glucose (obtained from the digestion of food) combines with oxygen of air in the cells of our, Ans., Mody to form carbon dioxide and water, and energy is released:, C,H12O, (aq), Why is respiration considered an exothermic reaction ? Explain., 6H,O (1) +, Energy, 6CO, (g), 60, (g), Oxygen, ), Water, Carbon dioxide, Glucose, Q.11. Why are, decomposition reactions called the opposite of combination reactions ? Write equations for these, reactions., 2H,0 (1), 2H, (g) + O2 (g)

Page 4 :
24, SCIENCE FOR TENTH CLASS : CHEMISTRY, In this reaction, two substances hydrogen and oxygen combine to form a single substance water, so this is a, combination reaction., (b) When acidified water is electrolysed, hydrogen and oxygen are formed :, 2H,0 (1), Electricity, 2H; (g) + O, (g), Here a single substance water breaks up into two simpler substances, hydrogen and oxygen. So, this is a, decomposition reaction. We can see from the above examples that a decomposition reaction is opposite of a, combination reaction., Q.12. Write one equation each for decomposition reactions where energy is supplied in the form of (a) heat, (b) light, and (c) electricity., Ans. (a) When calcium carbonate is heated, it decomposes to form calcium oxide and carbon dioxide :, Heat, CACO, (s), In this reaction, energy is supplied in the form of heat., CaO (s) + CO, (g), (b) When silver chloride is exposed to sunlight, it decomposes to form silver metal and chlorine gas:, Sunlight, 2AgCl (s), Here energy is supplied in the form of sunlight., (c) When acidified water is electrolysed, it decomposes to form hydrogen and oxygen :, 2Ag (s) + Cl, (g), 2H,0 (1), Electricity, 2H2 (g), O: (৪), In this case, energy is supplied in the form of electricity., 2.13. What is the difference between displacement and double displacement reactions ? Write equations for, these reactions., Ans. In a displacement reaction, a more reactive element displaces a less reactive element from its compound, whereas in a double displacement reaction, two compounds combine by an exchange of ions to form two, new compounds., () A displacement reaction takes place between copper sulphate solution and zinc to form zinc sulphate, solution and copper:, CUSO, (aq), Zn (s), ZnSO, (aq), Cu (s), (ii) A double displacement reaction takes place between barium chloride solution and sodium sulphate, solution to form a white precipitate of barium sulphate and sodium chloride solution:, BaCl, (aq), Na,SO, (aq), BaSO, (s), 2NACI (aq), 0.14. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by, copper metal. Write down the reaction involved., Cu(NO,)2 (aq), Copper nitrate, (solution), Ans., 2A9NO, (aq), Cu (s), 2Ag (s), Silver, Silver nitrate, Copper, (solution), 15. What do you mean by a precipitation reaction ? Explain by giving example., ns Any reaction in which an insoluble solid (called precipitate) is formed that suddenly separates from the, solution, is called a precipitation reaction. The reaction between silver nitrate solution and sodium chloride, solution to form silver chloride precipitate is an example of a precipitation reaction:, NaCI (aq), AGNO, (aq), Silver nitrate, AgCI (s), Silver chloride, NaNO, (aq), Sodium nitrate, Sodium chloride, (White ppt.), In this reaction, silver chloride is formed as a white, insoluble solid (called precipitate) which separates out, suddenly from the solution., 16. Explain the following in terms of gain or loss of oxygen with two examples each:, (b) Reduction, (a) Oxidation, n la) Oxidation : The gain of oxygen by a substance in a reaction is called oxidation

Page 5 :
O AND EXERCISES, NCERT DO, When magnesium is burned in air, then magnesium oxide is formed:, 325, 2Mg, Magnesium, O2, Oxygen, (From air), 2M O, Magnesium oxide, In this reaction, magnesium (Mg) has gained oxygen to form magnesium oxide (MgO), so magnestum, oxidised to magnesium oxide., a When copper oxide is heated with hydrogen, then copper metal and water are formed:, CuO, Hy, Hydrogen, Copper oxide, Cu, HO, Copper, In this case, hydrogen (H2) is gaining oxygen to form water (H,O), so hydrogen is getting oxidised to, Water, water., Reduction : The loss of oxygen by a substance in a reaction, is called reduction., o When zinc oxide is heated with carbon, then zinc metal and carbon monoxide are formed:, ZnO, Zinc oxide, Carbon, Zn, CO, Zinc, In this reaction, Zinc oxide (ZnO) is losing oxygen to form zinc metal (Zn), therefore, zinc oxide is, reduced to zinc., (i) When iron (III) oxide is heated with aluminium powder, then aluminium oxide and iron metal are, Carbon monoxide, formed:, Fe,O3, Iron (III) oxide, In this case, iron (II) oxide (Fe,O,) is losing oxygen to form iron metal (Fe), therefore, iron (III) oxide is, reduced to iron., 2AI, Al,O,, Aluminium oxide, 2Fe, Aluminium, Iron, 0.17. A shiny brown coloured element X on heating in air becomes black in colour. Name the element X and, the black coloured compound formed., Ans. See Sample Problem 5 on page 43 of this book., Q.18. Why do we apply paint on iron articles ?, Ans. Paint is applied on iron articles to prevent their rusting. When a coat of paint is applied to the surface of an, iron article, then air and moisture cannot come in contact with the iron metal of the article and hence no, rusting takes place., Q.19. Oil and fat containing food items are flushed with nitrogen. Why ?, Ans. The plastic bags containing oil and fat containing food items (such as potato chips) are flushed with an, unreactive gas nitrogen so as to prevent them from getting oxidised and turn rancid. This is because in the, presence of oxygen of air, the fats and oils present in food items get oxidised forming products having, unpleasant smell and taste which turn the foods rancid (making them unfit for eating). When air containing, oxygen is replaced by unreactive nitrogen gas, the packed food items do not get spoiled. They remain fresh, for a much longer time., Q.20. Explain the following terms with one example each :, (a) Corrosion (b) Rancidity, Ans. (a) Corrosion ne process, (such as an acid) on their surface, is called corrosion. Rusting of iron is the most common example of, corrosion. Rusting involves unwanted oxidation of iron metal which occurs in nature on its own. When, an iron object is left in damp air for a considerable time, it gets covered with a red-brown flaky substance, called 'rust'. The corrosion (or rusting) of iron is a continuous process which, if not prevented in time,, eats up the whole iron object., which metals are eaten up gradually by the action of air, moisture or a chemical, o7 Rancidity. Oxidation has damaging effect on foods containing fats and oils. When the food materials, prepared in fats and oil are kept for a long time, they start giving unpleasant smell and taste. The, condition produced by the aerial oxidation of fats and oil in foods marked by unpleasant smell and taste, a considerable time and makes them unfit for eating. For example, if potato chips prepared in oil are