Solubility of a gas increases as the pressure of that gas above the surface of the liquid increases. In other words, at high pressure the gas is forced into the liquid and the can therefore hold more gas than at a lower pressure.

As shown in the animation on the right, the number of gas molecules above the surface of the liquid decreases at high pressure while the number of gas molecules in the liquid increases.

Carbonated drinks are bottled under high pressure to increase the amount of carbon dioxide dissolved in the drink. When the bottle is opened, the pressure above the solution decreases. Click to see this phenomenon. When the bottle is opened, the solution effervesces and some of the carbon dioxide bubbles out.

1) Deep sea divers breath a mixture of helium and oxygen. Helium is only one fifth as soluble in blood as nitrogen. How does this mixture assist in preventing the bends?

Since helium is less soluble than nitrogen, there is less dissolved gas to form bubbles. Helium prevents other gases from dissolving in the blood. Helium form smaller bubbles than nitrogen and therefore is less life threatening.

2) Divers suffering from the bends are placed in a high pressure vessel. How does this help?

The high pressure makes breathing less frequent which causes the concentration of gas in the blood to decrease. The high pressure causes less gas into the blood. The high pressure makes the bubbles of gas formed in the blood dissolve.

3) Sports injuries often lead to hypoxia (low oxygen concentration) of injured tissue. This is overcome by a hyperbaric chamber where the patient breathes oxygen at high pressure. Explain how this works.

High pressure forces fluid out of the injured tissues. High pressure causes more gas to be dissolved in the blood. High pressure causes less oxygen to be absorbed into the blood.