Exemplar Questions and Solutions

Important CBSE Questions – Short Answer Questions

1) 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.

This is a combination reaction as two or more elements are combing to form a compound.

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

It is a balanced equation and is a double displacement reaction as two compounds (Sodium hydroxide and acetic acid) react by exchange of ions to form two new compounds.

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

It is already a balanced equation as the number of various atoms is equal on both sides. This reaction is a combination reaction as two or more elements are combing to form a compound.

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

Ans) C₂H₄ + 3O₂ → 2CO₂ + 2H₂O

It is a redox reaction as there is a change in the oxidation number of carbon and oxygen. Carbon is oxidized as it gains oxygen and C₂H₄ is the reducing agent while oxygen is reduced, as it loses oxygen and the oxidizing agent is O₂

2) 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) Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + Heat + Light

It is an exothermic redox reaction. As an incredible amount of heat is produced, it is an exothermic reaction. As aluminium is more reactive than iron, it displaces iron from ferric oxide (Fe₂O₃). The aluminium gains oxygen from ferric oxide and therefore, aluminium gets oxidized while iron is reduced as it loses oxygen during the redox reaction.

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

Ans) 3MgS + N₂ → 2Mg₃N₂

The above reaction is a combination reaction as two or more elements are combing to form a compound

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

Ans) 2KI + Cl₂ → 2KCl + I₂

The above reaction is a displacement reaction because chlorine being smaller than Iodine, it is more reactive than Iodine. Therefore, it displaces iodine from potassium iodide and forms potassium chloride.

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

Ans) C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + Heat

It is a redox reaction as there is a change in the oxidation number of carbon and oxygen. Carbon is oxidized as it gains oxygen and C2H5OH is the reducing agent while oxygen is reduced, as it loses oxygen and the oxidizing agent is O2. It is an exothermic reaction as there is release of heat energy.

3) Complete the missing components/variables given as given as x and y in the following reactions

a) Pb(NO₃)₂ (aq) + 2KI (aq) → PbI₂ (x) +2KNO₃ (y)

Ans) Pb(NO₃)₂ (aq) + 2KI (aq) → PbI₂ (s) + 2KNO₃ (aq)

b) Cu (s) + 2AgNO₃ (aq) → Cu(NO₃)₂ + X (s)

Ans) Cu (s) + 2AgNO₃ (aq) → Cu(NO₃)₂ + Ag (s)

c) Zn (s) + H₂SO₄ (aq) → ZnSO₄ (x) + H₂ (y)

Ans) Zn (s) + H₂SO₄ (aq) → ZnSO₄ (aq) + H₂ (g)

4) 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

Ans) (b) and (c) are exothermic as heat is released in these changes.

(a) and (d) are endothermic as heat is absorbed in these change

5) Identify the reducing agent in the following reactions:

a) 4NH₃ + 5O₂ → 4NO + 6H₂O

Ans) In this reaction, nitrogen of ammonia gains oxygen to form NO. It means that NH3 is oxidized. So, it acts as a reducing agent in the above given reaction. Therefore, substance oxidized is NH3 and thus it is the reducing agent while the substance reduced is O₂ and therefore, the oxidizing agent- is O₂.

b) H₂O + F₂ → HF + HOF

Ans) H2O is oxidized to HOF by losing hydrogen atom. Hence, H2O is the reducing agent. F2 is reduced to HF by gaining hydrogen atom. Hence, F₂ is the oxidizing agent.

c) Fe₂O₃ + 3CO → 2Fe + 3CO₂

Ans) In this reaction, oxygen is removed from Fe₂O₃ and is added to CO. So, Fe₂O₃ is reduced to Fe and CO is oxidized to CO₂. Fe₂O₃ is the oxidizing agent and CO is the reducing agent.

d) 2H₂ + O₂ → 2H₂O

Ans) In this reaction, 2H₂ gains oxygen (while reacting with O₂) and becomes 2H₂O. So, here 2H₂ gets oxidized and therefore, 2H₂ is the reducing agent.

