Question

How would you explain the following observations?
i.$BeO$ is almost insoluble but $BeS{O_4}$ is soluble in water
ii.$BaO$ is soluble but $BaS{O_4}$ is insoluble in water
iii.$LiI$is more soluble than $KI$ in ethanol

Answer 1

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, it is important to know that the lattice energy of a salt is high if the constituent ions are similar in size.

Complete step by step answer:
i.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.$BeO$ has a high lattice energy due to comparable sizes of the two atoms. Due to this and its covalent nature, it is insoluble in water whereas, $BeS{O_4}$ has a very low lattice energy and is thus readily soluble.
ii.Similarly, barium ion and sulphate ion are of comparable sizes. The lattice energy of barium sulphate is very high and that of barium oxide is low. Thus $BaO$ is soluble but $BaS{O_4}$ is insoluble in water
iii.Smaller cations have a high polarizing power as compared to larger cations. As a result, $L{i^ + }$ polarizes ${I^ - }$ to a larger extent as compared to ${K^ + }$. Thus, $LI$ has a higher covalent nature and is more soluble in ethanol which is an organic solvent.

Note:
Hydration energy can be termed as the amount of energy which is released when one mole of ions are hydrated. Lattice energy of a compound is termed as the energy released when ions combine to form a compound. It can also be said as the energy required to break the lattice of a salt in order to dissolve it in water.