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4 LIQUIDS AND SOLIDS eLearn.Punjab

This heat will only be utilized to break the intermolecular forces and convert the liquid into its
vapours. The amount of heat required to vapourize one mole of a liquid at its boiling point is
called its molar heat of vapourization. The molar heat of vapourization of water is 40.6 kjmol-1.
The boiling points of some commonly available liquids at one atmospheric pressure are shown in
the Table (4.5).

Table (4.5) Boiling points of some

common liquids at 760 torr.

Liquid B.P (0C) Liquids B.P (0C)

Acetic Acid 118.50 Carbontetrachloride 76.50

Acetone 56.00 Ethanol 78.26

Aniline 184.4 Naphthalene 218.00

Benzene 80.15 Phenol 181.80

Carbondisulphide 46.30 Water 100.00

The Fig. (4.12) shows the variation of vapour pressure of water, ethyl alcohol, ethylene glycol
and diethylether with temperature. It shows that the liquids reach upto their boiling points when their
vapour pressures are equal to 760 torr at
sea level. The way these curves start at 0
0C is interesting. Water takes start at 4.8
torr while diethyl ether at around 200 torr.
This is due to difference in the strengths
of their intermolecular forces. The curve
for water goes alongwith temperature
axis to a greater extent at the beginning
as compared to ether. It means that water
can hardly overcome its intermolecular
forces at low temperatures. It is clear
from the curves that the vapour pressure
increases very rapidly when the liquids
are closer to their boiling points.

Fig (4.12) Vapour pressures(torr) of four common liquids shown as a
function of temperature(°C).

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