Question

Carbonate which is insoluble in water:
A.Sodium
B.Potassium
C.Lithium
D.Cesium

Answer 1

Hint:The solubility of a salt depends majorly on its lattice energy and hydration energy. Smaller is the size of cation, more will be its hydration energy and thus more will be the solubility. To answer this question, you must recall any exceptional properties, if shown by any group 1 metal.

Complete answer:
If the hydration energy of the ions is large enough so as to overcome the lattice energy of the compound, the compound will be soluble in water. If the hydration energy of the ions does not exceed the lattice energy of the compound, it will not be soluble in water.
We know that lithium is the first element in the alkali metal series but behaves differently from other members of the group. This anomalous behaviour is shown by lithium due to its extremely small size.
In lithium, the valence electron lies very close to the nucleus and as a result has high ionization energy. Thus, lithium has less ionic character than the other alkali metals and it forms covalent bonds. Also, we know that lithium has polarizing power and can distort the electron cloud of the relatively large sized anions.
Thus, lithium carbonate is a covalent compound. The bond between the two species overpowers the attraction between these ions and water and is thus sparingly soluble in water whereas other alkali metal carbonates are highly soluble.

Thus, the correct option is C.

Note:
Apart from its anomalous behaviour, Lithium atom also shows diagonal relationship with alkaline earth metal magnesium. The electropositivity of an element increases down the group while it decreases from left to right in a period. Thus, due to a combined effect, magnesium metal has similar electropositivity as that of lithium.