Henry's Law of solubility

Meaning: A gas dissolves until the rate of molecules entering the liquid equals the rate leaving. Higher partial pressure above the liquid means more collisions with the surface, so the equilibrium dissolved concentration rises in proportion. The constant depends on the gas, the solvent, and the temperature; for most gases in water, solubility falls as temperature rises. The law fits dilute solutions best, and gases that react with the solvent can deviate.

Equation: C = kH × P, where C is the dissolved gas concentration (mol/m³), kH is Henry's constant for that gas, solvent and temperature (mol/(m³·Pa)), and P is the gas's partial pressure above the liquid (Pa). At fixed temperature, C₁/P₁ = C₂/P₂.

On the MCAT: Typical setups are a sealed soda being opened (CO₂ partial pressure drops, so gas bubbles out) and a diver whose blood holds more dissolved N₂ at depth. You may be asked to scale concentration when pressure changes, or to predict that a warm drink holds less gas.

Common confusion: Students use total pressure instead of the partial pressure of that gas. In a mixture, each gas dissolves according to its own partial pressure. Also, do not carry over the "hotter dissolves more" rule from most solids; for gases in water, solubility generally decreases with temperature.