A KCl solution containing 42 g of KCl per 100 g of water is cooled from 60 °C to 0 °C. What happens during cooling? (See Figure 13.4.)
Figure 13.4 Solubility of some ionic solids as a function of temperature
The solubility of a solid in a solution is dependent on temperature. Thus the solubility of most solids in water increases with increasing temperature.
From the figure 1., it is observed the solubility of KCl at 60 °C is 45 g per 100 g of water whereas the solubility of KCl at 0 °C is 28 g per 100 g of water. Thus, a KCl solution containing 42 g of KCl per 100 g of water is cooled at 60 °C forms unsaturated solution.
Therefore, the difference between the amount of KCl which could be dissolved at 60∘C and the amount of KCl which could be dissolved at 0∘C will precipitate out of solution.
Thus, when the KCl solution having 42 g of KCl per 100 g of water cools from from 60 °C to 0 °C, the solution becomes supersaturated and the excess solid precipitates out of solution. When the solution cools slowly,it forms crystals as it comes out of solution.