NCERT Exemplar for Class 10 Science Chapter 1 - Chemical Reactions and Equations (Book Solutions)

Multiple Choice Questions 

1. Which of the following is not a physical change? 

(a) Boiling of water to give water vapour 

(b) Melting of ice to give water 

(c) Dissolution of salt in water 

(d) Combustion of Liquefied Petroleum Gas (LPG) 

Ans: (d) Combustion of Liquefied Petroleum Gas (LPG) 

On combustion, Liquefied Petroleum Gas (LPG) reacts with oxygen to form carbon dioxide gas and water vapour. The chemical composition of hydrocarbons of Liquefied Petroleum Gas (LPG) changes to carbon dioxide and water so it is a chemical change.

In boiling of water to give water vapour, melting of ice to give water  and dissolution of salt in water, chemical composition remains the same only physical state changes hence, these undergo physical change. 

2. The following reaction is an example of a 

4 NH3(g) + 5 O2(g)   \[\to \]  4 NO(g) + 6 H2O(g) 

(i) displacement reaction 

(ii) combination reaction 

(iii) redox reaction 

(iv) neutralisation reaction 

(a) (i) and (iv) 

(b) (ii) and (iii) 

(c) (i) and (iii) 

(d) (iii) and (iv) 

Ans: (c) (i) and (iii) 

The given reaction undergoes displacement reaction and neutralisation reaction.

Displacement reaction: The oxygen displaces hydrogen from ammonia to form nitric acid and water.

Redox reaction: Ammonia combines with oxygen atom to undergo oxidation reaction whereas oxygen combines with hydrogen to undergo reduction.

3. Which of the following statements about the given reaction are correct? 

3 Fe(s) + 4 H2O(g)  \[\to \]  Fe3O4(s) + 4 H2(g) 

(i) Iron metal is getting oxidised 

(ii) Water is getting reduced 

(iii) Water is acting as reducing agent 

(iv)  Water is acting as oxidising agent 

(a) (i), (ii) and (iii) 

(b) (iii) and (iv) 

(c) (i), (ii) and (iv) 

(d) (ii) and (iv) 

Ans:  c. (i), (ii) and (iv) 

Fe3O4 contains two oxide of iron as FeO and Fe2O3 so the oxidation number of iron in Fe3O4 is  FeIIO  FeIII2O3. 

In the given reaction: 3 Fe(s) + 4 H2O(g)  \[\to \]  Fe3O4(s) + 4 H2(g) 

(a) Iron metal  is oxidised to +2 and +3 so iron metal  is getting oxidised.

(b) Water is reduced to hydrogen by removal of oxygen atom and also oxidation  changes from -2 to 0.

(c) Water oxidises the iron to iron (II,III) oxide so it acts as an oxidising agent.

4. Which of the following are exothermic processes? 

(i) Reaction of water with quicklime 

(ii) Dilution of an acid 

(iii) Evaporation of water 

(iv) Sublimation of camphor (crystals) 

(a) (i) and (ii) 

(b) (ii) and (iii) 

(c) (i) and (iv) 

(d) (iii) and (iv) 

Ans: a. (i) and (ii) 

The reaction of water with quicklime forms slaked lime with the evolution of large amounts of heat. So it is an exothermic reaction.  

Similarly, when water is added to acid on dilution, an excessive amount of heat is evolved. It is also an exothermic reaction.

5. Three beakers labelled as A, B and C each containing 25 mL of water were taken. A small amount of NaOH, anhydrous CuSO4 and NaCl were added to the beakers A, B and C respectively. It was observed that there was an increase in the temperature of the solutions contained in beakers A and B, whereas in case of beaker C, the temperature of the solution falls. Which one of the following statement(s) is(are) correct? 

(i) In beakers A and B, exothermic process has occurred. 

(ii) In beakers A and B, endothermic process has occurred. 

(iii) In beaker C exothermic process has occurred. 

(iv) In beaker C endothermic process has occurred.

(a) (i) only 

(b) (ii) only

(c) (i) and (iv) 

(d) (ii) and (iii) 

Ans: c. (i) and (iv) 

Exothermic reaction: An exothermic reaction is a reaction in which energy is released in the form of heat or light and  energy in the form of heat is transferred into the surroundings. Therefore, the temperature increases when an exothermic reaction takes place.

Example: Combustion, respiration,  dilution of acid, burning of wood, addition of NaOH and in anhydrous CuSO4 water etc.

Endothermic reaction: An endothermic reaction is a reaction in which energy is absorbed in the form of heat or light and energy in the form of heat is transferred from the surroundings to the system. 

Example: Melting of ice, photosynthesis, dissolving salt in water etc.

So, in beakers A and B, exothermic process has occurred and in beaker C endothermic process has occurred. 

6. A dilute ferrous sulphate solution was gradually added to the beaker containing acidified permanganate solution. The light purple colour of the solution fades and finally disappears. Which of the following is the correct explanation for the observation? 

(a) KMnO4 is an oxidising agent, it oxidises FeSO4 

(b) FeSO4 acts as an oxidising agent and oxidises KMnO4

(c) The colour disappears due to dilution; no reaction is involved 

(d) KMnO4 is an unstable compound and decomposes in presence of FeSO4 to a colourless compound. 

