Henry's law
/ HEN-reez law /
Open a fizzy drink and it hisses and bubbles. Inside the sealed bottle, the high pressure of carbon dioxide forced a lot of gas to dissolve in the liquid. Release the pressure and the gas comes rushing back out. Henry's law is the rule behind this: the amount of a gas that dissolves in a liquid is proportional to the pressure of that gas pushing on the surface.
Put precisely, for a dilute solution the partial pressure of a gas above the liquid equals its mole fraction in the liquid times a constant special to that gas-and-solvent pair (the Henry's law constant). Double the pressure of the gas above the liquid and you roughly double how much dissolves; halve it and half escapes. The constant tells you how soluble that particular gas is.
Henry's law explains carbonated drinks, the oxygen that fish breathe from water, and the dangerous nitrogen bubbles a diver can get if they surface too fast (the bends). It works best when little gas is dissolved and the gas does not react with the solvent; at high pressures or with reactive gases the simple proportionality breaks down.
A sealed bottle of soda is held under high CO2 pressure, so lots of gas stays dissolved. Pop the cap, the pressure drops, and the gas fizzes out of solution.
More gas pressure above the liquid means more gas dissolved within it.
Gas solubility also drops as temperature rises — warm water holds less dissolved oxygen, which is why a soda goes flat faster when warm and why thermal pollution can suffocate fish. Henry's law constants are quoted at a stated temperature for this reason.