Liquid State Vapour Pressure

Vapour Pressure Definition

The word vapour refers to particles of moisture that is suspended in the air; in other words, the particles of a liquid in a gaseous state. The pressure is basically force per unit area. So, vapour pressure meaning is the pressure of the vapour when in contact with its solid or liquid form. 


When a substance is put in a container that is constantly heated, the molecules of the substance are shown to travel in various directions at different speeds, which is attributed to the various levels of kinetic energy exhibited by the molecules of the liquid. The energy of the molecules increases as the material becomes heated; it gets lighter and rises. This is called evaporation.


The compounds that can be seen on the surface of the liquid are labelled vapours. The evaporation occurs at a steady pace, holding the temperature of the liquid stable.  If any molecules in the vapour process touch the walls of the containers, they are transformed back to the liquid phase. This is called condensation

Stage of Equilibrium

As time progresses, the amount of molecules in the vapour decreases gradually, while the rate of condensation increases. It hits a point where the evaporation rate is equivalent to the condensation rate. This phase is called the stage of equilibrium.

Vapour Pressure of Liquid

During the stage of equilibrium, the pressure of the molecules on the walls of the container is called the vapour pressure of the liquid. Vapour pressure is defined as the pressure exerted by the vapour above the liquid.

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Factors that Affect Evaporation

Nature of Liquid

Liquids have weak intermolecular forces. Heating the molecules of the liquid can help to break these forces and convert them to the vapour phase and thus increase the vapour pressure of the liquid. For e.g., acetone and benzene have a higher vapour pressure than water at a given temperature. 


The vapour pressure of the liquid increases with an increase in temperature because it increases the kinetic energy of the molecules enabling them to break away into the vapour phase faster.

Raoult's Laws

Raoult's law states that the partial vapour pressure of a solvent in a solution is equivalent to the vapour pressure of a pure solvent determined by its mole fraction in the solution.


It is represented by the equation: PSolution = Χsolvent x P0solvent , where, PSolution = Vapour Pressure of the solution, Χsolvent = mole fraction of the solvent, P0solvent = Vapour Pressure of the pure solvent. 


It is obvious from Raoult's rule that, as the mole fraction of the portion decreases, its partial pressure also decreases during the vapour phase.


Although this law is elegant, it can only be applied to ideal solutions. In an ideal solution, the solvent-solute interaction is identical to the solvent-solvent or solute-solute interaction. This implies that both the solute and the solvent require equal amounts of energy to emerge from the liquid phase to the vapour phase as they do in their pure forms.

Latent Heat of Vaporisation

When one mole of a liquid transforms into a gaseous state at its boiling point at the atmospheric pressure, the amount of heat taken up by the liquid to change into the vapour phase is known as the latent heat of vaporisation.

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As we supply heat to a liquid, its kinetic energy increases, resulting in an increase in the overall temperature. At the boiling point, the latent heat is used by the molecules to overcome the intermolecular forces of attraction in the liquid and to convert to the gaseous state, instead of increasing the temperature. This heat is known as latent heat because it is not reflected as a change in temperature but is required to break the intermolecular forces of attraction. 

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Vapour Pressure and Boiling Point

As we continue to increase the temperature of the liquid, its vapour pressure increases correspondingly. It reaches a stage where the vapour pressure of the liquid is equal to the atmospheric pressure. At this point, vapours at the surface tend to escape to the atmosphere, and the substance undergoes a phase change. The temperature is defined as the boiling point of the liquid. The boiling point of any given liquid is given corresponding to the pressure of 1 atm = 105 Pa. 

Relation Between Vapour Pressure and Boiling Point

Vapour Pressure and boiling point are inversely proportional. If the vapour pressure of a liquid is low, the boiling point is high, and vice-versa. Vapour Pressure is inversely proportional to intermolecular forces of attraction. 

  • Lesser intermolecular forces mean the liquid has a higher vapour pressure. Less heat energy needs to be applied to split the molecules, and the boiling point will be quite low. 

  • Conversely, if the intermolecular forces are strong, the molecules will be strongly attracted to each other. Fewer molecules break away and convert into vapours. Hence, the vapour pressure will be low. In this case, the boiling point will be high.

FAQ (Frequently Asked Questions)

1. What is the Difference Between Vapour Pressure and Atmospheric Pressure? 

The pressure exerted by the earth's atmosphere at any given point is the sum of the mass of the atmospheric column of the unit area above that point and of the gravitational acceleration at that level. This is atmospheric pressure. The atmospheric pressure is directly proportional to the boiling point of a liquid. When a liquid is trying to escape into a gaseous state, the atmospheric pressure limits its ability to break away by putting pressure on it.  

The vapour pressure is defined as the pressure exerted by the vapours above the liquid. The vapour pressure is inversely proportional to the boiling point of a liquid. When a liquid is trying to escape into a gaseous state, the vapour pressure makes the liquid molecules away from each other by exerting a force in an outwards direction from the liquid. 

2. What will Happen When Water is Placed in a Vacuum Chamber?

If you place a container filled with water in a vacuum chamber, the water will evaporate completely until it fills the entire vacuum chamber with water vapour. The vapour generated will push against the walls of the chamber as well as the water in the container. This vapour pressure will  continue to rise until it stops the water from giving out any more vapours.