Question

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

Answer 1

NaCl is a non volatile 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, the total vapour pressure over the solution and a decrease in boiling point of water results.