IB Chemistry - Equilibrium

IB Chemistry home > Syllabus 2016 > Equilibrium > Liquid - vapour equilibrium

Syllabus ref: 7.1

The water cycle is possibly one of the first things studied in science. Students are aware from an early age that water exists in the sea, the air and in the steam from a kettle.

Vapour pressure

The vapour pressure is the pressure exerted by a liquid vapour above its surface. The particles of vapour behave just like a normal gas in that they move, collide with each other and the walls of the container.

They have all the characteristics of a normal gas, i.e. temperature, pressure and volume.

The liquid does not have to be in an enclosed container; it has just the same vapour pressure in an open container. However, in an open container air draughts and diffusion can carry the vapour away (to then be replaced by more from the body of the liquid).


The nature of the liquid

The identity of the liquid is a very important factor, as volatile liquids form vapours more easily than non-volatile liquids. The intermolecular forces present in the liquid define the vapour pressure of the liquid.

Substances with weak intermolecular forces vaporise more easily and have higher vapour pressures at a given temperature. Hence the vapour pressure of ethoxyethane is much higher than that of water at the same temperature.

Volatile liquids produce vapour easily and have a greater vapour pressure at a given temperature. The volatility of a liquid depends on the intermolecular forces of attraction between the particles of the liquid.

Weak forces give rise to volatility and higher vapour pressures, as the particles of the liquid can more easily escape the forces holding them.

The volatility of a liquid can be assessed by considering its boiling point. Low boiling point liquids, such as ethoxyethane or ethanal, are volatile.

Ethanol, b.p. 79ºC is more volatile than water, b.p.100ºC, whereas methanol, b.p. 56ºC, is more volatile than ethanol.



The temperature is a measure of the average kinetic energy of the particles in a system. As the temperature increases so does the average kinetic energy. However, as we have seen earlier, the energy of the particles is not distributed evenly. The distribution is statistical, with very few particles having low or very high energies.

But, the proportion of particles with enough energy to escape the clutches of the attractions within a liquid increases as the temperature increases. This means that the rate of vaporisation increases with temperature.

If the system is open, the rate of evaporation is greater than before. This is why clothes dry faster on a warm day.

However, if the system is closed, the result is an equilibrium with more vapour and less liquid. This can be tested by measuring the pressure within a container as the temperature is increased.

The vapour pressure is only dependent on the temperature. It is independent of the volume of liquid, the surface area and the container.

Increasing the temperature provides more energy for the particles and increased the likelihood of the process:

liquid vapour

Particles are held in the liquid by the forces of attraction between them. When a particle gains sufficient energy it can overcome this force of attraction and leave the body of the liquid.

Clearly, it follows that substances which have large forces of attraction between particles cannot form vapour as easily. They have a lower vapour pressure at any given temperature.

The higher the temperature the more energy is available to the particles to break free from the liquid body. The relationship between vapour pressure and temperature is not linear, but is nearly so. This can be demonstrated using the following apparatus:

Vapour exerts a pressure just like any gas, that is due to collisions with the walls of any container and with other particles. The greater the concentrations of vapour the greater the pressure.