where a1 and a2 are the absorption coefficients at temperature T1 and T2, respectively, and A H is the change in enthalpy accompanying the solution of 1 mole of gas.

A practical illustration of the decreased solubility of gases with increase of temperature is the appearance of gas bubbles on the sides of a vessel containing water when the vessel is heated; the water is saturated with air at lower temperatures and the amount of air that it can contain decreases with increase of temperature, resulting in bubble formation.

2.4.2 Effect of pressure on solubility

The influence of pressure on solubility is expressed by Henry's law, which states that the mass of gas dissolved by a given volume of solvent at a constant temperature is proportional to the pressure of the gas in equilibrium with the solution. If w is the mass of gas dissolved by unit volume of solvent at an equilibrium pressure, p, then from Henry's law, w = kp

where k is a proportionality constant. Most gases obey Henry's law under normal conditions of temperature and at reasonable pressures, providing the solubility is not too high. If a mixture of gases is equilibrated with a liquid, the solubility of each component gas is proportional to its own partial pressure; that is, Henry's law may be applied independently to each gas.

In practice, Henry's law explains the often violent release of gas that occurs as a consequence of the decrease of solubility of a gas when the pressure above the gas is released suddenly, for example when the cap of a bottle of sparkling water is unscrewed quickly.

The application of Henry's law in the calculation of the effect of pressure on the solubility of gases in liquids is illustrated in Example 2.5.

EXAMPLE 2.5 Henry's law calculation

The solubility of oxygen in water at a partial pressure of 25 torr is 8.31 mgdm 3 at 25°C. Calculate the solubility if the partial pressure is increased to 100 torr at the same temperature.

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