Question

The boiling point of solution increases when 1 mol of NaCl is added to 1 litre of water while addition of 1 mol of methanol to one litre of water decreases its boiling point. Explain the above observations.

Hint:

Boiling point elevation is a colligative property, influenced by vapor pressure.
- Non-volatile solutes \(\rightarrow\) decrease vapor pressure \(\rightarrow\) increase boiling point.
- Volatile solutes \(\rightarrow\) increase vapor pressure \(\rightarrow\) decrease boiling point.

Answer 1

Observation 1: NaCl + Water (Boiling Point Increases)
- NaCl is a non-volatile solute.
- Its addition causes the vapour pressure of water to decrease.
- Hence, a higher temperature is required to reach atmospheric pressure, leading to boiling point elevation.
Observation 2: Methanol + Water (Boiling Point Decreases)
- Methanol is a volatile liquid.
- It increases the total vapour pressure of the solution.
- This results in the solution boiling at a lower temperature, i.e., boiling point depression.