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.

Explanation

Boiling point of a liquid depends on the vapour pressure of the liquid as compared to amspheric pressure. Lesser the vapour pressure higher would be the boiling point of a liquid or vice-versa, at a fixed atmospheric pressure. NaCl is a nonvolatile solute, therefore, addition of NaCl to water lowers the vapour pressure of water. As a result, boiling point of water increases. Methyl alcohol on the other hand is more volatile than water, therefore its addition increases, and the total vapour pressure over tha solution and a decrease in boiling point of water results.