Ans: a. KMnO4 is an oxidising agent, it oxidises FeSO4 

The chemical reaction that takes place for the given  reaction is:

FeIISO4   +  KMnO4  + H2SO4  \[\to \]  FeIII2(SO4)3 +  MnSO4  +   K2SO4  + H2O

KMnO4 acts as an oxidizing agent and oxidises iron from +2 oxidation state to +3 oxidation state. 

7. Which among the following is(are) double displacement reaction(s)? 

(i) Pb   +   CuCl2  \[\to \] PbCl2   +   Cu   

(ii) Na2SO4  +  BaCl2  \[\to \] BaSO4  +  2 NaCl 

(iii) C + O2  \[\to \]  CO2 

(iv) CH4 + 2 O2   \[\to \]  CO2  +  2 H2O 

(a) (i) and (iv) 

(b) (ii) only

(c) (i) and (ii) 

(d) (iii) and (iv)

Ans: b. (ii) only

In a double displacement reaction, one compound exchanges its ions with  the  ions of other compounds to form two new compounds.

In Na2SO4, Na+ ion combines with Cl- ion of BaCl2 whereas Ba+ ion combines with  SO42-ion to form BaSO4.

8. Which among the following statement(s) is(are) true? Exposure of silver chloride to sunlight for a long duration turns grey due to 

(i) the formation of silver by decomposition of silver chloride 

(ii) sublimation of silver chloride 

(iii) decomposition of chlorine gas from silver chloride 

(iv) oxidation of silver chloride 

(a) (i) only 

(b) (i) and (iii) 

(c) (ii) and (iii) 

(d) (iv) only 

Ans: (i) only

Silver chloride on exposure to sunlight for a long duration forms grey silver by decomposition of silver chloride.

            2 AgCl   $\xrightarrow{Sunlight}$    2 Ag   +   Cl2

            Silver Chloride          Silver(Grey)

9. Solid calcium oxide reacts vigorously with water to form calcium hydroxide accompanied by liberation of heat. This process is called slaking of lime. Calcium hydroxide dissolves in water to form its solution called lime water. Which among the following is (are) true about slaking of lime and the solution formed? 

(i) It is an endothermic reaction 

(ii) It is an exothermic reaction 

(iii) The pH of the resulting solution will be more than seven 

(iv) The pH of the resulting solution will be less than seven 

(a) (i) and (ii) 

(b) (ii) and (iii) 

(c) (i) and (iv) 

(d) (iii) and (iv) 

Ans: b. (ii) and (iii) 

When solid calcium oxide (commonly known as quicklime) is added to water,  it reacts vigorously to  form calcium hydroxide (commonly known as slaked lime) accompanied by liberation of heat. Calcium hydroxide is an alkali and hence, the pH of the solution will be more than seven. Due to liberation of heat, the reaction is an exothermic reaction. 

Slaking of lime:

                          CaO   +   H2O   \[\to \]    Ca(OH)2   +    Heat 

                       Calcium oxide                    Calcium Hydroxide

                           (Quicklime)                         (Slaked lime)

10. Barium chloride on reacting with ammonium sulphate forms barium sulphate and ammonium chloride. Which of the following correctly represents the type of the reaction involved? 

(i) Displacement reaction 

(ii) Precipitation reaction

(iii) Combination reaction 

(iv) Double displacement reaction 

(a) (i) only 

(b) (ii) only 

(c) (iv) only 

(d) (ii) and (iv) 

Ans: (d) (ii) and (iv) 

When you react, sodium sulphate  and barium chloride, double displacement reaction as the  Na+ ion combines with Cl- ion of BaCl2 whereas Ba+ ion combines with  SO42-ion to form BaSO4. 

The white precipitate of barium sulphate and aqueous solution of salt of sodium chloride are formed so the reaction is also a precipitation reaction.

Na2SO4  +  BaCl2  \[\to \] BaSO4 \[\downarrow\]  +    2 NaCl 

                                      White precipitate

In a double displacement reaction, one compound exchanges its ions with  the  ions of other compounds to form two new compounds.

Example: AB + CD    AD + CB

In a precipitate reaction, a precipitate of the insoluble compound is formed in the reaction when two or mole soluble salts combine.

11. Electrolysis of water is a decomposition reaction. The mole ratio of hydrogen and oxygen gases liberated during electrolysis of water is 

(a) 1:1 

(b) 2:1 

(c) 4:1 

(d) 1:2 

Ans: b. 2:1 

On electrolysis of water, water dissociates to liberate hydrogen and oxygen gas.

2 H2O  \[\to \]  2 H2(g)  +  O2(g)

2 moles of water dissociate to form 2 moles of hydrogen gas and 1 mole of oxygen gas on electrolysis of water. 

12. Which of the following is(are) an endothermic process(es)? 

(i) Dilution of sulphuric acid 

(ii) Sublimation of dry ice 

(iii) Condensation of water vapours 

(iv) Evaporation of water 

(a) (i) and (iii) 

(b) (ii) only 

(c) (iii) only 

(d) (ii) and (iv) 

Ans: d. (ii) and (iv) 

In an endothermic reaction,  the energy is absorbed in the form of heat or light. Also the energy in the form of heat is transferred from the surroundings to the system. 

Sublimation of dry ice, evaporation of water, melting of ice, photosynthesis, dissolving salt in water,  etc. are endothermic reactions as the energy in the form of heat is required for these processes.

