10 · GASES, LIQUIDS AND SOLIDSExperiments show that at pressures below 1 atm (101 kPa), the ratio PV/RTdoes
not deviate appreciably from 1 for most gases. This gives us a rough guide – most
real gases behave nearly ideally if their pressure is below 1 atm. Even at a pressure
of several atmospheres, many gases behave sufficiently ideally that the use of the
ideal gas equation remains justified.
A plot of PV/RTagainstPfor three gases is shown in Fig. 10.10. Deviations from
ideal behaviour occur because, above 1 atm pressure, the attractive forces between
the gas molecules can no longer be neglected. A single particle striking the wall of the
containing vessel will be slightly attracted (and therefore ‘held back’) by other
molecules in the bulk of the gas. As a consequence, the force with which the molecule
strikes the container wall is reduced. On a bulk scale this causes the pressure of the gas
to be lowerthan that of an ideal gas. Because Pis lower than that for an ideal gas, the
ratioPV/RTis smaller than it would be for an ideal gas. This is evident at moderately
high pressures in Fig. 10.10. For example, PV/RTfor CO 2 is less than 1 between 10
and 600 atm.164
Fig. 10.10Deviations from ideal gas behaviour. The behaviour of a gas following the ideal gas
equation is shown by a continuous line. (– – –) is for H 2 at 273 K; (····) is for N 2 at 273 K; (– · – ·)
is for CO 2 at 313 K.ideal gasDeviation from
ideal gas
behaviourWhy would you expect
carbon dioxide to deviate
from ideal behaviour more
than hydrogen or nitrogen?Exercise 10J
At yet higher pressures, it is no longer reasonable to assume that the volume of
the gas molecules is negligible compared with the container volume. This makes
PV/RTgreater than 1 because the volume Vis greater than that for an ideal gas. For
CO 2 , this is observed at pressures above 600 atm.Adsorption of gases on solids
Charcoal is used in gas masks to remove toxic gases. This is one example of a
general phenomenon known as adsorption.
In adsorption, a substance (the adsorbate) sticks to the surfaceof another material
(theadsorbent). This is distinct from absorption, in which a substance penetrates
another material. The difference between the two phenomena may be remembered by
thinking of a bath sponge. If dust sticks to the surface of the dry sponge then adsorp-
tion has occurred, but if water is taken into the sponge, absorption has occurred.
It is often impossible to decide whether adsorption, absorption or a mixture
of both are occurring. For this reason, the noncommittal term sorptionis used for
both processes. An example of this difficulty is found in chromatography, where a10.6