Which one of the following carbonates is water insoluble?

Calcium carbonate (CaCO₃) is a sparingly soluble or practically insoluble carbonate in water, unlike the carbonates of alkali metals such as sodium carbonate (Na₂CO₃), potassium carbonate (K₂CO₃), and ammonium carbonate ((NH₄)₂CO₃), which are highly soluble. The difference in solubility arises due to the balance between lattice energy and hydration energy of the ionic compound.

Lattice energy is the energy required to separate the ions in a crystalline solid, while hydration energy is the energy released when those ions become surrounded by water molecules. For a compound to dissolve, the hydration energy must be sufficient to overcome the lattice energy.

In the case of CaCO₃, the calcium ion (Ca²⁺) and carbonate ion (CO₃²⁻) form a very strong ionic lattice because both ions carry a +2 and –2 charge, respectively. This results in very high lattice energy, which is not compensated by the hydration energy when the compound interacts with water. Therefore, calcium carbonate remains insoluble under normal conditions.

On the other hand, Na₂CO₃, K₂CO₃, and (NH₄)₂CO₃ contain monovalent cations (Na⁺, K⁺, and NH₄⁺), which produce weaker ionic bonds and lower lattice energies. These compounds dissolve easily in water because their hydration energy is sufficient to overcome the lattice attraction between ions.

Calcium carbonate occurs naturally as limestone, chalk, and marble, and its low solubility plays an important role in natural processes like the formation of caves and the buffering of soil pH. It dissolves slightly in water containing dissolved carbon dioxide, forming calcium bicarbonate (Ca(HCO₃)₂), which is soluble.

Thus, among the given options, CaCO₃ (calcium carbonate) is the water-insoluble carbonate, illustrating an important solubility trend in Inorganic Chemistry.