Solubility of a Solid in a Liquid

Not every solid dissolves in every liquid. Common salt and sugar dissolve freely in water, yet naphthalene and anthracene barely dissolve in it at all. Reverse the solvent, and the picture flips: naphthalene and anthracene dissolve easily in benzene, while salt and sugar do not. The pattern behind these observations is that polar solutes tend to dissolve in polar solvents, and non-polar solutes in non-polar solvents. In broader terms, a solute dissolves well in a solvent when the intermolecular forces in the two substances are of a similar kind — a rule summed up in the phrase "like dissolves like."

Dissolution, crystallisation and dynamic equilibrium

When a solid solute is put into a solvent, particles leave the solid and spread through the liquid, so the concentration of dissolved solute rises. This forward process is called dissolution.

As dissolved particles move about, some of them collide with the surface of the undissolved solid and rejoin it, coming out of solution. This reverse process is called crystallisation.

Both processes go on at the same time. As more solute dissolves, crystallisation speeds up, until a point is reached where solute leaves the solution exactly as fast as it enters it. At this point the amount dissolved no longer changes, and the system is in dynamic equilibrium:

Saturated and unsaturated solutions

At equilibrium the concentration stays constant for the given temperature and pressure. A solution in which no more solute can dissolve at that temperature and pressure — one in dynamic equilibrium with the undissolved solid — is a saturated solution. It holds the maximum amount of solute the solvent can dissolve under those conditions, so the concentration of a saturated solution is exactly the solubility of the solute. A solution that can still dissolve more solute is unsaturated.

Effect of temperature

Because dissolution of a solid is a dynamic equilibrium, its response to temperature follows Le Chatelier's principle. The direction of the shift depends on whether dissolving absorbs or releases heat.