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.

(N/A) The boiling point of a liquid depends on its vapor pressure relative to the atmospheric pressure. At a fixed atmospheric pressure,a lower vapor pressure leads to a higher boiling point,and vice-versa.
$NaCl$ is a non-volatile solute. When added to water,it occupies the surface area and reduces the escaping tendency of water molecules,thereby lowering the vapor pressure of the solution. Consequently,the boiling point of water increases.
Conversely,methyl alcohol $(CH_3OH)$ is more volatile than water. Its addition increases the total vapor pressure of the solution,which leads to a decrease in the boiling point of the resulting mixture.