By definition, a supersaturated solution is a chemical solution that contains more solute than the solvent can hold. In other words, a supersaturated solution has more dissolved solute than a saturated solution. The process of forming a supersaturated solution is called supersaturation. Usually, supersaturation involves a solid solute dissolved in a liquid solvent, but the term also applies to gases in liquids and gas mixtures. Here is an explanation of how to make a supersaturated solution, a look at examples, and some uses of supersaturation.
How to Make a Supersaturated Solution
Supersaturation is all about solubility. A saturated solution contains the maximum amount of solute that dissolves in a solvent and reaches equilibrium. A supersaturated solution contains even more dissolved solute. Basically, you make a supersaturated solution by controlling the solubility of a solute in a solvent. Solubility depends on a few factors, but the easiest one to control is temperature.
Usually, solubility increases with temperature. So, you heat a solution, dissolve more solute, and carefully cool the resulting supersaturated solution. This is the process people use when growing crystals from aqueous solution, like rock candy or copper sulfate crystals.
Less commonly solubility decreases with temperature. For example, sodium sulfate or Glauber’s salt (Na2SO4) becomes less soluble as temperature increases (over a certain temperature range). In these cases, warming a solution changes it from a saturated solution into a supersaturated solution.
A supersaturated solution in metastable in that it maintains its concentration under set conditions, but is not actually at equilibrium. Once the solution overcomes the thermodynamic barrier required to change phase, it moves from supersaturated to saturated. Some supersaturated solutions spontaneously crystallize when disturbed. More often, adding a seed crystal to a supersaturated solution induces crystallization.
Keep in mind, crystallization only reduces the concentration of the solution to the point where it is at equilibrium. This is a saturated solution. So, you can’t remove all of the solute from a solution via crystallization.
For solutions involving gases, supersaturating often involves pressure. Increasing pressure generally increases solubility. Carefully releasing the pressure allows for supersaturated solutions.
Examples of Supersaturated Solutions
Since they are not stable, you might think supersaturated solutions are rare. However, there are several examples in the everyday world.
- The sugars in clear honey are supersaturated. Crystals tend to form slowly at room temperature, although you can knock sugar out of solution quickly if you refrigerate honey.
- Crystals grown by dissolving a solute in water rely on supersaturation. The first step is dissolving the solid in warm or hot water. As the solution cools to room temperature, it becomes supersaturated. Adding a seed crystal promotes crystal growth. Otherwise, imperfections on the container surface or minor impurities in the solution act as nucleation sites.
- Carbon dioxide is supersaturated in soft drinks. In this case, pressure forces more carbon dioxide into the solvent (water) than ordinarily dissolves. Opening the can releases the pressure and lets some of the dissolved carbon dioxide escape as gas bubbles.
- Similarly, decompression sickness or “the bends” occurs when gases dissolved in blood are supersaturated at depth. Slow decompressions helps prevent dissolved gases from turning into bubbles.
- Cloudbursts result from supersaturation of water in air. A change in temperature or pressure triggers the conversion from water vapor into liquid water.
Uses of Supersaturated Solutions
Supersaturation has both entertaining and practical applications.
- It enables crystal growth, both for pleasure and for study of crystal structure.
- Crystallization from a supersaturated solution is one method of increasing chemical purity, as the crystal excludes many contaminants.
- Some drugs are supersaturated in a solvent to allow precise doses or deliver drugs that have only low solubility in water.
- The study of oxygen supersaturation in water is an indicator of photosynthetic activity and ecosystem health.
References
- Coquerel, Gérard (2014-03-10). “Crystallization of molecular systems from solution: phase diagrams, supersaturation and other basic concepts”. Chemical Society Reviews. 43 (7): 2286–2300. doi:10.1039/c3cs60359h
- IUPAC (1997). “Supersaturation”. Compendium of Chemical Terminology (the “Gold Book”) (2nd ed.). Oxford: Blackwell Scientific Publications. ISBN 0-9678550-9-8. doi:10.1351/goldbook.S06146
- Linnikov, O. D. (2014). “Mechanism of precipitate formation during spontaneous crystallization from supersaturated aqueous solutions”. Russian Chemical Reviews. 83 (4): 343–364. doi:10.1070/rc2014v083n04abeh004399
- Löffelmann, M.; Mersmann, A. (October 2002). “How to measure supersaturation?”. Chemical Engineering Science. 57 (20): 4301–4310. doi:10.1016/S0009-2509(02)00347-0
- Tomlinson, Charles (1868). “On Supersaturated Saline Solutions”. Philosophical Transactions of the Royal Society of London. 158: 659–673. doi:10.1098/rstl.1868.0028