Write an equation in each case to represent the change that occurs when the following carbonates are heated. A: Na2CO3, B: MgCO3.

The $\text{Mg}^{2+}$ ion has a smaller ionic radius and a higher charge (+2) compared to the $\text{Na}^{+}$ ion, which has a larger ionic radius and a lower charge (+1). This gives $\text{Mg}^{2+}$ a significantly higher charge density than $\text{Na}^{+}$.

Due to its high charge density, the $\text{Mg}^{2+}$ ion has a strong electrostatic attraction for the lone pairs of electrons on the oxygen atom of water molecules. This strong attraction allows $\text{Mg}^{2+}$ to effectively polarize water molecules and form strong coordinate bonds, leading to the formation of stable hydrated ions.

The $\text{Na}^{+}$ ion, with its lower charge density, does not attract water molecules strongly enough to form stable hydrated complexes. Therefore, $\text{MgCl}_2$ readily forms hydrates (e.g., $\text{MgCl}_2 \cdot 6\text{H}_2\text{O}$), while $\text{NaCl}$ typically exists in its anhydrous form.