Chemistry, Third edition

SOLUBILITY OF GASES IN WATER 185

Variation of the solubility of gases with the temperature

of the solution

Figure 11.7 shows the concentration of dissolved oxygen in water (in equilibrium

with dry air containing 20.9% O 2 by volume at 1 atm total pressure) at different tem-

peratures. As expected, the concentration of dissolved O 2 falls with increasing tem-

perature. The concentrations vary from 14.6 to 7.6 mg dm^3 between 0 and 30C.

This means that over this temperature range, 1 dm^3 of water contains between 10

and 6 cm^3 of dissolved oxygen gas.

The presence of dissolved solids in water further reduces the solubility of

oxygen. Sea water contains roughly 10 000 mg dm^3 NaCl. At 25C, this reduces the

solubility of oxygen from 8.4 to 7.6 mg dm^3. Typically, fish growth is inhibited

below about 6 mg dm^3 of dissolved oxygen.

Demonstrations involving gas solubility

1. Lemonade bottle

A bottle of lemonade is essentially a solution of carbon dioxide in flavoured water

and so Henry’s law applies:

c(CO 2 )KHpCO 2

wherec(CO 2 ) is the concentration of CO 2 dissolved in the water. When a new bottle

of lemonade is opened, some of the carbon dioxide gas above the liquid escapes.

SincepCO 2 falls, so does the concentration of CO 2 in the water – the CO 2 is lost as

bubbles which rush out of the water.

2. Carbonating water

If we try and carbonate a bottle of water which contains drink concentrate it will

spray everywhere. This is because the presence of concentrate in the water reduces

the solubility of the carbon dioxide – the excess CO 2 has nowhere to go and so it

forces some of the liquid out from the bottle. A less dramatic effect is observed if you

carbonate water first and add concentrate later – but note the release of CO 2 as the

concentrate is added.

3. Diving

As divers descend in water, the pressure from the column of sea water above them

increases. To counteract this, the divers’ regulators provide them with air from

cylinders at a pressure equal to the pressure from the surrounding sea. This means

that scuba divers breathing from a cylinder of air (containing 21% O 2 and the rest

N 2 ) will be breathing higher partial pressures of oxygen underwater than they would

Fig. 11.7Variation of the

concentration of dissolved oxygen in

pure water with temperature.

CO 2 fizzy drinks maker. CO 2 is

injected from the gas cylinder

at above atmospheric

pressure. The dissolved CO 2

makes the drinks fizzy, and

contributes to the tangy taste.

For more about

Henry’s Law and

the effect of

temperature on gas

solubility see

Appendix 11 on the

website.