Sublimation Reaction:                   Solid  $\xrightarrow{Heat}$   Gas

Evaporation Reaction:                   Liquid  $\xrightarrow{Heat}$  Gas

13. In the double displacement reaction between aqueous potassium iodide and aqueous lead nitrate, a yellow precipitate of lead iodide is formed. While performing the activity if lead nitrate is not available, which of the following can be used in place of lead nitrate? 

(a) Lead sulphate (insoluble) 

(b) Lead acetate 

(c) Ammonium nitrate 

(d) Potassium sulphate 

Ans: b. Lead acetate

When the  reaction between aqueous potassium iodide and aqueous lead nitrate,  double displacement  reaction takes place a yellow precipitate of lead iodide is formed.

2 KI (aq)    +   Pb(NO3)2(aq) \[\to \]  2 PbI2(s)  +    2 KNO3(aq)

Yellow ppt.

Lead acetate can be used when lead nitrate is not available to obtain the yellow precipitate of lead Iodide and following  reaction will occur:

2 KI (aq)    +   Pb(CH3COO)2(aq) \[\to \] 2 PbI2(s)  +    2 CH3COOK(aq)

Yellow ppt.

14. Which of the following gases can be used for storage of fresh samples of an oil for a long time? 

(a) Carbon dioxide or oxygen 

(b) Nitrogen or oxygen 

(c) Carbon dioxide or helium 

(d) Helium or nitrogen 

Ans: d. Helium or nitrogen 

Helium is a noble gas, does not react with fats and oil and protects oil from its oxidation or rancidity. Nitrogen gas has triple bond between two nitrogen atoms due to the presence of triple bond nitrogen also acts as noble gas and does not react with fats and oils and also their rancidity.

15. The following reaction is used for the preparation of oxygen gas in the laboratory 

2 KClO3 (s) \[\to \] 2 KCl (s) + 3O2 (g) 

Which of the following statement(s) is(are) correct about the reaction? 

(a) It is a decomposition reaction and endothermic in nature 

(b) It is a combination reaction 

(c) It is a decomposition reaction and accompanied by release of heat 

(d) It is a photochemical decomposition reaction and exothermic in nature 

Ans: a. It is a decomposition reaction and endothermic in nature 

The given reaction is a thermal decomposition reaction, as the KClO3 decomposes to KCl and O2 on heating.

In the given reaction, the heat is supplied to the reaction mixture, so it is also an endothermic reaction.

16. Which one of the following processes involve chemical reactions? 

(a) Storing of oxygen gas under pressure in a gas cylinder 

(b) Liquefaction of air 

(c) Keeping petrol in a china dish in the open 

(d) Heating copper wire in presence of air at high temperature 

Ans: d. Heating copper wire in presence of air at high temperature 

Copper metal  reacts with oxygen when heated at high temperature  to form black color copper Oxide as shown in the reaction below:

2 Cu   +   O2  \[\to \]   2 CuO

The chemical composition is changed during the course of reaction, so it involves a chemical change. 

17. In which of the following chemical equations, the abbreviations represent the correct states of the reactants and products involved at reaction temperature? 

a. 2 H2(l)    +    O2(l)  \[\to \] 2 H2O(g)  

b. 2 H2(g)   +    O2(l)  \[\to \] 2 H2O(l) 

c. 2 H2(g)   +    O2(g)  \[\to \] 2 H2O(l)   

d. 2 H2(g)   +    O2(g)  \[\to \] 2 H2O(g)   

Ans: c. 2 H2(g)   +    O2(g)  \[\to \] 2 H2O(l)   

The hydrogen and oxygen both in gaseous physical state react, for the formation of water in liquid state.

18. Which of the following are combination reactions? 

(i) 2 KClO3 (s) $\xrightarrow{Heat}$ 2 KCl (s) + 3O2 (g)  

(ii) MgO    +     H2O    \[\to \]  Mg(OH)2

(iii) 4 Al       +     3 O2    \[\to \] 2 Al2O3 

(iv) Zn        +     FeSO4   \[\to \]  ZnSO4   +   Fe 

(a) (i) and (iii) 

(b) (iii) and (iv) 

(c) (ii) and (iv) 

(d) (ii) and (iii) 

Ans: d. (ii) and (iii)

The reactions (ii) and (iii) are the combination reaction as the reactants combine to form the single product.

Short Answer Questions 

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. 

Ans:  2 N2 (g) + 3 H2(g) $\xrightarrow{773K}$ 2 NH3 (g) 

Nitrogen gas and hydrogen gas combines at 773K to form ammonia gas by Haber process, is a combination reaction as a single product is formed from two reactants.

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

Ans:   NaOH(aq)   +   CH3COOH(aq) \[\to \]   CH3COONa (aq)  +  H2O(aq)

Sodium hydroxide is an alkali which reacts with acetic acid to form sodium acetate salt  and water. 

This reaction is a neutralisation reaction and double displacement reaction also.

(c) Ethanol is warmed with ethanoic acid to form ethyl acetate in the presence of concentrated H2SO4 . 

Ans:   C2H5OH (aq) +  CH3COOH (aq) $\xrightarrow{{{H}_{2}}S{{O}_{4}}}$  CH3COOC2H5(aq) + H2O

Ethanol undergoes esterification on reacting with ethanoic acid to form ester ethyl acetate and water is released. This reaction is an esterification reaction.

