Question 7(iv)
Give one relevant observation for each of the following reactions.
(a) Zinc carbonate is heated.
(b) Excess of ammonia is passed through an aqueous solution of lead nitrate.
(c) Sodium hydrogen carbonate is added to nitric acid.
(a) On heating, zinc carbonate, which is a light amorphous white solid, changes to pale yellow and gives off a colourless and odourless gas that turns lime water milky. The milkiness disappears on passing excess of gas. This proves that the gas evolved is carbon dioxide.
ZnCO3 ZnO + CO2 ↑
(b) Chalky white precipitate of lead hydroxide is formed which is insoluble in excess of ammonium.
Pb(NO3)2 + 2NH4OH ⟶ 2NH4NO3 + Pb(OH)2 ↓
(c) When sodium hydrogen carbonate is added to nitric acid, a colourless and odourless gas is evolved that turns lime water milky. The milkiness disappears on passing excess of gas. Hence, the gas evolved is carbon dioxide.
NaHCO3 + HNO3 ⟶ NaNO3 + H2O + CO2 ↑