Chemistry, Third edition

186 11 · SOLUTIONS AND SOLUBILITY

be at sea level. This can cause CO 2 poisoning because the higher oxygen pressure

triggers less frequent breathing which allows CO 2 levels in the diver’s blood to build

up. To match this, divers use ‘air’ mixtures which contain smaller percentages of

oxygen, the exact percentage depending on the depth at which they operate.

Operating underwater (at higher pressure) causes more gas (notably nitrogen) to

dissolve in the blood. If the diver returns to the surface too quickly, the nitrogen

bubbles out from the blood, and the diver suffers ‘the bends’. The bends can be

reduced by replacing the nitrogen in the diver’s cylinder with helium gas which (as

shown in Table 11.5) is less soluble in water. Even so, divers often have to allow their

bodies to slowly equilibrate with the gases they breathe. This is accomplished by

slowly descending or ascending.

4. Floating candle

If we float a candle on water and cover the candle with a jam jar, the candle eventu-

ally goes out. During this period, the water level rises (Fig. 11.8).

The burning of the candle consumes oxygen. Therefore, we might expect the

volume of air to be reduced by about 21%, i.e. for the water level to rise by about

one-fifth. The observed reduction in volume is much less than this because (i) the

candle goes out before the oxygen gas contained in the jar is exhausted, and (ii) the

carbon dioxide that is produced in the burning is only slightly soluble in water. (If

CO 2 werecompletely insoluble, the production of one mole of CO 2 for every mole of

O 2 consumed would produce no change in the overall volume of gas in the jar.)

5. Fountain experiment

Very soluble gases (such as ammonia and hydrogen chloride) can be made to per-

form the ‘fountain experiment’. Figure 11.9 shows the experiment carried out with

HCl(g). A flask is fitted with a cork and tube and filled with dry hydrogen chloride.

Without delay, the flask is lowered into a deep bucket of water. So much gas dissolves

in the water rising up the tube that a partial vacuum is created in the flask. This draws

Water

+ blue litmus

HCl(g)

Red fountain

Fig. 11.9The fountain experiment.

Fig. 11.8A burning candle consumes oxygen.