Question

The effect of adding a non-volatile solute to a solvent is:
(this question has multiple correct options)
a. To lower the vapour pressure
b. To increase its freezing point
c. To increase its boiling point
d. To decrease its osmotic pressure

Answer 1

Hint: Apply the concept of colligative properties in this question.Colligative properties are those which depend on the number of solute particles present in the solution. A non-volatile solute is the one which does not evaporate into gas.

Complete step by step answer:
Certain properties of dilute solutions containing non-volatile solute do not depend on the nature of the solute dissolved but depend only on the concentration. Such properties are called colligative properties.
There are four colligative properties –
1. Vapour pressure
- It follows Raoult’s law.
- When a non-volatile substance is dissolved in a liquid, the vapour pressure of the liquid or solvent is lowered.
- Therefore, option (a) is correct.
2. Freezing point
- When a non-volatile substance is dissolved in a liquid, the freezing point of the liquid or solvent is lowered.
- This is known as depression in freezing point.
- Therefore, option (b) is incorrect.
3. Boiling point
- When a non-volatile substance is dissolved in a liquid, the boiling point of the liquid or solvent increases.
- This is known as elevation in boiling point.
- Therefore, option (c) is correct.
4. Osmotic pressure
- When a non-volatile substance is dissolved in a liquid, the osmotic pressure of the liquid or solvent increases.
- Therefore, option (d) is incorrect.

Therefore, the answer is – option (a) and option (c).

Note: We can keep the following points in mind –
1. Study of elevation in boiling point of a liquid in which a non-volatile solute is dissolved is known as ebullioscopy.
2. Study of depression in freezing point of a liquid in which a non-volatile solute is dissolved is known as cryoscopy.