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

Ans:  C2H4(g) +  3 O2 (g) $\xrightarrow{{{H}_{2}}S{{O}_{4}}}$  2 CO2(g) + 2 H2O(g) + heat + light

Ethene undergoes combustion when burnt in presence of oxygen to form carbon dioxide gas and water vapour and energy is released in the form of heat and light.

This reaction is a combustion reaction and also an exothermic reaction.

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. 

Ans:  2 Al(s)  +   Fe2O3 (s)   $\xrightarrow{{{H}_{2}}S{{O}_{4}}}$    Al2O3(s)  +   Fe(l)   +   heat 

In thermite reaction, aluminium metal reacts with iron(II) oxide, aluminium being more reactive than iron displaces iron from iron oxide. The reaction is an exothermic reaction as a large amount of heat is evolved due to which iron is produced in molten state.

This is a displacement, exothermic and redox reaction.

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

Ans:    3 Mg(s)   +   N2(g)   \[\to \]   Mg3N2(s)

The magnesium ribbon combines with nitrogen to form a single product magnesium nitride. This reaction is a combination reaction.

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

Ans:     2 KI(s)   +   Cl2(g)   \[\to \]   2 KCl (aq)  +  I2 (s)

Chlorine gas reacts with potassium iodide in which chlorine being more reactive displaces iodine from potassium iodide to form potassium chloride. 

This reaction is a displacement reaction.

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

Ans:    C2H5OH(l)   +  3O2(g)   \[\to \]  2CO2(g)    +  3H2O(l)  +    Heat

Ethanol when burnt in air undergoes combustion and reacts with oxygen to form carbon dioxide gas and water with the evolution of heat. 

This is a combustion reaction and exothermic reaction.

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

(a) Pb(NO3)2 (aq)   +   2 KI(aq)  \[\to \]   PbI2 (x)   +   2 KNO3(y) 

Ans: Pb(NO3)2 (aq)   +   2 KI(aq)   \[\to \]   PbI2 (s)   +   2 KNO3(aq) 

When lead nitrate reacts with aqueous potassium iodide, it undergoes precipitation reaction and yellow precipitate of lead iodide is produced in aqueous solution of potassium nitrate due to displacement of nitrate by iodine.

(b) Cu(s)   +   2  AgNO3(aq) \[\to \]   Cu(NO3)2(aq)   +   x(s) 

Ans:   Cu(s)   +   2  AgNO3(aq)  \[\to \]   Cu(NO3)2(aq)   +   2 Ag(s) 

Copper metal is more reactive than silver and displaces silver from silver nitrate aqueous solution and forms copper nitrate aqueous solution and silver metal.

(c) Zn(s)   +   H2SO4(aq)    \[\to \]   ZnSO4(x)  +   H2(y) 

Ans:  Zn(s)   +   H2SO4(aq)   \[\to \]   ZnSO4(aq)  +   H2(g) 

When zinc metal is added to the sulphuric acid, hydrogen gas is evolved and the aqueous solution of zinc sulphate is obtained.

(d) CaCO3(s) $\xrightarrow{X}$  CaO(s) + CO2(g) 

Ans:   CaCO3(s) $\xrightarrow{Heat}$  CaO(s) + CO2(g) 

Calcium carbonate undergoes decomposition reaction on heating to form calcium oxide and carbon dioxide gas.

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

(a) Decomposition of ferrous sulphate 

Ans: Endothermic reaction

2 FeSO4(s)  $\xrightarrow{\Delta }$   Fe2O3(s)  +   SO2(g) +   SO3(g) 

For decomposition of ferrous sulphate, heat is supplied to it  to form iron(III) oxide, sulphur dioxide gas  and sulphur trioxide gas.

(b) Dilution of sulphuric acid 

Ans:  Exothermic reaction

Dilution of acid is an exothermic reaction and sometimes the reaction mixture results in boiling. Sulphuric acid is a strong acid that has a great affinity for water and liberates large amounts of heat when diluted.

(c) Dissolution of sodium hydroxide in water 

Ans: Exothermic reaction

Sodium hydroxide is a strong base when added to water, the reaction mixture container becomes hot. It is advised to mix the NaOH in water carefully as the reaction is an exothermic reaction, 

(d) Dissolution of ammonium chloride in water 

Ans:  Endothermic reaction

Ammonium chloride in water absorbs the  heat from the surroundings and  forms the product, the temperature of the reaction mixture is lower and hence, it is an endothermic reaction.

23. Identify the reducing agent in the following reactions 

(a) 4 NH3  +  \[\text{5O}_{\text{2}}^{\text{0}}\to \] 4 NO   +   6 H2O-2

Ans: Ammonia 

Ammonia reduces the oxygen to water and the oxidation of oxygen changes from 0 to -2.

(b) H2O  +  \[\text{F}_{\text{2}}^{\text{0}}\to \]   HF-1   +   HOF 

Ans: H2O

Water (H2O) reduces the fluorine to HF and the oxidation state changes from 0 to -1.

(c) Fe2+3O3   +  3 CO  \[\to \]  2 Fe0    +   3 CO2 

Ans: CO

Carbon monoxide (CO) reduced iron (III) oxide to iron metal as the oxidation of iron (III) oxide changes from +3 to 0.

