The prefix "super-" means to surpass, whereas the term saturate means to put in as much of something as it is ordinarily possible for something else to hold. A supersaturated solution is produced when a substance — a solute — is dissolved in water or another solvent to a degree not ordinarily possible. Most substances form a supersaturated solution only with difficulty. A supersaturated solution is generally achieved through changing the conditions of a saturated solution. Evaporating some of the solvent in a very clean environment may accomplish the purpose, as may an appropriate change in solution temperature.
As some of the solvent evaporates from a solution, that solution develops an increased solute-to-solvent ratio. If the starting solution was previously saturated, it then becomes supersaturated. Most substances increase in solubility with a rise in temperature and decrease in solubility when the temperature is lowered. For those substances, a saturated solution becomes supersaturated when the temperature is lowered. The reverse is true for substances that decrease in solubility with rising temperature and increase in solubility with lowering temperature; if one of them was in a saturated state, it will no longer be at a lower temperature.
Supersaturated solutions are inherently unstable, and respond in ways similar to those of super-cooled liquids; many persons have experienced the phenomenon of supercooled water. If pure water is chilled sufficiently and the container is kept still and is clean and scratch-free, a temperature below the freezing point may be attained without the formation of ice — the water is supercooled. Place a scratched or dirty object into the water and it may solidify in mere moments. This is because scratches and particles provide point and line interfaces onto which crystals may nucleate and grow. Similarly, introduce a few dust particles or tiny crystals into a supersaturated solution, and in just moments, solid forms at the bottom of the container, leaving an ordinary, saturated solution.
One commercial product for outdoor use employs the property of supersaturation. It is a chemical heating pad or hand-warmer, using the simple compound sodium acetate trihydrate. This chemical salt readily forms a supersaturated solution, if kept free of dust and crystals. If a closed pouch containing this solution is then has a few small crystals of the acetate dropped into it, the supersaturated solution quickly reverts to a saturated one by throwing down the excess solute. Since the reaction is exothermic — heat producing — the solution warms the hands or pockets of the person holding the pouch.
