Solubility and Henry's law

Solubility is the largest amount of a substance that can dissolve in a stated amount of solvent at a stated temperature. It depends on the nature of solute and solvent, on temperature and, for gases, on pressure.

Like dissolves like: sodium chloride and sugar dissolve readily in water but not in benzene, while naphthalene and anthracene dissolve in benzene but not in water. A solute dissolves when its intermolecular forces resemble the solvent's.

In a saturated solution, dissolution and crystallisation run at equal rates, so the solution sits in dynamic equilibrium with undissolved solute and holds the most solute possible at that temperature and pressure. An unsaturated solution can still take more.

Solid in liquid: by Le Chatelier's principle, a nearly saturated solution whose dissolving is endothermic (ΔsolH > 0) dissolves more on heating, and one whose dissolving is exothermic dissolves less. Pressure has no significant effect, because solids and liquids are almost incompressible.

Henry's law: at a fixed temperature, how much of a gas a liquid holds is directly proportional to the gas's partial pressure over it. The form used most is p = K_H x, where x is the mole fraction of the gas in the solution.

K_H depends on the gas and on the temperature. At a given pressure, the larger K_H is, the less the gas dissolves: CO₂ (1.67 kbar at 298 K) is far more soluble in water than N₂ (76.48 kbar at 293 K).

K_H for N₂ and O₂ rises with temperature, so gases dissolve less in warm water. Dissolving a gas releases heat, as condensation does, so by Le Chatelier's principle warming drives gas out; this is why aquatic species are more comfortable in cold water.

Henry's law at work: soft drinks are bottled under high CO₂ pressure; divers who surface quickly form nitrogen bubbles in the blood (the bends), so their tanks carry air diluted with helium (11.7% He, 56.2% N₂, 32.1% O₂); at high altitude the low partial pressure of oxygen lowers blood oxygen and can cause anoxia.