Which of the following metal carbonates decomposes on heating? a) MgCO3 b) Na2CO3 c) K2CO3 d) Rb2CO3.
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The solution is as follows:

1. Correct option is  (a) MgCO₃.

2. This is because, the group of alkali metals have thermally stable carbonates as we move down the group. But only Lithium carbonate is not thermally stable. As Mg shares a diagonal relationship with Li, it has less thermally stable carbonate.

3. Also, the size of Mg is much smaller as compares to carbonate, which leads to size incompatibility and there is a weaker bond which breaks in heating. The reaction is as follows:

        MgCO₃(s) $\underset{∆}{\to }$ MgO (s) + CO₂(g)