(d) 2 H2   + \[\text{O}_{\text{2}}^{\text{0}}\to \]   2H2O-2 

Ans: H2

Hydrogen (H2) gas reduces the oxygen gas to water and acts as a reducing agent, changing the oxidation state of oxygen from 0 to -2

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

(a) Pb3O4   +   8 HCl-1   \[\to \]  3 PbCl2  +   Cl20   +   4 H2O

Ans: Pb3O4 

Pb3O4  acts as an oxidant and oxidises hydrochloric acid to chlorine gas and changes the oxidation number of chloride ion to chlorine from -1 to 0.

(b) 2 Mg   +   O2  \[\to \]  2 MgO 

Ans: O2

Oxygen gas acts as oxidant and oxidises magnesium metal to magnesium oxide and changes the oxidation state of magnesium from +2 to 0.

(c) CuSO4   +    Zn0 \[\to \]  Cu   +   Zn+2SO4 

Ans:  CuSO4

Copper sulphate oxidises the zinc metal to zinc sulphate and changes its oxidation number from 0 to +2 by losing  electrons and hence CuSO4 acts as oxidant.

(d) V2O5   +   5 Ca0 \[\to \] 2 V + 5 Ca+2O

Ans: V2O5 is an oxidant and oxidises the calcium metal to calcium oxide and changes its oxidation state from 0 to +2.

(e) 3 Fe0   +   4 H2O   \[\to \]   Fe3O4   +   4 H2 

Ans: H,2O

Water (H2O) is an oxidising agent in the given reaction  and oxidises iron metal to iron (II,III) oxide and evolves hydrogen gas. 

(f) CuO   +   H20 \[\rightarrow\]   Cu    +   4  H+12O 

Ans: CuO

Copper oxide oxidises hydrogen gas to water and gets reduced to copper metal. The oxidation of each hydrogen atom changes from 0 to +1.

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

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

Ans: Na2CO3 + HCl \[\to \] NaCl + NaHCO3

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

Ans: NaHCO3+HCl \[\rightarrow\] NaCl+ H2O + CO2

(c) Copper sulphate on treatment with potassium iodide precipitates cuprous iodide (Cu2I2), liberates iodine gas and also forms potassium sulphate. 

Ans:   2  CuSO4 + 4 KI \[\rightarrow\] Cu2I2  + I2 + 2 K2SO4

26. A solution of potassium chloride when mixed with silver nitrate solution, an insoluble white substance is formed. Write the chemical reaction involved and also mention the type of the chemical reaction? 

Ans: When a solution of potassium chloride is mixed with silver nitrate solution, an insoluble  white precipitate of silver chloride is formed.

KCl (s)    +     AgNO3(aq)   \[\rightarrow\]  AgCl (s) + KNO3(aq)

The type of  reaction are: 

Double displacement reaction: In the given reaction, both the compounds exchange their ions to form two new compounds.

Precipitation reaction: The given reaction, forms an insoluble white precipitate of silver chloride, so it is a precipitation reaction also.

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

Ans: 2 FeSO4(s)  \[\xrightarrow{\Delta }\]   Fe2O3(s)  +   SO2(g) +   SO3(g) 

Ferrous sulphate undergoes decomposition reaction on heating to form ferric oxide and evolves sulphur dioxide gas and sulphur trioxide gas which have characteristic odour of burning sulphur.

The given reaction is a thermal decomposition reaction as well as endothermic reaction.

28. Why do fireflies glow at night? 

Ans: Fireflies glow at night due to chemical reactions taking place inside their body to emit light in the form of energy.

Fireflies store a luciferin protein, which is an organic compound. The oxygen  combines with calcium, adenosine triphosphate (ATP) and the chemical luciferin in the presence of luciferase, a bioluminescent enzyme to form a new substance (oxyluciferin) along with the evolution of energy in the form of light.   This type of light production is called bioluminescence.

29. Grapes hanging on the plant do not ferment but after being plucked from the plant can be fermented. Under what conditions do these grapes ferment? Is it a chemical or a physical change? 

Ans: Grapes when hanging on the plant, do not ferment due to their active immune system. After grapes are plucked from the plant, it  ferments as  microbes start acting on the sugar present in grapes and cause fermentation of them. Due to fermentation the sugar in grapes is converted into ethanol and carbon dioxide. As the chemical composition of sugar in grapes changes, it is a chemical change.

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

(a) Evaporation of petrol 

Ans: Physical changes

The physical state changes from liquid state to gaseous of the petrol, hence its a physical change.

(b) Burning of Liquefied Petroleum Gas (LPG) 

Ans: Chemical changes

Liquefied Petroleum Gas (LPG) is a mixture of hydrocarbons that  undergoes combustion on burning and forms carbon dioxide gas and water and releases large amounts of heat energy.

(c) Heating of an iron rod to red hot. 

Ans: Physical changes

The iron rod is made of iron metal and remains as iron even if heated to red hot so no chemical change is taking place hence this is a physical change.

(d) Curdling of milk 

Ans: Chemical changes

Milk contains casein protein and this protein reacts with the lactic acid when small amount of curd is added to milk. Lactic acid breaks the chemical structures/bonds of the milk and forms new bonds thus giving rise to a semi-solid form of milk called curd.

We saw in the entire process that the reaction between lactic acid and milk involved breaking of bonds and forming of new bonds. Therefore, there is a chemical change associated with it and hence this change is a chemical change.

Hence, curdling milk is a chemical change.

(e) Sublimation of solid ammonium chloride 

Ans: Physical change

Sublimation is the property of converting a  substance in solid  state into vapour state directly or vice versa. Ammonium chloride in solid state is sublimed without changing its chemical composition , so it is a physical change.

