Explain in why on addition of 1 mole of NaCl to 1L of water, the boiling point of water increases, while addition of 1 mole of methyl alcohol to 1 L of water decreases its boiling point .

Explanation

NaCl is a non-volatile solute. On adding NaCl to water, vapour pressure of lowered and hence boiling point of water increases. Methyl alcohol is more volatile than water. On adding methyl alcohol into water, vapour pressure of solution is greater than that of water. Hence, boiling point of water decreases.