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

Step 1: Recall the link between vapour pressure and boiling point.
A liquid boils when its vapour pressure equals atmospheric pressure. Lower vapour pressure means the solution needs more heat to boil (higher b.p.); higher vapour pressure means it boils at a lower temperature (lower b.p.).
Step 2: Effect of KCl on boiling point of water.
KCl is a non-volatile ionic solute: $KCl \rightarrow K^+ + Cl^-$. These ions do not escape into the vapour phase. By Raoult's law, adding a non-volatile solute reduces the mole fraction of water and lowers the vapour pressure of the solution: \[ P_{solution} = X_{water} \cdot P^*_{water} < P^*_{water} \] Reduced vapour pressure means a higher boiling point. This is the colligative property of elevation of boiling point. 1 mol KCl gives 2 mol particles (van't Hoff factor $i=2$), significantly raising the boiling point.
Step 3: Summarise the KCl effect.
KCl is a non-volatile solute. It lowers the vapour pressure of water. Lower vapour pressure raises the boiling point. So the boiling point of water INCREASES on addition of KCl.
Step 4: Effect of methyl alcohol on boiling point of water.
Methyl alcohol (methanol, $CH_3OH$) is a VOLATILE liquid with its own significant vapour pressure (b.p. $64.7^\circ C$). When added to water, both water and methanol contribute to the total vapour pressure: \[ P_{total} = X_{water}P^*_{water} + X_{methanol}P^*_{methanol} \] Since methanol has a high vapour pressure, $P_{total}$ of the mixture is higher than $P^*_{water}$ alone at any temperature. Higher vapour pressure means the solution reaches atmospheric pressure at a lower temperature, so the boiling point DECREASES.
Step 5: Key distinction.
KCl: ionic, non-volatile, reduces vapour pressure, raises b.p. Methanol: molecular, volatile, increases total vapour pressure, lowers b.p. The volatility of the solute is the key difference.
Step 6: State the conclusions.
KCl (non-volatile) lowers vapour pressure of water, elevating the boiling point. Methyl alcohol (volatile) raises the total vapour pressure of the solution, reducing the boiling point. \[ \boxed{\text{KCl (non-volatile): raises b.p.;\; Methanol (volatile): lowers b.p.}} \]