NCERT Exemplar Solutions
Class 10 - Science - CHAPTER 1: Chemical Reactions and Equations
Short Answer Questions

Question. 19

19. Write the balanced chemical equations for the following reactions and identify the type of reaction in each case.

(a) Nitrogen gas is treated with hydrogen gas in the presence of a catalyst at 773K to form ammonia gas.

(b) Sodium hydroxide solution is treated with acetic acid to form sodium acetate and water.

(c) Ethanol is warmed with ethanoic acid to form ethyl acetate in the presence of concentrated \(H_2SO_4\).

(d) Ethene is burnt in the presence of oxygen to form carbon dioxide, water and releases heat and light.

Answer:

(a) \(N_2 + 3H_2 \xrightarrow{Fe,773K} 2NH_3\) (Combination reaction)
(b) \(CH_3COOH + NaOH \to CH_3COONa + H_2O\) (Neutralisation reaction)
(c) \(C_2H_5OH + CH_3COOH \xrightarrow{conc.H_2SO_4} CH_3COOC_2H_5 + H_2O\) (Esterification)
(d) \(C_2H_4 + 3O_2 \to 2CO_2 + 2H_2O + \, heat\) (Combustion reaction)

Question. 20

20. Write the balanced chemical equations for the following reactions and identify the type of reaction in each case.

(a) Thermit reaction, iron (III) oxide reacts with aluminium and gives molten iron and aluminium oxide.

(b) Magnesium ribbon is burnt in an atmosphere of nitrogen gas to form solid magnesium nitride.

(c) Chlorine gas is passed in an aqueous potassium iodide solution to form potassium chloride solution and solid iodine.

(d) Ethanol is burnt in air to form carbon dioxide, water and releases heat.

Answer:

(a) \(Fe_2O_3 + 2Al \to 2Fe + Al_2O_3\) (Displacement reaction)
(b) \(3Mg + N_2 \to Mg_3N_2\) (Combination reaction)
(c) \(Cl_2 + 2KI \to 2KCl + I_2\) (Displacement reaction)
(d) \(C_2H_5OH + 3O_2 \to 2CO_2 + 3H_2O + \, heat\) (Combustion reaction)

Question. 21

21. Complete the missing components/variables given as x and y in the following reactions:

(a) \(Pb(NO_3)_2(aq) + 2KI(aq) \to PbI_2(x) + 2KNO_3(y)\)

(b) \(Cu(s) + 2AgNO_3(aq) \to Cu(NO_3)_2(aq) + x(s)\)

(c) \(Zn(s) + H_2SO_4(aq) \to ZnSO_4(x) + H_2(y)\)

(d) \(CaCO_3(s) \xrightarrow{x} CaO(s) + CO_2(g)\)

Answer:

(a) \(x = (s), \, y = (aq)\)
(b) \(x = Ag\)
(c) \(x = (aq), \, y = (g)\)
(d) \(x = Heat\)

Question. 22

22. Which among the following changes are exothermic or endothermic in nature?

(a) Decomposition of ferrous sulphate

(b) Dilution of sulphuric acid

(c) Dissolution of sodium hydroxide in water

(d) Dissolution of ammonium chloride in water

Answer:

(a) Decomposition of ferrous sulphate → Endothermic
(b) Dilution of sulphuric acid → Exothermic
(c) Dissolution of NaOH in water → Exothermic
(d) Dissolution of NH_4Cl in water → Endothermic

Question. 23

23. Identify the reducing agent in the following reactions:

(a) \(4NH_3 + 5O_2 \to 4NO + 6H_2O\)

(b) \(H_2O + F_2 \to HF + HOF\)

(c) \(Fe_2O_3 + 3CO \to 2Fe + 3CO_2\)

(d) \(2H_2 + O_2 \to 2H_2O\)

Answer:

(a) Reducing agent: \(NH_3\)
(b) Reducing agent: \(H_2O\)
(c) Reducing agent: \(CO\)
(d) Reducing agent: \(H_2\)

