Supersaturation occurs when a solute is dissolved in a solvent to a degree not normally possible, often achieved by changing conditions of a saturated solution. Supersaturated solutions are unstable and can quickly return to a saturated state if introduced to particles or crystals. A chemical heating pad or hand warmer uses the supersaturation property of sodium acetate trihydrate to produce heat.
The prefix “super-” means to surpass, while the term saturate means to put in as much of something as something else is normally able to take. A supersaturated solution is produced when a substance, a solute, is dissolved in water or other solvent to a degree not normally possible. Most substances form a supersaturated solution only with difficulty. A supersaturated solution is usually obtained by changing the conditions of a saturated solution. The evaporation of part of the solvent in a very clean environment can achieve this, as well as an appropriate variation of the temperature of the solution.
When some of the solvent evaporates from a solution, that solution develops an increase in the solute-solvent ratio. If the starting solution was previously saturated, then it becomes supersaturated. Most substances increase in solubility as temperature increases and decrease in solubility as temperature decreases. For such substances, a saturated solution becomes supersaturated as the temperature drops. The opposite is true for substances which decrease in solubility with increasing 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 similar ways to supercooled liquids; many people have experienced the phenomenon of supercooled water. If pure water is chilled enough and the container is held still and is clean and free of scratches, it is possible to reach a temperature below freezing without ice forming: the water is supercooled. Put a scratched or dirty object in the water and it could solidify in moments. This is because scratches and particles provide point and line interfaces upon which crystals can nucleate and grow. Similarly, introduce a few particles of dust or tiny crystals into a supersaturated solution and within moments, solids form at the bottom of the container, leaving a normal saturated solution.
A commercial product for external use uses the supersaturation property. It is a chemical heating pad or hand warmer, using the simple compound sodium acetate trihydrate. This chemical salt easily forms a supersaturated solution if kept free of dust and crystals. If a few small crystals of acetate have fallen into a sealed bag containing this solution, the supersaturated solution quickly returns to saturated, throwing off the excess solute. Because the reaction is exothermic, producing heat, the solution warms the hands or pockets of the person holding the bag.
Protect your devices with Threat Protection by NordVPN
