When 1 mol of NaCl is added to 1 litre of water, the boiling point of water increases. On the other hand, addition of 1 mol of methyl alcohol to one litre of water decreases the boiling point of water. Explain why does this happen.
The vapour pressure above the liquid is inversely proportional to the boiling point of the liquid i.e. higher the vapour pressure above the liquid, lower is the boiling point of the liquid.
NaCl is a non-volatile substance, it decreases the vapour pressure above water. Thus, increasing the boiling point of the water.
Methyl alcohol is volatile in nature, so it increases the vapour pressure above water when added to it. Thus, it decreases the boiling point of the water.