Henry's Law
Definition and meaning of Henry's Law in chemistry.
Henry's law states that, at a given temperature, the concentration of a gas dissolved in a liquid is directly proportional to the partial pressure of that gas above the liquid.
In more detail
The relationship is written C = kH·P, where C is the equilibrium concentration of dissolved gas, P is its partial pressure, and kH is the Henry's law constant, a value specific to each gas-solvent pair that decreases as temperature rises. The law holds only for dilute solutions in which the gas does not react chemically with the solvent, so it fails for gases like CO2 or NH3 at high concentrations, where significant reaction with water occurs. It underlies practical phenomena including beverage carbonation, gas exchange between blood and alveolar air in the lungs, and nitrogen narcosis and decompression sickness in scuba diving.
Key facts
| Field | Physical Chemistry |
|---|---|
| Equation | C = kH·P |
| Proposed by | William Henry, 1803 |
| Validity | Dilute, non-reacting gas-liquid systems |
A sealed soda bottle is manufactured with CO2 gas at high pressure, forcing a large amount of CO2 to dissolve in the liquid. When the cap is removed, the partial pressure of CO2 above the liquid drops sharply toward atmospheric levels, so the dissolved CO2 comes out of solution as bubbles until a new, much lower equilibrium concentration is reached.
Frequently asked questions
Why does soda go flat after opening?
Opening the bottle lowers the partial pressure of CO2 above the liquid, so by Henry's law the equilibrium concentration of dissolved CO2 drops, and excess gas escapes as bubbles over time.
Does Henry's law apply to every gas equally?
No. It works well for gases that stay chemically unchanged in solution, but gases that react with the solvent, such as CO2 forming carbonic acid or NH3 forming ammonium hydroxide, deviate from it at higher concentrations.