Henry's law states that the mass of gas which is dissolved by a volume of liquid, at constant temperature, is directly proportional to the pressure of the gas. The law is not followed exactly if there is any interaction between the gas and the liquid other than London dispersion forces.

The equation which describes Henry's law is C_g = kP_g where C_g is the solubility of the gas in the solution phase, usually expressed as molarity, k is the Henry's law constant, and P _g is the partial pressure of the gas over the solution. The Henry's law constant is different for each solute and solvent pair, it is measured in moles per liter atmosphere.

Henry's law is used to indicate how much of a gas will dissolve in a liquid. By increasing the partial pressure of the gas more gas can be dissolved in the liquid. One practical effect of this law is shown in the production of carbonated drinks. These are bottled or canned under a pressure of carbon dioxide greater than one atmosphere. When the drinks are opened to the air the partial pressure of the carbon dioxide is decreased above the drink, the solubility of carbon dioxide decreases and carbon dioxide bubbles are produced in the drink.