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PHASE EQUILIBRIA INVOLVING TWO-COMPONENT SYSTEMS: PARTITION 205

5.4 Phase equilibria involving two-component
systems: partition

Why does a fizzy drink lose its fizz and go flat?

Equilibrium constants of partition

Drinks such as lemonade, orangeade or coke contain dissolved CO 2 gas. As soon as
the drink enters the warm interior of the mouth, CO 2 comes out of solution, imparting
a sensation we say is ‘fizzy’.
The CO 2 is pumped into the drink at the relatively high pressure of about 3 bar.
After sealing the bottle, equilibrium soon forms between the gaseous CO 2 in the space
above the drink and the CO 2 dissolved in the liquid drink (Figure 5.14). We say the
CO 2 is partitioned between the gas and liquid phases.
The proportions of CO 2 in the space above the liquid and in the liquid are fixed
according to an equilibrium constant, which we call the partition constant:
amount of CO 2 in phase 1
K (partition) = (5.8)
amount of CO 2 in phase 2

We need to note how the identities of phases 1 and 2 must be
This equilibrium con-
defined before K can be cited. We need to be aware that K (partition)
is only ever useful if the identities of phases 1 and 2 are defined. stant is often incor-
On opening the drink bottle we hear a hissing sound, which rectly called a ‘partition
function’ – which is in
occurs because the pressure of the escaping CO 2 gas above the
fact a term from statis-
liquid is greater than the atmospheric pressure. We saw in Chapter 4
tical mechanics.
that the molar change in Gibbs function for movement of a gas is
given by

p (final)
G = RT ln (5.9)
p (initial)

Carbon dioxide
in the gas phase

Carbon dioxide
dissolved in solution

Figure 5.14 In a bottle of fizzy drink, carbon dioxide is partitioned between the gas and the
solution phases