Henry’S Law is a relationship governing the partition of a substance in equilibrium between the atmosphere and the aqueous phase. It states that at constant temperature, the solubility of a substance is directly proportional to the partial pressure p of that substance:
Where c is the concentration in aqueous solution (such as beer) and kH is its Henry’s law constant. In beer, Henry’s law governs carbonation and the evolution of carbon dioxide (CO2) out of the liquid during dispense. The law is named after William Henry, an English chemist from Manchester who in 1802 described experiments on the quantity of gases absorbed by water at different temperatures and under different pressures.
Henry’s law is most applicable with gases that have not reached their saturation limits, and that do not react with the solvent. Different units of kH exist in the literature and care must be taken when performing relevant calculations. By comparing kH for different solutes, one can predict relative solubilities in a given liquid. kH can also be used to compute the equilibrium concentration of a substance in either the atmosphere or the aqueous phase. Values of kH are obtained empirically and the approximation is reliable only for substances of low solubility.
One may observe the effects of Henry’s law when opening a bottle of beer (or other carbonated beverage). Carbonation levels in typical beers are around 4 to 5 g/l [2 to 2.5 volumes], whereas the equilibrium concentration of CO2 at serving temperature is around 2.5 g/l. Upon opening a bottle, therefore, the system will strive for equilibrium and bubbles are formed as CO2 is released into the gaseous phase.