31. During the reaction of some metals with dilute hydrochloric acid, following observations were made. 

(a) Silver metal does not show any change 

Ans: Silver metal is placed below hydrogen in the reactivity series, so it is less reactive to displace hydrogen from HCl and hence silver metal does not show any change when reacted with dilute hydrochloric acid.

(b) The temperature of the reaction mixture rises when aluminium (Al) is added. 

Ans:  The reaction of aluminium metal  with dilute hydrochloric acid is exothermic and it is advised that the reaction should be carried out carefully as the hydrogen gas is also evolved and can splash the hot reaction mixture.

(c) The reaction of sodium metal is found to be highly explosive 

Ans: Sodium is a highly reactive metal as it is placed at the top in the reactivity series. When  sodium metal reacts with dilute HCl, it evolves a large  amount of heat along with hydrogen gas  which catches fire  and hence is found to be  highly explosive.

(d) Some bubbles of a gas are seen when lead (Pb) is reacted with the acid. Explain these observations giving suitable reasons. 

Ans: When  lead (Pb) is reacted with the dilute hydrochloric acid, bubbles of hydrogen gas are observed as lead is placed above hydrogen in reactivity series as lead is more reactive to displace hydrogen from HCl acid.

32. A substance X. which is an oxide of a group 2 element, is used intensively in the cement industry. This element is present in bones also. On treatment with water it forms a solution which turns red litmus blue. Identify X and also write the chemical reactions involved. 

Ans: Calcium is an element of group 2 and is present in bones also. Calcium oxide, commonly known as quicklime, is used intensively in the cement industry. Calcium on treatment with water forms alkali calcium hydroxide which turns the red litmus blue.

CaO(s)  +  H2O(l) \[\to \] Ca(OH)2 (aq)

33. Write a balanced chemical equation for each of the following reactions and also classify them. 

(a) Lead acetate solution is treated with dilute hydrochloric acid to form lead chloride and acetic acid solution. 

Ans: (CH3COO)2Pb   +  2 HCl \[\to \]    PbCl2   +   2 CH3COOH (double displacement reaction + Precipitation reaction)

(b) A piece of sodium metal is added to absolute ethanol to form sodium ethoxide and hydrogen gas. 

Ans: 2 Na      +      C2H5OH     \[\to \]   2 C2H5ONa   +  H2 (Displacement Reaction)

(c) Iron (III) oxide on heating with carbon monoxide gas reacts to form solid iron and liberates carbon dioxide gas. 

Ans: Fe2O3 + CO \[\to \] 2 Fe +3 CO2          (Redox    Reaction)

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

Ans: H2S + O2  \[\to \] 2 S + 2 H2O     (Redox reaction)

34. Why do we store silver chloride in dark coloured bottles? 

Ans: Silver chloride undergoes decomposition reaction in presence of sunlight to form silver and evolve chlorine gas. 

2 AgCl    \[\xrightarrow{\text{Sunlight}}\]   2 Ag   +   Cl2

To prevent the decomposition of silver chloride by sunlight, it is stored in dark coloured bottles.

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

(a) Mg(s) + Cl2(g)  \[\to \]  MgCl2(s) 

Ans:  Mg(s) + Cl2(g)  \[\to \] MgCl2(s)      

This is a combination reaction.

(b) HgO(s)    \[\xrightarrow{\text{Heat}}\]  Hg(l) + O2(g) 

Ans:   2 HgO(s)   \[\xrightarrow{\text{Heat}}\]   2 Hg(l) + O2(g) 

This is a thermal decomposition reaction.

(c) Na(s) + S(s)  \[\xrightarrow{\text{Fuse}}\] Na2S(s) 

Ans:  2 Na(s) + S(s) \[\xrightarrow{\text{Fuse}}\] Na2S(s)    

This is combination reaction

(d) TiCl4 (l)  +  Mg (s)   \[\to \]   Ti(s)  +  MgC12(s) 

Ans:  TiCl4 (l)  +  2 Mg (s)   \[\to \]    Ti(s)  +  2 MgC12(s) 

This is a displacement reaction

(e)  CaO(s)   +   SiO2(s)   \[\to \]   CaSiO3(s) 

Ans:  CaO(s)   +   SiO2(s)   \[\to \]  CaSiO3(s) 

This is a combination reaction

(f) H2O2(1)     \[\xrightarrow{\text{UV}}\]    H2O(1)   +   O2(g) 

Ans: 2  H2O2(1)     \[\xrightarrow{\text{UV}}\]    2 H2O(1)   +   O2(g) 

This is a photo-decomposition reaction.

36. A magnesium ribbon is burnt in oxygen to give a white compound X accompanied by emission of light. If the burning ribbon is now placed in an atmosphere of nitrogen, it continues to burn and forms a compound Y. 

(a) Write the chemical formulae of X and Y. 

Ans: Magnesium  reacts with oxygen when burnt in air to form white anhydrous  magnesium oxide (compound X) accompanied by emission of light. 

                                Mg(s)     +     O2(g)  \[\to \]  MgO(s)

                           Magnesium                            Magnesium Oxide

When magnesium is burnt in the nitrogen atmosphere, it forms magnesium nitride (compound Y), which continues to burn.

