Exemplar Questions and Solutions

Important CBSE Questions – Short Answer Questions

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.