6) Identify the oxidising agent (oxidant) in the following reactions

a) Pb₃O₄ + 8HCl → 3PbCl₂ + Cl₂ + 4H₂O

Ans) In the above given reaction, Pb₃O₄ loses oxygen and reduces to PbCl₂. So, Pb₃O₄ is an oxidizing agent as it undergoes reduction and it oxidises HCl to chlorine and water.

b) 2Mg + O₂ → 2MgO

Ans) In the above reaction, Mg gains oxygen and becomes 2MgO. So, here 2Mg gets oxidized and therefore 2Mg is the reducing agent. O₂ loses oxygen and reduces to 2MgO. Hence O₂ is the oxidizing agent.

c) CuSO₄ + Zn → Cu + ZnSO₄

Ans) In this reaction, CuSO4 loses oxygen and reduces to Cu. So, CuSO4 is an oxidizing agent as it undergoes reduction.

d) V₂O₅ + 5Ca → 2V + 5CaO

Ans) In this reaction, V₂O₅ loses oxygen and reduces to 2V. So, V₂O₅ is an oxidizing agent as it undergoes reduction.

e) 3Fe + 4H₂O → Fe3O₄ + 4H₂

Ans) Oxygen is added to iron, thus, iron is oxidized. So, 3Fe is the reducing agent 4H₂O. 4H₂O loses oxygen and becomes 4H₂.

Therefore, 4H₂O is reduced and is thus the oxidizing agent.

f) CuO + H₂ → Cu + H₂O

Ans) Copper oxide (CuO) loses oxygen and is reduced to copper (Cu). So, CuO is the oxidizing agent. While, hydrogen (H₂) gains oxygen and gets oxidised to water (H₂O). So, H₂ is the reducing agent

7) 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 hydrogen carbonate.

Ans) Na₂CO₃ + HCl → NaCl + NaHCO

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

Ans) NaHCO₃ + HCl → NaCl + H₂O + CO₂

(c) Copper sulphate on treatment with potassium iodide precipitates cuprous iodide (Cu₂ I₂), liberates iodine gas and also forms potassium sulphate

Ans) 2CuSO₄ + 4KI → Cu₂I₂ + I₂ + 2K₂SO₄

8) 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) KCl + AgNO₃ → AgCl + KNO₃

Since AgCl is a white precipitate, we can therefore call the above reaction as a precipitation reaction. It is a double displacement reaction as two compounds (potassium chloride and silver nitrate) react by exchange of ions to form two new compounds.

9) 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.

Sulphur dioxide evolves in this smells like burning of sulphur. This reaction is a thermal decomposition reaction because a single substance decomposes to two or more simpler substances on heating.

10) Why do fire flies glow at night?

Ans) Fireflies produce a chemical reaction inside their bodies that allows them to light up. Fireflies contain a chemical in their abdomen called luciferin. When that chemical combines with oxygen and with an enzyme called luciferase, the ensuing chemical reaction causes their abdomen to light up.

11) 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 are living while they are hanging on the plant and thus their immune system is actively functioning. This immune system of the grape plant prevents the microorganisms from fermenting the sugar present inside grape fruit. But, once the fruits are picked from the plant their immune system stop functioning and thereby making fermentation possible. Fermentation of grapes takes place under anaerobic condition by microorganisms like yeast. This is a chemical change.

12) Which among the following are physical or chemical changes?

(a) Evaporation of petrol

Ans) Physical change. Petrol kept in the sun evaporates. When the petrol vapours are collected, the vapours change to liquid petrol.

(b) Burning of Liquefied Petroleum Gas (LPG)

Ans) Burning of LPG involves both physical and chemical changes. Burning of LPG in our kitchen is another such example in which a physical change occurs when LPG come out of cylinder and is converted from liquid to gaseous state and a chemical change occurs when this gas burn in air.

