The phenomenon of lowering of vapour pressure is defined s :

Options

A. Decrease in vapour pressure of a solvent on addition of a volatile non-electrolyte solute in it.
B. Decrease in vapour pressure of a solvent on addition of a non-volatile non-electrolyte solute in it.
C. Decrease in vapour pressure of a solvent on addition of a volatile electrolyte solute in it.
D. Decrease in vapour pressure of a solvent on addition of a non-volatile solute in it.

Answer

**Step-by-Step Text Solution:**

1. **Understanding Vapor Pressure:**
- Vapor pressure is the pressure exerted by the vapor of a liquid in equilibrium with its liquid phase at a given temperature. It indicates how readily a substance will evaporate.

2. **Definition of Lowering of Vapor Pressure:**
- The lowering of vapor pressure refers to the phenomenon where the vapor pressure of a solvent decreases when a non-volatile solute is added to it.

3. **Role of Non-Volatile Solute:**
- A non-volatile solute is a substance that does not readily evaporate and thus does not contribute to the vapor pressure of the solution. When it is added to a solvent, it occupies space at the surface of the liquid, reducing the number of solvent molecules that can escape into the vapor phase.

4. **Colligative Properties:**
- The lowering of vapor pressure is one of the colligative properties of solutions, which depend on the number of solute particles in a given amount of solvent, rather than the identity of the solute.

5. **Conclusion:**
- Therefore, the phenomenon of lowering of vapor pressure is defined as the decrease in the vapor pressure of a solvent when a non-volatile electrolyte solute is dissolved in it.

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