Equilibrium involving dissolution of solids or gases in liquids

Physical equilibrium is not limited to a single pure substance changing phase -- it also governs how a solute dissolves into a solvent.

Solid in liquid. Adding sugar to water at a fixed temperature dissolves it to form a sugar solution. Keep adding sugar, though, and eventually a point is reached where any further sugar added simply remains undissolved, sitting as solid at the bottom -- the solution is now saturated. Just as in the phase-change equilibria above, this is a genuine dynamic equilibrium between solute molecules in the solid phase and solute molecules dissolved in solution:

with the rate of dissolution of solute equal to the rate of crystallisation of solute. …