In an ideal solution of two liquids, both components obey Raoult's
Law. However, it has been experimentally observed that, for
real solutions at low concentrations, although the solvent (the
major component of the solution) usually obeys Raoult's Law,
the solute (the minor component of the solution) does not. The
vapour pressure of the solute is proportional to its
mole fraction, but the constant of proportionality is not the
vapour pressure of the pure substance. This relationship is
defined in Henry's Law:
Note the solute is labeled as substance B to avoid confusion
with the solvent, labeled A. xB is the mole fraction
of the solute, and KB is an empirically determined
constant with the dimensions of pressure, chosen so that on
a graph of the vapour pressure of B against its mole fraction,
the Law gives a tangent to the experimental curve at xB
= 0 :
Mixtures for which the solute obeys Henry's Law and the solvent
obeys Raoult's Law are called ideal-dilute
solutions. Note KB may be greater or less
The reason that the behaviours of solvent and solute differ
so markedly at low concentrations is intuitively quite obvious.
The solvent is in large excess, so solvent molecules are likely
to be surrounded by other solvent molecules. Their environment
is very much like that of the pure liquid, and consequently
the behaviour of the solvent is very like that of the pure liquid.
The solute, on the other hand, is in low concentration, so solute
molecules are likely to be surrounded by solvent molecules.
Thus their environment is quite different from in the pure solute,
and consequently their behaviour is greatly modified.
Exceptions arise when the solvent and solute are of very similar
structure. In this instance, though the solute molecules are
still surrounded by solvent molecules, their environment is
not dissimilar to that in pure solute, their behaviour will
not be greatly altered, and both components of the mixture will
tend to obey Raoult's Law.
This also explains why the greatest deviations from ideality
are observed for strongly dissimilar liquids.