| The
boiling point diagram shows how the
equilibrium compositions of the components in a liquid mixture
vary with temperature at a fixed pressure. Consider an example
of a liquid mixture containing 2 components (A and B) - a binary
mixture. This has the following boiling point diagram.
 The boiling point of A is that at which the mole fraction of A is
1. The boiling point of B is that at which the mole fraction
of A is 0. In this example, A is the more volatile component
and therefore has a lower boiling point than B. The upper curve
in the diagram is called the dew-point
curve while the lower one is called the bubble-point
curve.
The
dew-point is the temperature at which the saturated vapour
starts to condense.
The
bubble-point is the temperature at which the liquid starts
to boil.
The
region above the dew-point curve shows the equilibrium composition
of the superheated
vapour
while the region below the bubble-point curve shows the equilibrium
composition of the subcooled
liquid.
For
example, when a subcooled liquid with mole fraction of A=0.4
(point A) is heated, its concentration remains constant until
it reaches the bubble-point (point B), when it starts to boil.
The vapours evolved during the boiling has the equilibrium composition
given by point C, approximately 0.8 mole fraction A. This is
approximately 50% richer in A than the original liquid.
This
difference between liquid and vapour compositions is the basis
for distillation operations.
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