(c) Heating of an iron rod to red hot

Ans) Physical change. If you heat an iron bar until it glows red hot, it is still chemically the same iron.

(d) Curdling of milk

Ans) Chemical change. Curd is produced from milk. The curd formed is a new substance having entirely different chemical properties. It is irreversible, as milk cannot be got back from curd. Hence, setting of curd is a chemical change.

(e) Sublimation of solid ammonium chloride

Ans) Sublimation of solid ammonium chloride involves both physical and chemical changes Physical change. Sublimation is the process of change of state from solid directly into gaseous state. On heating, solid ammonium chloride changes directly to vapour state. On cooling, the vapours form solid ammonium chloride again.

Chemical change. On heating strongly above 340^oC, the white solid ammonium chloride, thermally decomposes into a mixture of two colourless gases ammonia and hydrogen chloride.

The products ammonia and chlorine recombine to form ammonium chloride. This is also a chemical change. It is also a reversible reaction.

13) During the reaction of some metals with dilute hydrochloric acid, following observations were made. Explain these observations giving suitable reasons.

(a) Silver metal does not show any change

Ans) No change takes place because silver being a noble metal does not react with hydrochloric acid under normal situations. Metals like copper, silver, gold and platinum are placed below hydrogen in the reactivity series and hence will not react with dilute acid. They cannot displace hydrogen from acids.

Ag(s) + HCl(aq) → No reaction

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

Ans) When aluminium is added to dilute HCl, it displaces hydrogen from the acid as it is more reactive than hydrogen and forms aluminium chloride and hydrogen gas. The reaction of Al with dil. HCl is exothermic, i.e. heat is released in the reaction thereby increasing the temperature of the reaction mixture.

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

Ans) The reaction of sodium metal is found to be highly explosive because it is an exothermic reaction. Sodium being very reactive metal, reacts vigorously with dil HCl the evolution of heat too.

(d) Some bubbles of a gas are seen when lead (Pb) is reacted with the acid.

Ans) Lead reacts very slowly with dilute hydrochloric acid as it is placed just above hydrogen in the reactivity series. Lead displaces the H from HCl and there is the evolution of colourless bubbles of a gas. The bubbles formed when lead reacts with dilute Hcl shows that a gas is being made. The gas is hydrogen.

Pb(s) + 2HCl (aq) → PbCl₂(s) + H₂(g)

14) 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) Element ‘X’ is calcium. When calcium oxide reacts with water it forms calcium hydroxide (slaked lime). Since calcium hydroxide is a base, thus it turns red litmus blue.

Calcium belongs to group 2 of the periodic table. It is used in cement industry and is also found in bones.

15) 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) Pb(CH₃COO)₂ + 2HCl → PbCl₂ + CH₃COOH

It is a double displacement reaction as two compounds (Lead acetate and hydrochloric acid) react by exchange of ions to form two new compounds.

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

Ans) 2Na + 2C₂H₅OH → 2C₂H₅ONa + H₂

It is a displacement reaction sodium metal displaces the hydrogen in the –OH group to form sodium ethoxide and hydrogen gas.

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

Ans) Fe₂O₃ + 3CO → 2Fe + 3CO₂

It is a redox reaction as each carbon atom increases its oxidation number from +2 to +4, so each carbon atom in CO(g) is oxidized, and CO(g) is the reducing agent. Each iron atom in Fe2O3 decreases its oxidation number from +3 to 0, so each Fe atom in Fe₂O₃ is reduced and Fe₂O₃ is the oxidizing agent

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

Ans) 2H₂S + O₂ → 2S + 2H₂O

The above given chemical reaction is a redox reaction. S atoms in H2S are oxidized, O atoms in O₂ are reduced, O₂ is the oxidizing agent, and H₂S is the reducing agent.

16) Why do we store silver chloride in dark coloured bottles?

