In physics, Henry's Law is one of the gas laws formulated by William Henry in 1803. It states that at a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.
An equivalent way of stating the law is that the solubility of a gas in a liquid is directly proportional to the pressure of the gas (Figure 1). Henry's Law has since been shown to apply for a wide range of dilute solutions, not merely those of gases.
An everyday example of Henry's Law is given by carbonated soft drinks. Before the bottle or can is opened, the gas above the drink is almost pure carbon dioxide at a pressure slightly higher than atmospheric pressure. The drink itself contains dissolved carbon dioxide. When the bottle or can is opened, some of this gas escapes, giving the characteristic hiss (or "pop" in the case of a sparkling wine bottle). Because the pressure above the liquid is now lower, some of the dissolved carbon dioxide comes out of solution as bubbles. If a glass of the drink is left in the open, the concentration of carbon dioxide in solution will come into equilibrium with the carbon dioxide in the air, and the drink will go "flat." Note that the pressure acting above the drink in the sealed container must come from the partial pressure of carbon dioxide. If the gas is only air it would not produce the same effect even if the pressure value is the same. A slightly more exotic example of Henry's law is in decompression and decompression sickness of divers.
Henry's Law can be put into mathematical terms (at constant temperature), as shown in .
Where p is the partial pressure of the solute in the gas above the solution, c is the concentration of the solute, and kH is a constant with the dimensions of pressure divided by concentration. The constant, known as the Henry's Law constant, depends on the solute, the solvent, and the temperature.
Some values for kH for gases dissolved in water at 298 K include: oxygen (O2) : 769.2 L·atm/mol carbon dioxide (CO2) : 29.41 L·atm/mol hydrogen (H2) : 1282.1 L·atm/mol.
There are other forms of Henry's Law, each of which defines the constant kH differently and requires different dimensional units. In particular, the "concentration" of the solute in solution may also be expressed as a mole fraction or as a molality.
It should also be noted the Henry's Law is a limiting law that only applies for sufficiently dilute solutions. The range of concentrations in which it applies becomes narrower the more the system diverges from ideal behavior. Roughly speaking, that is the more chemically different the solute is from the solvent.
It also only applies simply for solutions where the solvent does not react chemically with the gas being dissolved. A common example of a gas that does react with the solvent is carbon dioxide, which forms carbonic acid (H2CO3) to a certain degree with water.