# NCERT Exemplar Problems and their Solutions

Reviewed By:
Krishna Kant Majee
M.Sc., B.Ed.

### Chapter 1-Chemical Reactions and Equations

Q.39. On heating blue coloured powder of copper (II) nitrate in a boiling tube, copper oxide (black), oxygen gas and a brown gas X is formed.
(a) Write a balanced chemical equation of the reaction.
(b) Identify the brown gas X evolved.
(c) Identify the type of reaction.
(d) What could be the pH range of aqueous solution of the gas X?

$\ce{2Cu(NO3)2 ->[{Heat}] 2CuO + O2 + 4NO2}$
(b) X is nitrogen dioxide gas (NO2) that is evolved as brown, choking fumes.
(c) A thermal decomposition reaction.
(d) The aqueous solution of the $NO_{2}$ forms nitrous acid and nitric acid so pH of solution would be less than 7 as medium will be acidic.

Q.40. Give the characteristic tests for the following gases.
(a) $CO_{2}$
(b) $SO_{2}$
(c) $O_{2}$
(d) $H_{2}$

(a) $CO_{2}$ gas: When $CO_{2}$ gas is passed through lime water; it forms insoluble calcium carbonate which turns the solution milky. This is called as lime water test.
$Ca(OH)_{2}(aq) +$CO_{2}\rightarrow CaCO_{3} + H_{2}O$The solution becomes clear in excess of CO¬2 because of formation of soluble calcium bicarbonate.$CO_{2}+ H_{2}O + CaCO_{3}\rightarrowCa(HCO_{3})_{2}$(b)$SO_{2}$gas: Due to acidic nature, sulphur dioxide gas turns moist litmus paper from blue to red. It also changes the orange colour of acidified potassium dichromate solution to green.$ K_{2}Cr_{2} + 3SO_{2} + H_{2}SO_{4}\rightarrow Cr_{2}(SO_{4})+ ~~K_{2}SO_{4} +H_{2}O$(c) O2 gas: Oxygen gas burns brightly with wooden splinter that proves the combustible nature of oxygen gas.$\ce{ C + O2-> CO2 + Heat + Light}$(d)$\ce{H2}$gas: In the presence of atmospheric oxygen, hydrogen gas burns with a poping sound. Question. 41. What happens when a piece of (a) Zinc metal is added to copper sulphate solution? (b) Aluminium metal is added to dilute hydrochloric acid? (c) Silver metal is added to copper sulphate solution? Also, write the balanced chemical equation, if reaction occurs. Answer. (a) When zinc metal is added to copper sulphate solution, the colour of CuSO4 solution disappears and colourless zinc sulphate solution is formed. Solid brown copper is deposited as it displaces byZn.$\ce{Zn(s) + CuSO4(aq)-> ZnSO4(aq) + Cu(s)}$(b) Aluminium displaces hydrogen from HCl and hydrogen gas is evolved with soluble aluminium chloride.$\ce{2Al(s) + 6HCl(aq)->2AlCl2(aq) + 3H2(g)}$(c) Silver is a Nobel metal and does not exhibit any reaction with copper sulphate. Question. 42. What happens when zinc granules are treated with dilute solution of$\ce{H2SO4, HCl, HNO3, NaCl and NaOH,}$also write the chemical equations, if reaction occurs? Answer. Dilute$\ce{H2SO4}$: Zn reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas.$ Zn(s) + H_{2}SO_{4}(aq)\rightarrow ZnSO_{4} + H_{2}\uparrow$Dilute HCl: Zn reacts with HCl and form zinc chloride with evolution of hydrogen gas.$\ce{Zn(s) + 2HCl(aq)-> ZnCl2(aq) + H2}$Dilute HNO3: Zinc reacts with cold and dilute nitric acid and form zinc nitrate, water and nitric oxide.$\ce{ 3Zn(s) + 8 HNO3 (aq) -> 3Zn (NO3)_{2}(aq) + 4H2O (l) + 2NO (g)}$NaCI solution: No reaction will take place as sodium is more reactive than Zn and cannot replace by it.$Zn (s) + NaCl (aq) \rightarrow No reaction $NaOH solution: Zinc reacts with NaOH solution and form sodium tetrahydroxidozincate and hydrogen gas.$Zn + 2 NaOH + 2 H_{2}O\rightarrow Na_{2}[Zn(OH)_{4}] + H_{2}$Question. 43 .On adding a drop of barium chloride solution to an aqueous solution of sodium sulphite, white precipitate is obtained. (a) Write a balanced chemical equation of the reaction involved. (b) What other name can be given to this precipitation reaction? (c) On adding dilute hydrochloric acid to the reaction mixture, white precipitate disappears. Why? Answer. (a)$\ce{Na2SO3 (aq) + BaCl2 (aq) -> BaSO3(s) + 2NaCl (aq)}$(b) Double displacement reaction (c) Addition of HCl dissolves the white precipitate of$BaSO_{3}$and form soluble$BaCl_{2}$.$\ce{BaSO_3 (s) + 2HCl (aq) ->BaCl2 (aq) + H2O (l) + SO2 (g)}$Question. 44. You are provided with two containers made up of copper and aluminium. You are also provided with solutions of dilute HCl, dilute$HNO_{3}$,$ ZnCl_{2} and H_{2}O$. In which of the containers these solutions can be kept? Answer. (i) Reactions of copper with (a) Dilute HCI: Copper exhibits no reaction with HCl so it can store in copper container. (b) Dilute$HNO_{3}$: With dilute nitric acid, copper metal forms copper nitrate with nitric oxide and water so dilute nitric acid cannot store in copper container.$\ce{Cu(s) + 8HNO3 (dil) -> 3Cu(NO3)2 (aq) + 2NO (g) + 4H2O(l)}$(c)$ZnCl_{2}$: Since Cu metal is less reactive than Zn so it cannot displace Zn from its compounds and there will be no reaction. Hence zinc chloride can store in copper container. (d)$H_{2}O$: There is no reaction between water and copper metal at room temperature so water can store in copper container. (ii) Reactions of aluminium with (a) Dilute HCI: Aluminium reacts with Al reacts with dilute HCI and form aluminium chloride with hydrogen gas so it cannot be store in aluminium container.$\ce{2Al + 6HCl -> 2AlCl3+ 3H2}$(b) Dilute$HNO_3$: Nitric acid is an oxidizing agent and in the presence of it, aluminium metal forms a protective layer of oxide so it will not react further. Hence dilute nitric acid can store in aluminium container. (c)$ZnCl_2$: Aluminium readily reacts with zinc chloride and forms aluminium chloride with zinc metal.$\ce{2Al + 3ZnCl2 -> 2AlCl3 + 3Zn}$Hence it cannot store in zinc container. (d)$H_{2}O$: At room temperature, there will be no reaction between aluminium metal and water so it can store in aluminium container. Answer: Reaction of Mg with oxygen from MgO:$\ce{2Mg(s) + O2-> 1MgO +Light }$Reaction of Mg with atmospheric nitrogen from$Mg_{3}N_{2}\ce{3Mg(s) + N(2)->Mg3N2(s)}$(a) The chemical formula of$\ce{X = MgO: Y=Mg3N2}$(b)$\ce{MgO + H2O -> Mg(OH)2}\$