To study the effect of heat on the solubility of a substance.
Half-fill a test tube with distilled water and add to it a small quantity of potassium chloride. Shake well. If it dissolves, add more until on stirring, the solution can dissolve no more and excess salt settles at the bottom. The solution is now saturated. Now warm the solution and see what happens. Add more salt until the solution is saturated again at the higher temperature. Repeat the experiment with nitre and slaked lime.
Observe that in the case of potassium chloride and nitre, the solution which is saturated at room temperature becomes unsaturated when heated and can take more of the solid. In other words, they are more soluble in hot water than in cold. The reverse is noticed in the case of slaked lime, for, instead of the solid in the solution saturated at room temperature disappearing on heating, it seems that more solid is deposited at the higher temperature. i.e. slaked lime is less soluble in hot water than in cold.
Generally, most substances are considerably more soluble in hot water than in cold. There are only a very few exceptions e.g. slaked lime (calcium hydroxide) and sodium sulphate.