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4 LIQUIDS AND SOLIDS eLearn.Punjab

The number of molecules leaving the surface is just equal Animation 4.6 : Vapour Pressure
to the number of molecules coming back into it at a constant Source & Credit: mecaflux
temperature. The molecules which are in the liquid state at any
moment may be in vapour state in the next moment.
The magnitude of vapour pressure does not depend upon the
amount of liquid in the container or the volume of container. It also
does not depend on surface area of a liquid. The larger surface area
also presents a larger target for returning the molecules, so the
rate of condensation also increases.

Vapour Pressure Increases with Temperature

The values of vapour pressures of various liquids depend fairly upon the nature of

liquids i.e. on the sizes of molecules and intermolecular forces, but the most important parameter

which controls the vapour pressure of a liquid is its temperature. At an elevated temperature, the

kinetic energy of molecules is enhanced and capability to leave the surface increases.

It causes the increase of vapour pressure. Table(4.3) Tablc (4.3) Vapour pressures of water
shows change in vapour pressure of water at different (torr) at various temperatures

temperatures. The Table (4.3) shows that increases

of vapour pressure goes on increasing for the same Temperature Vapour
Pressure
difference of temperature from 0°C to 100°C for water. There is (0C)
(Torr)
increase of vapour pressure from 4.579 torr to 9.209 torr for change 4.579
9.209
of temperature from 0°C to 10°C. But the increase is from 527.8 torr 0 17.54
to 760 torr when temperature changes from 90°C to 100°C. 10
The difference in the strength of intermolecular forces in different 20

liquids is directly related to their vapour pressures at a particular 30 31.82

temperature. The stronger the intermolecular forces the lower 37 47.07
the vapour pressure. The following Table (4.4) shows that at 20 °C 40 55.32
isopentane has the highest vapour pressure, while glycerol has the 50 92.51
lowest. 60 149.4
70 233.7

80 355.1

90 527.8

100 760.0

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