Chemistry, Third edition

VAPOUR PRESSURE

combination of solute absorption and adsorption may bring about the separation of

a mixture on a chromatographic column (Chapter 19).

The forces that bind the adsorber to adsorbent may be physical (i.e. intermolecu-

lar forces) or chemical (when chemical bonds are formed). The adsorption of gases

by charcoal is physical, whereas the adsorption of gases on some catalysts is chemi-

cal. Toxic gases adsorb to charcoal better than the oxygen and nitrogen of the air

because they are often relatively large molecules and are frequently polar. This

increases the intermolecular forces between adsorbate and adsorbent.

The charcoal used in gas masks is called ‘activated charcoal’, and is made by

heating wood or coal in carbon dioxide, water vapour or in a limited supply of air.

Activated charcoal has an enormous surface area – crucial to adsorption – as high as

1000 m^2 of gas per gram of charcoal! At a given temperature and pressure, a sample of

activated charcoal will adsorb only a fixed mass of adsorbate, but the mass increases

as the pressure of adsorbate gas is increased or if the temperature is lowered. The

charcoal may be regenerated by heating, which drives off the adsorbate.

Adsorption will also occur in solution. For example, charcoal beds are used to

adsorb trace organic compounds (such as pesticides and dyes) from water.

Vapour pressure

Meaning of vapour pressure

A vapour is a gas in contact with a liquid of the same substance. For example, the

water ‘gas’ above the surface of liquid water is described as water vapour. The gas

pressure of the water vapour is known as its vapour pressure.

Suppose that liquid water is placed into a container and that the gas (mainly air)

above the water is pumped away using a powerful pump, and the container sealed

without allowing air to re-enter. Some of the liquid water then evaporates so that the

only gas in the space above the liquid water is water vapour. Figure 10.11(a) shows

how a pressure measuring device might be used to measure the vapour pressure of

the water.

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Gas masks contain activated charcoal which absorbs toxic

gases in the surrounding air. This photograph of a family out

shopping in London was taken during a tear gas test in 1941

(World War II).