Solubility of Gases in Liquids

Introduction to Solubility of gases in liquids

Most of the gases dissolve in water.  Oxygen dissolves only to a small extent in water.  It is this dissolved oxygen which sustains all aquatic life.  On the other hand, hydrogen chloride gas (HCl) is highly soluble in water. Solubility of gases in liquids is greatly affected by pressure and temperature.  The solubility of gases increases with the increase of pressure. 

The lower part is solution and the upper part is gaseous system at pressure p and temperature T. Assume this system to be in a state of dynamic equilibrium, i.e., under these conditions rate of gaseous particles entering and leaving the solution phase is the same.  Now increase the pressure over the solution phase by compressing the gas to a smaller volume in the above figure.  This will increase the number of gaseous particles per unit volume over the solution and also the rate at which the gaseous particles are striking the surface of solution to enter it. The solubility of the gas will increase until a new equilibrium is reached resulting in an increase in the pressure of a gas above the solution and thus its solubility increases.


Henry’s law


The dissolution of a gas in a liquid is governed by Henry’s law, according to which, at a given temperature, the solubility of a gas in a liquid (p) is directly proportional to the partial pressure of the gas(x).  And is given by

p = KH x

 KH is the Henry’s law constant.  Different gases have different KH values at the same temperature.  This suggests that KH is a function of the nature of the gas. It is obvious from equation that higher the value of KH at a given pressure, the lower is the solubility of the gas in the liquid. KH values for both N2 and O2 increase with increase of temperature indicating that the solubility of gases increases with decrease of temperature. It is due to this reason that aquatic species are more comfortable in cold waters rather than in warm waters.

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Henry’s law finds several applications in industry . These are


To increase the solubility of CO2 in soft drinks and soda water, the bottle is sealed under high pressure.

Scuba divers must cope with high concentrations of dissolved gases while breathing air at high pressure underwater.  Increased pressure increases the solubility of atmospheric gases in blood.  When the divers come towards surface, the pressure gradually decreases.  This releases the dissolved gases and leads to the formation of bubbles of nitrogen in the blood.  This blocks capillaries and creates a medical condition known as bends, which are painful and dangerous to life.  To avoid bends, as well as, the toxic effects of high concentrations of nitrogen in the blood, the tanks used by scuba divers are filled with air diluted with helium (11.7% helium, 56.2% nitrogen and 32.1% oxygen).

At high altitudes the partial pressure of oxygen is less than that at the ground level.  This leads to low concentrations of oxygen in the blood and tissues of people living at high altitudes or climbers.  Low blood oxygen causes climbers to become weak and unable to think clearly, symptoms of a condition known as anoxia.


Effect of Temperature on solubility of gases in liquids


Solubility of gases in liquids decreases with rise in temperature.  When dissolved, the gas molecules are present in liquid phase and the process of dissolution can be considered similar to condensation and heat is evolved in this process.  As dissolution is an exothermic process, the solubility should decrease with increase of temperature.