The solubility of a substance is the maximum amount that can dissolve to form a stable homogeneous solution under specified conditions, especially temperature and, for gases, pressure.

A solution containing less dissolved solute than this limit is unsaturated: additional solute can still dissolve. A saturated solution has reached the solubility limit under the stated conditions. If excess solid is present, dissolution and return of dissolved particles to the solid can continue microscopically even though the dissolved concentration remains constant overall.

Suppose the solubility of a solid at a certain temperature is $36,\mathrm g$ per $100,\mathrm g$ of water. If $50,\mathrm g$ of the solid is mixed with $100,\mathrm g$ of water and enough time is allowed for the system to settle, at most $36,\mathrm g$ can remain dissolved under those conditions; about $14,\mathrm g$ remains as a separate solid phase.

A supersaturated solution contains more dissolved solute than the ordinary solubility limit. Such a state can sometimes persist temporarily, but it is unstable with respect to crystallization: a seed crystal, scratch or other disturbance can trigger rapid formation of solid until the dissolved concentration returns toward saturation.

For two liquids, miscible means they can form one homogeneous liquid solution over the full composition range; immiscible liquids have very limited mutual solubility and separate into phases.

Solubility depends on molecular interactions. Strong favorable solute-solvent interactions often increase solubility, but simple rules such as “like dissolves like” are only heuristics. Temperature can increase or decrease the solubility of a solid depending on the particular system. For many gases dissolved in liquids, increasing temperature lowers solubility, while increasing the pressure of the gas above the liquid raises it.

Solubility should not be confused with dissolution rate. A substance can have high solubility yet dissolve slowly, or low solubility yet reach its small stable dissolved concentration quickly.