Question. 24

24. Identify the oxidising agent (oxidant) in the following reactions:

(a) \(Pb_3O_4 + 8HCl \rightarrow 3PbCl_2 + Cl_2 + 4H_2O\)

(b) \(2Mg + O_2 \rightarrow 2MgO\)

(c) \(CuSO_4 + Zn \rightarrow Cu + ZnSO_4\)

(d) \(V_2O_5 + 5Ca \rightarrow 2V + 5CaO\)

(e) \(3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2\)

(f) \(CuO + H_2 \rightarrow Cu + H_2O\)

Answer:

(a) \(Pb_3O_4\)   (b) \(O_2\)   (c) \(Cu^{2+}\) in \(CuSO_4\)   (d) \(V_2O_5\)   (e) \(H_2O\)   (f) \(CuO\)

Question. 25

25. Write the balanced chemical equations for the following reactions:

(a) Sodium carbonate with hydrochloric acid (equal molar) gives sodium chloride and sodium hydrogencarbonate.

(b) Sodium hydrogencarbonate with hydrochloric acid gives sodium chloride, water and carbon dioxide.

(c) Copper sulphate on treatment with potassium iodide precipitates cuprous iodide (\(Cu_2I_2\)), liberates iodine gas and also forms potassium sulphate.

Answer:

(a) \(Na_2CO_3 + HCl \rightarrow NaHCO_3 + NaCl\)
(b) \(NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2\)
(c) \(2CuSO_4 + 4KI \rightarrow Cu_2I_2\downarrow + I_2 + 2K_2SO_4\)

Question. 26

26. Potassium chloride solution mixed with silver nitrate solution gives an insoluble white substance. Write the chemical reaction and mention its type.

Answer:

\(KCl(aq) + AgNO_3(aq) \rightarrow AgCl(s)\downarrow + KNO_3(aq)\)
Type: Double displacement (precipitation) reaction.

Question. 27

27. Ferrous sulphate decomposes with evolution of a gas having the odour of burning sulphur. Write the reaction and identify the type.

Answer:

On heating: \(2FeSO_4 \xrightarrow{\Delta} Fe_2O_3 + SO_2 + SO_3\). The smell is due to \(SO_2\).
Type: Thermal decomposition.

Question. 28

Why do fireflies glow at night?

Answer:

Due to a chemiluminescent (bioluminescence) reaction—oxidation of luciferin catalysed by luciferase, producing light without appreciable heat.

Question. 29

29. Grapes on the plant do not ferment, but after plucking they can. Under what conditions do they ferment? Is it a chemical or a physical change?

Answer:

Fermentation occurs in the presence of yeast and absence of air (anaerobic conditions), converting sugars to ethanol and \(CO_2\). It is a chemical change.

Question. 30

30. Which among the following are physical or chemical changes?

(a) Evaporation of petrol   (b) Burning of LPG   (c) Heating an iron rod to red hot   (d) Curdling of milk   (e) Sublimation of solid ammonium chloride

Answer:

(a) Physical   (b) Chemical   (c) Physical   (d) Chemical   (e) Physical.

Question. 31

31. During reactions of some metals with dilute HCl the following were observed. Explain with reasons:

(a) Silver metal does not show any change.

(b) Temperature of mixture rises when aluminium is added.

(c) Sodium metal reaction is highly explosive.

(d) Bubbles are seen when lead is reacted with the acid.

Answer:

(a) Ag is below hydrogen in the reactivity series → does not displace \(H_2\) from HCl.
(b) Al reacts (after oxide film breaks) forming \(AlCl_3\) and \(H_2\); reaction is exothermic, so temperature rises.
(c) Na is extremely reactive; its reaction with acid is highly exothermic and the liberated \(H_2\) ignites.
(d) Pb is above H in the series and reacts slowly with HCl, giving \(PbCl_2\) (sparingly soluble) and \(H_2\) bubbles; product layer slows the reaction.

