Question

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

Hint:

Non-volatile solutes elevate the boiling point of a solvent, whereas volatile solutes may lower the boiling point if they increase the total vapour pressure of the solution.

Answer 1

Step 1: KCl is non-volatile and raises boiling point.
KCl dissociates: $\text{KCl} \rightarrow \text{K}^+ + \text{Cl}^-$. Being non-volatile, the ions do not contribute to vapour pressure. By Raoult's law, $P_{\text{solution}} < P^\circ_{\text{water}}$. A higher temperature is needed to match atmospheric pressure, so boiling point INCREASES ($\Delta T_b = iK_bm$, with $i \approx 2$ for KCl).
Step 2: Methyl alcohol is volatile and lowers boiling point.
$\text{CH}_3\text{OH}$ is a volatile liquid with its own vapour pressure. Both components contribute to the total: \[P_{\text{total}} = P_{\text{water}} + P_{\text{methanol}} > P^\circ_{\text{water}}\] The elevated vapour pressure allows atmospheric pressure to be reached at a LOWER temperature. Boiling point DECREASES.
Step 3: Key principle.
Non-volatile solutes reduce vapour pressure and raise the boiling point (colligative effect). Volatile solutes raise total vapour pressure and lower the boiling point. The volatility of the added substance is the deciding factor.