Ans) When silver chloride is exposed to light, it breaks down into elemental silver and chlorine gas:

2AgCl → 2Ag + Cl₂

The white colour of silver colour changes into grey colour during the chemical reaction. We store silver chloride in dark coloured bottles because the dark coloured bottles interrupt the path of light and it almost becomes impossible for the light to reach silver chloride in the bottles and thus, its decomposition is prevented. Thus, to increase the shelf life of silver chloride, by preventing a photochemical decomposition reaction, substances like silver chloride are kept in dark bottles in the laboratory.

17) Balance the following chemical equations and identify the type of chemical reaction.

a) Mg + Cl₂ → MgCl₂

Ans) The original equation is already balanced

Mg + Cl₂ → MgCl₂

It is a combination reaction because two or more substances (Mg and Cl₂) combine to form a single substance (MgCl₂), are called combination reactions.

The above given reaction is also a redox reaction. Magnesium loses two electrons and gets oxidized. It is therefore, the reducing agent while and chlorine gains two electron and is reduced. Therefore, chlorine is the oxidizing agent.

The above given reaction is an example of thermal decomposition reaction because decomposition {a single substance(HgO) decomposes to two or more simpler substances (Hg and O)}is carried out by heating.

It is a combination reaction as two or more substances (Sodium and Sulphur) are combined to form new substance(Sodium sulphide).

d) TiCl₄ + Mg → Ti + MgCl₂

Ans) TiCl₄ + 2Mg → Ti + 2MgCl₂

It is a displacement reaction because Magnesium displaces titanium (Ti) from Titanium tetrachloride(TiCl₄) as Mg is more reactive than Ti.

e) CaO + SiO₂ → CaSiO₃

Ans) CaO + SiO2 → CaSiO3

The above reaction is a combination reaction as two or more substances(CaO and SiO₂) are combined to form new substance namely Calcium silicate(CaSiO₃).

The above reaction is a decomposition reaction as a single substance namely Hydrogen peroxide(H₂O₂) decomposes to form two substances(water and oxygen)

18) 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) The chemical formula of X is MgO (Magnesium oxide) and Y is Mg₃N₂(Magnesium nitride)

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

Ans) As we know, X is Magnesium oxide. When Magnesium oxide dissolves in water, it gives Magnesium hydroxide.

MgO + H₂O → Mg(OH)₂

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

Ans) The activity series of metals is very useful in predicting the products formed during a displacement reaction and also the reactivity of metals in other reactions. While taking a look into the position of zinc and copper in the reactivity series, we can find that zinc is placed much above copper. This means that the metals above hydrogen are more reactive than hydrogen and the ones below it. These metals can easily displace hydrogen from acids or water and liberate hydrogen gas whereas those below hydrogen are less reactive than hydrogen and they do not replace hydrogen from water or acid.

So, zinc will react with hydrochloric acid to produce zinc chloride and hydrogen gas.

Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)

Zinc will reduce the hydrogen to diatomic hydrogen gas and get oxidized in the process.

In case of copper, no reaction takes place when copper is placed in hydrochloric acid as copper is less reactive than hydrogen and thus can’t displace hydrogen from its compound.

Cu(s) + HCl(aq) → NO REACTION

20) 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.

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

Ans) a)The atmospheric air is made of moisture and gases like nitrogen, carbon dioxide, oxygen, argon, etc along with air pollutants such as sulphur dioxide, hydrogen sulphide, oxides of nitrogen, chlorides etc. When silver is exposed to air, the silver starts reacting with the oxygen present in the atmosphere and forms silver oxide. In the presence of moisture and hydrogen sulphide, it combines with sulfur and forms silver sulfide. Silver sulfide is black solid and when this silver sulfide forms a thin coating on the surface of silver, it darkens the silver. This phenomenon is called tarnishing of silver. Reaction involve in this is given here:

2Ag + H₂S → Ag₂S + H₂

(b) The black substance formed over the silver metal is silver sulphide. The chemical formula of silver sulphide is Ag₂S.