Science — CBSE Class 10 board question

(i) Lead (II) nitrate on heating decomposes to give lead monoxide, nitrogen dioxide (brown fumes) and oxygen. It is a thermal decomposition reaction.

$$2\text{Pb(NO}_3)_2(s) \xrightarrow{\Delta} 2\text{PbO}(s) + 4\text{NO}_2(g) + \text{O}_2(g)$$

(ii) Iron (II) sulphate on heating decomposes to give iron (III) oxide, sulphur dioxide and sulphur trioxide.

$$2\text{FeSO}_4(s) \xrightarrow{\Delta} \text{Fe}_2\text{O}_3(s) + \text{SO}_2(g) + \text{SO}_3(g)$$

(iii) Silver chloride on exposure to sunlight decomposes to give silver metal and chlorine gas. The white silver chloride turns grey. It is a photochemical decomposition reaction.

$$2\text{AgCl}(s) \xrightarrow{\text{Sunlight}} 2\text{Ag}(s) + \text{Cl}_2(g)$$

All three are decomposition reactions.

Source: Chemical Reactions and Equations, Chapter 1

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• All three are examples of decomposition reactions — thermal decomposition for (i) and (ii), photochemical decomposition for (iii). Examiners expect you to state what is observed (brown fumes, grey colour, etc.) and write a correct balanced equation for each. Identifying the type of reaction (decomposition) adds value and may fetch bonus credit.
• The brown fumes of NO₂ in (i) are a key observable fact to mention.
• For (iii), stating the colour change (white → grey) demonstrates understanding.