A Breakdown | What Is Vapour Pressure?

April 23, 2021

Vapour pressure, Vp, is a pressure where vapour and liquid phases are in equilibrium at a given temperature. In short, it is a measure of the tendency for a substance to change into a gaseous/vapour state.

This property does not depend upon quantity. It can be calculated by using the Antoine equation which expresses vapour pressure as a function of temperature which can be seen below:

LogP =  A – [B/(T +C)]


T is the temperature of liquid or substance

P is the vapour pressure of a liquid or substance.

A , B and C are constant.

The equation can be rearranged to calculate temperature as follows:

T = [B/(-LogP + A )] – C

In fractional distillation, this property plays an important role as the design of the column depends upon vapour pressure differences.

What Are The Units Of Vapour Pressure?

In general value of Vp is measured in the same units of pressure. As we know that there are different units available for a measure of pressure like kg/cm2 , PSI , newton per meter square, kPa, bar, pascal, and much more.

Factors That Affect Vapour Pressure

Some of the main and key factors which affect the vapour pressure are:

  • Temperature
  • Solute concentration and nature
  • Boiling point


As you increase the temperature of the solid or liquid in a system then its Vp will also increase and vice versa for when it decreases.

Solute concentration and nature

If you add more non-volatile solute to dissolve into a volatile solvent then the vapour pressure of the solvent will reduce, hence, in this example the more solute you add the lower the vapour pressure of the solute gets. 

Factors That Do Not Affect Vapour Pressure


Vp does not increase or decrease with respect to the volume of its system.

Surface Area

The surface area of the solid or liquid in contact with the gas will have no effect on the vapour pressure of the system.

What is Raoult’s Law? 

This is a law that states the relationship between pressure and temperature of a molecule or liquid in an ideal solution. It essentially states that the vapour pressure of a solvent above a solution is equal to the vapour pressure of the pure solvent at the same temperature and this is scaled by the mole fraction of the solvent that is present. It can be represented in the formula below:

P_{solution} = X_{solvent}.P_{solvent}

Where the P solution is the vapour pressure of the solution, X solvent is the mole fraction of the solvent, and P solvent is the vapour pressure of the pure solvent.

Raoult’s law can be used to estimate the contribution of individual components of a liquid or solid mixture to the total pressure exerted by the system.

We can use Raoult’s Law to calculate the vapour pressure of a given liquid. So Raoult’s Law is very helpful in the design of distillation columns. Using Raoult’s Law we can calculate the required temperature under a given vacuum in a distillation system. 

Vapour Pressure of Water

Vp value of water at 25 degrees Celsius is, 23.8 mm of mercury at 100 degrees Celsius water will be boiled so the Vp value is equivalent to one atmosphere.

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