Page 241 - Physical chemistry understanding our chemical world

208 PHASE EQUILIBRIA

Aside

The reason we need to shake the two solutions together when partitioning is because
the solute only passes from one solvent to the other across the interface between them,
i.e. across the meniscus.
The meniscus is quite small if the funnel is kept still, and partitioning is slow.
Conversely, shaking the funnel generates a large number of small globules of solvent,
which greatly increases the ‘active’ surface area of the meniscus. Therefore, we shake
the funnel to increase the rate of partitioning.

Why is an ice cube only misty at its centre?

The temperature dependence of partition

Most ice cubes look misty at their centre, but are otherwise quite clear. The ice from
which the ice cubes are made is usually obtained from the tap, so it contains dissolved
impurities such as chlorine (to ensure its sterility) and gases from the atmosphere. The
mist at the centre of the ice cube comprises millions of minute air bubbles containing
these gases, principally nitrogen and oxygen.
Gaseous oxygen readily partitions with oxygen dissolved in solu-
Afish wouldnot be tion, in much the same way as the partitioning of CO 2 in the
able to ‘breathe’ in fizzy-drink example above. The exact amount of oxygen in solu-
water if it contained no tion depends on the value of K (partition) , which itself depends on the
oxygen gas. temperature.
Tap water is always saturated with oxygen, the amount depend-
ing on the temperature. The maximum concentration of oxygen in
Like all other equilib- −3 ◦
rium constants, the water – about 0.02 mol dm – occurs at a temperature of 3 C.
The amount of oxygen dissolved in water will decrease below
value of K (partition)
depends strongly on this temperature, since K (partition) decreases. Accordingly, much dis-
temperature. solved oxygen is expelled from solution as the water freezes, merely
to keep track of the constant decreasing value of K (partition) .
The tap water in the ice tray of our fridge undergoes some interesting phase changes
during freezing. Even cold water straight from a tap is warmer than the air within a
freezer. Water is a poor thermal conductor and does not freeze evenly, i.e. all at once;
rather, it freezes progressively. The first part of the water to freeze is that adjacent to
the freezer atmosphere; this outer layer of ice gradually becomes thicker with time,
causing the amount of liquid water at the cube’s core to decrease during freezing.
But ice cannot contain much dissolved oxygen, so air is expelled from solution
each time an increment of water freezes. This oxygen enters any liquid water nearby,
which clearly resides near the centre of the cube. We see how the oxygen from the
water concentrates progressively near the cube centre during freezing.
Eventually, all the oxygen formerly in the water resides in a small volume of water
near the cube centre. Finally, as the freezing process nears its completion and even