Vapour pressure | Factors of vapour pressure

The pressure exerted by the vapours of liquid in equilibrium with the liquid at a given temperature is called vapour pressure.

Example

Vapour pressure of water at 100 centigrade in one atm.

Condensation

The process in which gas molecules are converted into liquid is called condensation.

Dynamic equilibrium

When the rate of evaporation and the rate of condensation are equal to each other, dynamic equilibrium is established is called dynamic equilibrium.

Vapour pressure is independent of the surface area of liquid. Why?

Evaporation increases with the increase of temperature and condensation is a process in which gas molecules are converted into liquid.Which is equal to the evaporation if the surface area is same.So number of molecules of a liquid per unit surface area are same.Hence the vapour pressure is independent of the surface area and the amount of liquid. 

Factors of vapour pressure

There are two factors of vapour pressure.

Temperature

Temperature is directly proportional to vapour pressure.If temperature is increased, the vapour pressure also increases and vice versa.

Example

At zero degree centigrade temperature vapour pressure of water is 4.579 torr and at 10 degree centigrade temperature, vapour pressure of water is 9.209 torr.

Intermolecular forces

Intermolecular forces are inversely proportional to pressure.Vapour pressure is increased with the decrease in intermolecular forces and vice versa. 

Example

At 20 degrees centigrade temperature, the vapour pressure of water is 17 torr and vapour pressure of ether is 4.42 torr. Due to the presence of hydrogen bonding in water and absence of hydrogen bonding in ether.