Question

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

Hint:

Non-volatile solute raises boiling point; volatile solute can lower boiling point by increasing vapour pressure.

Answer 1

Concept:
Boiling point depends on vapour pressure. A liquid boils when its vapour pressure becomes equal to atmospheric pressure. Non-volatile solutes lower the vapour pressure of solvent and therefore increase the boiling point. Volatile solutes may increase the total vapour pressure of solution and therefore decrease the boiling point.

Step 1: Effect of adding KCl to water.
KCl is a non-volatile electrolyte. It dissociates in water: \[ KCl \rightarrow K^+ + Cl^- \] Since KCl is non-volatile, it does not contribute to vapour pressure. It lowers the vapour pressure of water. To make vapour pressure equal to atmospheric pressure, the solution must be heated more. Therefore, boiling point increases. \[ \boxed{KCl \Rightarrow \text{boiling point increases}} \]

Step 2: Effect of adding methyl alcohol to water.
Methyl alcohol is volatile. \[ CH_3OH \] It contributes its own vapour pressure to the solution. So the total vapour pressure of solution may increase. Because of higher vapour pressure, the solution reaches atmospheric pressure at a lower temperature. Therefore, boiling point decreases. \[ \boxed{CH_3OH \Rightarrow \text{boiling point decreases}} \] Hence: \[ \boxed{\text{KCl raises boiling point because it is non-volatile.}} \] \[ \boxed{\text{Methyl alcohol lowers boiling point because it is volatile.}} \]