Explain why on addition of 1 mol of NaCl to 1 litre of water, the boiling point of water increases, while addition of 1 mol of methyl alcohol to one litre of water decreases its boiling point.

Addition of a non-volatile solute like NaCl to solvent like water increases the boiling point of the solution. On the other hand, methyl alcohol is more volatile than water, hence addition of methyl alcohol as solute to water decreases the boiling point.