Why is the vapour pressure of liquid constant at a constant temperature ?

Explanation

**Step-by-Step Solution:**

1. **Understanding Vapour Pressure:**
- Vapour pressure is defined as the pressure exerted by the vapor of a liquid when it is in equilibrium with its liquid phase. This means that the rate of evaporation of the liquid equals the rate of condensation of the vapor back into the liquid.

2. **Equilibrium State:**
- At equilibrium, the number of molecules escaping from the liquid to the vapor phase is equal to the number of molecules returning from the vapor phase to the liquid. This dynamic process results in a constant pressure of the vapor above the liquid.

...