168 10 · GASES, LIQUIDS AND SOLIDS
Critical temperature and pressure
If the volume of a gas (or vapour) is reduced by compressing it (as in a piston), or
cooling it, or both, the gas may condense to a liquid. Experiments show that a gas
cannot be liquefiedby pressure alone unless it is at (or below) a temperature known as
thecritical temperature. Put another way, the critical temperature is the highest
temperature at which a liquid can exist.
At a molecular level, molecules in a sample at a temperature above the critical
temperature simply have too much energy to ‘stick together’ – no matter what the
applied pressure.
The pressure needed to liquefy a gas at its critical temperature is called its critical
pressure. The critical temperatures and pressures for some gases are given in Table
10.3. For example, if ethanol vapour is above 243 C, no amount of pressure will
convert the vapour to liquid. If the ethanol vapour were exactly at a temperature of
243 C, a pressure of 63 atm would need to be applied to the vapour in order to force
it to condense to a liquid.10.8
BOX 10.3
Flash point of solvents
Table 10.2 Flash points and ignition temperatures of common solvents
Solvent Flash point/°C Boiling point/°C Auto-ignition
temperature/°C
Propanone (acetone) 20 56 538
Benzene 11 80 536
Ethoxyethane 40 34.5 170
(diethyl ether)
Ethanol 13 78 425
30–40 petroleum spirit 51 30–40 230
Butan-1-ol 29 118 365Many solvents are good fuels, and will readily
burn in air. This can be a safety hazard in labora-
tories and factories. The temperature at which a
solvent catches fire in the absenceof a flame is
called its auto-ignition temperature. This is sur-
prisingly high for most solvents (Table 10.2).
The temperature at which the vapour
pressure of a pure solvent in air is sufficient
to be ignited by a naked flame is called the
flash pointof the solvent (Fig. 10.14). This
can be very low (Table 10.2). For example,
the flash point of petroleum ether (a fraction
obtained from the distillation of crude oil and
which boils between 30 and 40 °C) is 51 C.
This means that petroleum ether stored in an
open (or leaking) container will still represent
a serious fire hazard even if stored in a
freezer operating at 51 C. Flash points are
used to classify the flammabilityof solvents.Aflammableliquid is one with a flash point
less than 55°C, a highly flammableliquid is
one with a flash point less than 21°C and an
extremely flammableliquid is one with a
flash point less than 0°C and a boiling point
of 35°C or less.Fig. 10.14Solvent flash point.Critical
temperature and
pressure(i)A mixture of hydrogen
and oxygen at 119 C
is subjected to a very
high pressure. Would
you expect liquid
hydrogen and liquid
oxygen to condense?
(ii)Historically, hydrogen
was one of the last
gases to be liquefied in
the laboratory – why?
(HintWhat is the boiling
point of H 2 (I)?)Exercise 10M