A KNO3 solution containing 45 g of KNO3 per 100 g of water is cooled from 40 °C to 0 °C. What happens during cooling?(see Figure 1)
Figure 1 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 KNO3 at 40 °C is 62 g per 100 g of water whereas the solubility of KNO3 at 0 °C is 12 g per 100 g of water. Thus, a KNO3 solution containing 45 g of KCl per 100 g of water is cooled at 40 °C forms unsaturated solution.
Therefore, the difference between the amount of KNO3 which could be dissolved at 40∘C and the amount of KNO3 which could be dissolved at 0∘C will precipitate out of solution.
Thus, when the KNO3 solution having 45 g of KNO3 per 100 g of water cools from from 40 °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.