                             3 Mg(s)     +     N2(g)     \[\to \]  Mg3N2(s)

                           Magnesium                            Magnesium Nitride

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

When magnesium oxide (compound X) is dissolved in water, alkali solution of magnesium hydroxide is formed which turns red litmus blue.

                                MgO(s)     +     H2O(g)     \[\to \]  Mg(OH)2(s)

                           Magnesium Oxide                           Magnesium Hydroxide

37. Zinc liberates hydrogen gas when reacted with dilute hydrochloric acid, whereas copper does not. Explain why? 

Ans: Zinc metal is placed above hydrogen in the reactivity series and hence more reactive to  displace hydrogen from dilute hydrochloric acid and liberate hydrogen gas. 

                                Zn(s)     +     HCl(aq)     \[\to \]     ZnCl2 (aq)    +     H2(g)

                                 Zinc                                                                             Hydrogen

Copper metal is placed below hydrogen in the reactivity series and hence less reactive to  displace hydrogen from dilute hydrochloric acid and  no reaction takes place. 

                                Cu(s)     +     HCl(aq)     \[\to \]     No reaction

                               Copper                                                                           

38. A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining. 

(a) Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved. 

Ans: Silver metal articles  when kept in open air for a few days, react with sulphur compounds present in the atmosphere and turn black  due to the formation of  Ag2S. This phenomenon is corrosion.

                                2 Ag(s)     +     H2S(g)        \[\to \]           Ag2S(s)    +     H2(g)

                                 Silver         Hydrogen sulphide              Silver Sulphide                                                             

Toothpaste contains calcium carbonate, aluminium hydroxide which can remove the black layer of silver sulphide and silver shines again.

                              3 Ag2S      +     2 Al        \[\to \]         6 Ag        +         Al2S3

              Silver Sulphide     Aluminium                     Silver           Aluminium Sulphide                

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

Ans: The black substance is silver sulphide and has a chemical formula as H2S.

Long Answer Questions 

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. 

Ans: The copper (II) nitrate decomposes on heating to form black copper oxide and oxygen gas is liberated along with brown nitrogen gas.

           2 Cu(NO3)2(s)   \[\xrightarrow{\text{Heat}}\]   2 CuO(s)     +    4 NO2   +     O2   

      Copper Nitrate                                                   Copper Oxide      Nitrogen Dioxide 

            (blue)                                                                 (black)                   (brown)

(b) Identify the brown gas X evolved. 

Ans: The brown gas X is nitrogen dioxide (NO2).

(c) Identify the type of reaction. 

Ans: The type of reaction is thermal  decomposition reaction as the single reactant decomposes on heating to give  three products.

(d) What could be the pH range of aqueous solution of the gas X? 

Ans: The pH of the oxides of non-metal is acidic in nature, nitrogen is a non metal so nitrogen dioxide gas which  is  gas  X is acidic and the range of aqueous solution of nitrogen dioxide is 0-7.

40. Give the characteristic tests for the following gases:

(a) CO2 

Ans: Carbon dioxide gas shows the characteristic of turning limewater milky due to formation of insoluble precipitate of calcium carbonate. 

                    Ca(OH)2(aq)    +     CO2(g)       \[\to \]          CaCO3(s)    +     H2O(g)

                        Limewater           Carbon Dioxide               Calcium Carbonate  

(b) SO2 

Ans: Sulphur dioxide gas turns the purple color acidic solution of potassium permanganate colourless. The SO2 acts as a reducing agent and forms colourless potassium sulphate and colorless manganese sulphate.

KMnO4(aq)   +  H2O(l)  + 5 SO2(g)   \[\to \]  K2SO4(aq)    +   2 H2SO4   +     MnSO4 

Potassium Permanganate                    Potassium Sulphate             Manganese Dioxide

     (Purple)                                                               (Colorless)                                (Colorless)

(c) O2 

Ans: When a matchstick is brought near the  oxygen gas, it burns with more intensity and with bright flames as the oxygen gas supports the burning.

(d) H2 

Ans: When a matchstick is brought near the hydrogen gas, it burns with a ‘pop’ sound. 

41. What happens when a piece of:

(a) zinc metal is added to copper sulphate solution? 

Ans: When zinc metal is added to blue copper sulphate solution, zinc being more reactive than copper (as zinc is placed above copper in the reactivity series) displaces copper metal from copper sulphate solution to form colorless solution of zinc sulphate and reddish brown copper metal settles at the bottom.

      Zn(s)      +     CuSO4(aq)          \[\to \]            ZnSO4(aq)    +     Cu  

       Zinc             Copper Sulphate                 Zinc Sulphate            Copper

                               (Blue)                              (Colorless)       (Reddish-brown)

(b) Aluminium metal is added to dilute hydrochloric acid? 

Ans: When aluminum metal is added to dilute hydrochloric acid, aqueous solution of aluminium chloride is formed and hydrogen gas is evolved.

     2 Al(s)        +     6 HCl(aq)            \[\to \]           2 AlCl3(aq)           +      3  H2(g)

Aluminium            Hydrochloric Acid                   Aluminium Chloride               Hydrogen

(c) silver metal is added to copper sulphate solution? 

Also, write the balanced chemical equation if the reaction  occurs 

Ans: Silver metal is less reactive than copper, so it will not displace copper from copper sulphate solution and hence, no reaction takes place when silver is added to copper sulphate solution.

