A situation of constant (static) macroscopic properties while having two opposing changes take place at an equal rate is termed dynamic equilibrium [Local].

For a system to reach equilibrium, it must be in a closed system. If a sugar solution is open to the atmosphere, water will evaporate at some rate. Eventually, as the water evaporates, sugar crystallizes from the solution. This process can be used to grow crystals of rock candy. In the end, there is not equilibrium; instead, there is rock candy, and water in the atmosphere.

Two different types of observations give evidence for a dynamic state in saturated solutions.

Consider a saturated solution made by dissolving lead(II) iodide in distilled water. At some point, the amount of solid present will stop decreasing and a saturated solution will have formed. Once the solution is saturated we commonly say no more solid will dissolve. That is true on a macroscopic level only. With the solution saturated, a sample of solid lead(II) iodide that contains radioactive iodide (iodine-131) is added. After a period of a few hours, the solution is carefully filtered and the liquid phase only is tested with a radiation detector. If the system had been static, the liquid would be non-radioactive. However, the liquid portion shows a measurable amount of radioactive iodide-131. The only way it could be in the liquid is if some of the added solid dissolved in the saturated solution. (If you can access the Equilibrium ChemStudy film, this experiment is demonstrated.)

If a 'perfect' crystal has been grown but becomes damaged on a surface or edge, this imperfection can be 'healed' by suspending the crystal in a saturated solution together with small crystals on the bottom. Even at a constant temperature in a sealed container, the damaged edges or surface slowly are repaired. In a dynamic equilibrium, both dissolving and crystallizing are taking place. It turns out that the large crystals are preferred relative to smaller ones.