Chemistry, Third edition

10 · GASES, LIQUIDS AND SOLIDS

The vapour pressure increases as soon as the container is sealed, but the rate of

increase slows down after a few seconds because, although water molecules continue

to evaporate, some water molecules also condense back to form liquid water. After a

few more seconds the vapour pressure levels off and reaches a maximum called the

saturated vapour pressure– so-called because the air is saturated with water vapour

at that temperature. For pure water at 20C, the saturated vapour pressure is

2.33 kPa. Even at this stage, molecules continue to evaporate and condense, but a

dynamic equilibriumhas been achieved in which the number of molecules evaporat-

ing per second equals the number of molecules which are condensing per second

(Fig. 10.11(b)). For this reason, saturated vapour pressure is often called equilibrium

vapour pressure. We represent the equilibrium by the equation

liquid vapour

If the container was open to the air, the water vapour would continually diffuse

away and the vapour pressure would not reach its equilibrium value. More and more

water would slowly evaporate into the air until (after several days) no more water

was left in the container.

Figure 10.12 shows the saturated vapour pressure of water at various tempera-

tures. The figure also shows the way that the saturated vapour pressure of ethanol

varies with temperature. The vapour pressure of water and ethanol (like that of all

liquids) increases with temperature. This is expected, since at the higher tempera-

ture more liquid molecules have sufficient energy to escape the attractive forces

holding molecules in the liquid.

At the boiling point of a liquid, the saturated vapour pressure equalsthe external air

pressure. At sea level, the air pressure is 1 atm (101 kPa). The boiling point of a liquid

at 1 atm external pressure is called the normal boiling pointof the liquid. The normal

boiling points of water and ethanol are 100C and 78C, respectively.

166

Fig. 10.11Vapour pressure: (a) experimental measurement of saturated vapour pressure;

(b) the dynamic equilibrium that gives rise to the saturated vapour pressure.

Fig. 10.12The vapour pressure of

ethanol and water between 10 °C and

100 °C.

Boiling point

Explain the following:

(i)The boiling point of

water drops with

increasing altitude.

(ii)Salty water boils at a

higher temperature than

pure water.

Exercise 10K

For more on boiling,

see Appendix 10 on

the website