     Ag(s)        +       CuSO4(aq)            \[\to \]         No Reaction

     Silver                   Copper Sulphate                

42. What happens when zinc granules are treated with dilute solution of H2SO4, HC1, HNO3, NaC1 and NaOH, also write the chemical equations if reaction occurs. 

When zinc granules are treated with dilute solution of the following:

(i) H2SO4

When zinc granules are treated with dilute solution of sulphuric acid, zinc sulphate solution is formed along with hydrogen gas.

        Zn(s)        +    H2SO4(aq)            \[\to \]      ZnSO4(aq)  +    H2(g)

          Zinc               Sulphuric Acid           Zinc Sulphate       Hydrogen 

(ii) HCl

When zinc granules are treated with dilute solution of hydrochloric acid, zinc chloride solution is obtained with the evolution of hydrogen gas.

       Zn(s)        +    2 HCl(aq)             \[\to \]          ZnCl2(aq)     +    H2(g)

     Zinc               Hydrochloric Acid              Zinc Chloride      Hydrogen 

(iii) HNO3

When zinc granules are treated with dilute solution of  nitric acid,  an aqueous solution of zinc nitrate is formed along with water and nitrous oxide.

      4 Zn(s)        +    10 HNO3(aq)        \[\to \]      4 Zn(NO3)2(aq)    +    H2O(l)    +   N2O

          Zinc                    Nitric Acid                 Zinc Nitrate     

(iv) NaCl

Zinc granules on treatment with NaCl, do not undergoes any reaction as sodium is a reactive metal than zinc (sodium is placed above zinc in the reactivity series), and cannot displace sodium from its salt solution of sodium chloride.

        Zn(s)        +    NaCl(aq)             \[\to \]        No Reaction

          Zinc               Sodium Chloride                    

(v) NaOH:

Zinc metal reacts with alkali to form salt and evolve hydrogen gas.

       Zn(s)        +    2 NaOH(aq)        \[\to \]          Na2ZnO2(aq)    +    H2(g)   

        Zinc                 Sodium Hydroxide                   Sodium Zincate    

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

Ans: Barium chloride, on adding to an aqueous solution of sodium sulphite, a white precipitate of barium sulphite is formed.

       BaCl2(aq)        +    Na2SO3(aq)        \[\to \]          BaSO3(s)    +   2 NaCl(aq)   

        Zinc                        Sodium Sulphite                      Barium Sulphite   

                                                                                         (White ppt.)

(b) What other name can be given to this precipitation reaction? 

Ans: The other name that can be given to this precipitation reaction is double displacement reaction.

(c) On adding dilute hydrochloric acid to the reaction mixture, white precipitate disappears. Why?

Ans: On adding  dilute hydrochloric acid to the reaction mixture, white precipitate  of barium sulphite disappears due to formation of aqueous solution of barium chloride, water and sulphur dioxide gas is evolved.

       BaSO3(s)        +      2  HCl(aq)         \[\to \]      BaCl2(aq)    +    SO2(g)   +    H2O(l)  

     Barium Sulphite        Hydrochloric Acid              Barium Chloride  

44. You are provided with two containers made up of copper and aluminium. You are also provided with solutions of dilute HC1, dilute HNO3, ZnCl2 and H2O. In which of the above containers these solutions can be kept? 

Ans: Copper is a less reactive metal and placed below hydrogen and zinc metal in the reactivity series and hence cannot displace hydrogen from acids and water aw well as zinc from zinc chloride, hence all the solution of  dilute HC1, dilute HNO3, ZnCl2 and H2O can be kept in copper vessel.

Aluminium is reactive metal and placed above hydrogen in the reactivity series hence reacts with the following:

(i) dilute HCl:

      2 Al(s)        +      6  HCl(aq)         \[\to \]      AlCl3(aq)    +     H2(g)  

 Auminium            Hydrochloric Acid              Aluminum Chloride  

Aluminium reacts with dilute HCl to form aluminium chloride solution and hydrogen gas is evolved. So, dilute HCl solution cannot be stored in aluminium vessels.

(ii) dilute HNO3:

Aluminium reacts with a dilute solution of nitric acid to form the protective layer of aluminium oxide (Al2O3) which prevents the further reaction of acid with aluminium vessels and hence, dilute nitric acid can be stored in the aluminium vessel.

(iii) ZnCl2 

      2 Al(s)        +      3 ZnCl2(aq)         \[\to \]      2 AlCl3(aq)    +     3 Zn(s)  

  Aluminium                    Zinc chloride                 Aluminum Chloride  

Aluminum is a more reactive metal than zinc (aluminium is placed above zinc in the reactivity series), displaces zinc from zinc chloride solution to form aluminium chloride and zinc metal. Zinc chloride solution cannot be stored in aluminium vessels.

(iv) H2O

Aluminium metal reacts with cold water but can react with steam to form a protective layer of aluminium oxide (Al2O3) which prevents further reaction with water hence, water can be stored in aluminium vessels.

NCERT Exemplar for Class 10 Science - Chemical Reactions and Equations

Class 10 Science NCERT Exemplar Book:  

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NCERT has provided Class 10 Chemistry Exemplar for NCERT and Class 11 Chemistry Exemplar for NCERT. The exemplars give you an idea about the important topics of Chemistry which you need to cover in your examinations. The NCERT Exemplar for Class 10 Science will have the right level of complexity and it will have questions of varying difficulty levels. It is recommended to practice NCERT Class 10 Chemistry Exemplar for Class 10 Science at least once before the examination.