Question. 32

32. A substance X, an oxide of a group 2 element used in cement industry and present in bones, forms a solution turning red litmus blue on treatment with water. Identify X and write the reactions.

Answer:

X is calcium oxide, \(CaO\) (quick lime).
\(CaO + H_2O \rightarrow Ca(OH)_2\) (lime water, basic → turns red litmus blue).

Question. 33

33. Write a balanced chemical equation for each reaction and classify them.

(a) Lead acetate solution with dilute HCl → lead chloride and acetic acid.

(b) Sodium metal added to absolute ethanol → sodium ethoxide and hydrogen gas.

(c) Iron(III) oxide heated with carbon monoxide → iron and carbon dioxide.

(d) Hydrogen sulphide reacts with oxygen gas → solid sulphur and liquid water.

Answer:

(a) \((CH_3COO)_2Pb + 2HCl \rightarrow PbCl_2\downarrow + 2CH_3COOH\) — Double displacement (precipitation).
(b) \(2C_2H_5OH + 2Na \rightarrow 2C_2H_5ONa + H_2\) — Displacement/Redox.
(c) \(Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2\) — Redox (reduction of iron(III) oxide).
(d) \(2H_2S + O_2 \rightarrow 2S + 2H_2O\) — Redox (oxidation of \(H_2S\)).

Question. 34

Why is silver chloride stored in dark coloured bottles?

Answer:

\(AgCl\) undergoes photodecomposition in light: \(2AgCl \xrightarrow{light} 2Ag + Cl_2\). Dark bottles prevent light from initiating the reaction.

Question. 35

35. Balance the following equations and identify the type of reaction.

(a) \(Mg + Cl_2 \rightarrow MgCl_2\)

(b) \(HgO \xrightarrow{Heat} Hg + O_2\)

(c) \(Na + S \xrightarrow{Fuse} Na_2S\)

(d) \(TiCl_4(l) + Mg(s) \rightarrow Ti(s) + MgCl_2(s)\)

(e) \(CaO(s) + SiO_2(s) \rightarrow CaSiO_3(s)\)

(f) \(H_2O_2(l) \xrightarrow{UV} H_2O(l) + O_2(g)\)

Answer:

(a) \(Mg + Cl_2 \rightarrow MgCl_2\) — Combination.
(b) \(2HgO \xrightarrow{\Delta} 2Hg + O_2\) — Thermal decomposition.
(c) \(2Na + S \rightarrow Na_2S\) — Combination.
(d) \(TiCl_4 + 2Mg \rightarrow Ti + 2MgCl_2\) — Displacement/Redox.
(e) \(CaO + SiO_2 \rightarrow CaSiO_3\) — Combination.
(f) \(2H_2O_2 \xrightarrow{UV} 2H_2O + O_2\) — Photochemical decomposition.

Question. 36

36. A magnesium ribbon burns in oxygen to give a white compound X with light. In nitrogen it continues to burn forming Y.

(a) Write the formulae of X and Y.

(b) Write a balanced equation when X is dissolved in water.

Answer:

(a) \(X=MgO\), \(Y=Mg_3N_2\).
(b) \(MgO + H_2O \rightarrow Mg(OH)_2\).

Question. 37

Zinc liberates hydrogen with dilute HCl whereas copper does not. Explain.

Answer:

Zn is above hydrogen in the reactivity series and displaces \(H^+\) from HCl: \(Zn + 2HCl \rightarrow ZnCl_2 + H_2\). Copper is below hydrogen, so it cannot displace \(H_2\) from dilute acid.

Question. 38

38. A silver article turns black after a few days in open air and regains shine on rubbing with toothpaste.

(a) Why does it turn black? Name the phenomenon.

(b) Name the black substance formed and give its chemical formula.

Answer:

(a) Silver reacts with sulphur compounds (e.g., \(H_2S\)) in air forming a black layer—tarnishing (a kind of corrosion).
(b) The black substance is silver sulphide, \(Ag_2S\).
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