Solubility and Henry’s law

The essentials

Solubility is the maximum amount of a substance that dissolves in a given amount of solvent at a given temperature and pressure. "Like dissolves like": polar solutes dissolve in polar solvents and non-polar solutes in non-polar solvents. When no more solute can dissolve, the solution is saturated and a dynamic equilibrium exists between dissolved and undissolved solute.

For solids in liquids, pressure has almost no effect because solids and liquids are nearly incompressible. Temperature has an effect that follows Le Chatelier's principle: if dissolving is endothermic, solubility rises with temperature; if exothermic, it falls.

Key points

• Henry's law: the partial pressure of a gas above a solution is proportional to its mole fraction in the solution, p = K_H × x.
• Equivalently, at constant temperature the solubility of a gas in a liquid is directly proportional to the pressure of the gas above it.
• K_H depends on the nature of the gas and on temperature. A higher K_H means lower solubility.
• K_H generally increases with temperature, so gases are less soluble in warm water. This is why aquatic animals are more comfortable in cold water.
• Applications: soft drink bottles are sealed under high CO₂ pressure; deep-sea divers can suffer "bends" when nitrogen dissolved in blood at high pressure forms bubbles on rapid ascent, so air tanks are diluted with helium; climbers at high altitude can suffer anoxia because of low oxygen partial pressure.

Exam tips

• If the pressure of a gas is doubled at the same temperature, its solubility doubles.
• When comparing gases, the one with the smallest K_H at a given temperature is the most soluble.

Common mistakes

• Assuming larger K_H means more soluble; it is the reverse.
• Applying Henry's law to the solubility of solids, where pressure has negligible effect.