What Is Vapour Pressure
Consider a liquid in a closed container. heat is given to the container. So liquid inside the container starts to evaporate. Now the molecules of liquid in the gaseous state remain in the container above the surface of the liquid.
These gases molecule continue to remain in a random motion. They collide with each other and also with a wall of a container and again return to the liquid state. This process is opposite to the evaporation called as condensation. We can say that the process of evaporation and the process of condensation takes place simultaneously.
After some interval of time, the equilibrium is established between these processes and the rate of evaporation becomes equal to the rate of condensation.
The pressure exerted by the gaseous state of that substance when it is in equilibrium with the solid or liquid phase is called as the vapour pressure of the substance.
Vapour pressure of liquid increases with an increase of temperature. The concept of the boiling point also depends on the vapour pressure and it is defined as the temperature at which vapour pressure of liquid becomes equal to external pressure. External pressure may be atmospheric pressure.