The following figure shows the solubility curves of NaCl, KNO₃ and hydrated calcium sulphate.

(a) Identify and label the curves with the salt it represents.

(b) State the factors on which the solubility depends.

(c) Solubility of which salt(s) shows :

(i) Endothermic process

(ii) Exothermic process?

(a) The labelled curves are shown below:

(b) Solubility of a solid in a solvent depends on the following factors:

1. Size of solute particles — The smaller the size of the solute particles, the greater is its total surface area exposed to the solvent. Therefore, greater is the solubility of that solute.
2. Stirring — This brings more of the solvent in contact with the solute and thus increases the rate of formation of solution.
3. Temperature — The solubility of a gas in a liquid always decreases with rise in temperature. But the solubility of most solids in water usually increases with rise in temperature.

In case of NaCl, solubility increases only a little with increase in temperature.

In case of KNO₃, solubility increases considerably with increase in temperature.

In case of calcium sulphate, solubility decreases (after attaining a certain temperature) with further rise in temperature.

(c) (i) In an endothermic process, the solubility of a solute increases with an increase in temperature.

For example: solubility of KNO₃ increases with rise in temperature and solubility of NaCl increases only a little with increase in temperature.

(ii) In an exothermic process, the solubility increases on lowering the temperature.

For example: solubility of calcium sulphate in water decreases on increasing the temperature.