States that, at equilibrium and constant temperature, the concentration of a dissolved non-reactive gas in a liquid is proportional to the partial pressure of that gas above the liquid.

Principle

A gas's solubility in a solvent is governed by a proportionality constant (Henry's constant): c = k_H · p (for dilute, non-reacting systems at fixed T).

Demonstration

A sealed beverage bottle contains CO2 at high partial pressure; when the bottle is opened the gas partial pressure above the liquid drops and dissolved CO2 concentration decreases until a new equilibrium is reached, producing effervescence.

Misapplication

Using a single Henry constant for a reactive gas (e.g., CO2 forming carbonic acid), for very high pressures, or across large temperature ranges where k_H changes significantly leads to large errors.

Consequence

Given the partial pressure and an appropriate k_H at the temperature of interest, one can predict dissolved-gas concentration and design gas–liquid processes (stripping, carbonation) under the law's assumptions.

Reversal

If pressure above the liquid is reduced (or temperature raised), dissolved-gas concentration falls and the gas exsolves; the inverse proportional change in concentration follows the same proportionality only within the law's limits.

Boundary

Applies to dilute solutions and gases that do not react chemically with the solvent; assumes equilibrium, low to moderate pressures, and constant temperature. Does not apply to strongly interacting or chemically reacting systems, to phases with significant non-ideality, or when interfacial kinetics dominate.

Semantic Tension

Often confused with Raoult's law (relating vapor pressure to mole fraction of a volatile solvent) or with general solubility rules; Henry's law concerns a gas's dissolved concentration proportional to its partial pressure, not the vapor pressure of a solvent component.

Synthesis

Henry's law is the equilibrium proportionality between gas partial pressure and its dissolved concentration in a dilute, nonreactive liquid at constant temperature; it is a practical tool for predicting dissolution or exsolution when its idealizing assumptions hold.