Alum is the least soluble until the temperature of the water increases to about 65 °C.
Alum is any of a group of hydrated double salts, usually consisting of aluminium sulfate, the water of hydration, and the sulfate of another element.
The first test is to find the melting temperature of the compound and compare this value with the accepted (published) value for alum (92.5°C).
The second test determines the water of hydration present in the alum crystals.
The third test is a chemical test to determine the per cent sulfate in your sample of alum.
Hence, alum is the least soluble until the temperature of the water increases to about 65 °C. After that point, the solubility of alum is greater than that of salt but never as great as sugar.
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