WAEC Chemistry 2025 Paper

Concept:The thermal stability of a metal carbonate depends on the nature of the metal. Carbonates of alkali metals are highly stable to heat.Explanation:Most metal carbonates decompose on strong heating to form the metal oxide and carbon dioxide gas.For example:$$\mathrm{CaCO_3 \xrightarrow{\Delta} CaO + CO_2}$$$$\mathrm{MgCO_3 \xrightarrow{\Delta} MgO + CO_2}$$These carbonates are not stable to heat because their metal cations have high charge density, which weakens the carbonate ion.In contrast, $\mathrm{Na_2CO_3}$ is an alkali metal carbonate. Alkali metal carbonates have very low charge density on the metal ion, so they are thermally stable and do not decompose even when heated strongly.Therefore, among the given options, only $\mathrm{Na_2CO_3}$ remains unchanged on strong heating.Answer:D. $\mathrm{Na_2CO_3}$