Explain the vapour pressure of a liquid.

(N/A) Equilibrium or Saturated Vapour Pressure: If an evacuated container is partially filled with a liquid,a portion of the liquid evaporates to fill the remaining volume of the container with vapour.
Initially,the liquid evaporates and the pressure exerted by the vapours on the walls of the container (vapour pressure) increases. After some time,it becomes constant,and an equilibrium is established between the liquid phase and the vapour phase. The vapour pressure at this stage is known as 'equilibrium vapour pressure' or 'saturated vapour pressure'.
Since the process of vaporisation is temperature-dependent,the temperature must be mentioned while reporting the vapour pressure of a liquid.
Boiling and Boiling Point:
- Boiling: When a liquid is heated in an open vessel,it vaporises from the surface. At the temperature where the vapour pressure of the liquid becomes equal to the external pressure,vaporisation occurs throughout the bulk of the liquid,and vapours expand freely into the surroundings. This condition of free vaporisation throughout the liquid is called boiling.
- Boiling Point: The temperature at which the vapour pressure of a liquid is equal to the external pressure is called the boiling point temperature at that pressure.
Normal and Standard Boiling Point: At $1 \text{ atm}$ pressure,the boiling temperature is called the 'normal boiling point',and at $1 \text{ bar}$,the boiling point is called the 'standard boiling point'. Note that the standard boiling point $(1 \text{ bar})$ is less than the normal boiling point $(1 \text{ atm})$.
e.g.,$\text{Standard boiling point of } H_2O = 99.6^{\circ}C \ (372.6 \ K) < \text{Normal boiling point of } H_2O = 100^{\circ}C \ (373 \ K)$,because $1 \text{ bar} < 1 \text{ atm}$.
Effect of Pressure on Boiling Point: If external pressure decreases,the boiling point decreases; if external pressure increases,the boiling point increases.
Examples:
$1$. On mountains,atmospheric pressure is lower,so the boiling point of water is lower,making it difficult to cook food properly.
$2$. In a pressure cooker,pressure is increased,which increases the boiling point of water,allowing for faster and more efficient cooking.
$3$. In hospitals,surgical instruments are sterilized in autoclaves where the boiling point of water is increased by raising the pressure above atmospheric pressure.