In chemistry, a saturated solution is a chemical solution that contains the maximum amount of solute dissolved in the solvent. The saturation point is the point of maximum concentration. Additional solute won’t dissolve in a saturated solution or past the saturation point.
Factors That Affect Saturation
The amount of solute that dissolves in a solvent depends on multiple factors. Some of the key factors affecting solubility are:
- Temperature: Increasing temperature increases solubility, up to a point. For example, more salt dissolves in hot water than in cold water. A saturated solution at a cold temperature has a lower concentration than a saturated solution at a higher temperature.
- Pressure: Increasing pressure forces more solute into solution. One application is dissolving gases into liquids, such as carbon dioxide into soda.
- Chemical Composition: The nature of the solute and solvent affect solubility. So does the present of other compounds in the solution. For example, you can dissolve more sugar in water than salt in water.
- pH: The acidity or basicity of a solution affects whether or not ions dissociate, so it influences solubility.
Saturated vs Supersaturated Solutions
Controlling these factors allows for supersaturation. A supersaturated solution is an unstable solution that contains more solute than should dissolve in the solvent. For example, if you prepare a saturated solution of sugar in hot water and then cool the solution, it becomes supersaturated when the temperature changes. Disturbing the solution or adding a nucleation point (like a seed crystal or even a scratch on the container) induces crystal growth.
Examples of Saturated Solutions
Saturated solutions are common in every life, not just in a laboratory! Here are some familiar examples:
- A soda is a saturated solution of carbon dioxide in water. When pressure decreases by opening the container, the solubility of carbon dioxide decreases and it bubbles out of the solution.
- Adding sugar to coffee or tea until it stops dissolving forms a saturated solution.
- Adding salt to melted butter up to the point where the grains stop dissolving forms a saturated solution.
- Honey is a saturated solution of sugars (glucose and fructose) in water. If you refrigerate honey, it crystallizes because lowering the temperature lowers sugar solubility.
- Stirring powdered cocoa mix into water or milk until it stops dissolves forms a saturated solution.
- You can add powdered soap to water until no more dissolves, making a saturated solution.
How to Make a Saturated Solution
There is more than one way to prepare a saturated solution:
- Add solute to a solvent until no more dissolves.
- Evaporate solvent from an unsaturated solution until it reaches the saturation point.
- Add a seed crystal to a supersaturated solution to induce crystallization. Excess solute deposits onto the crystal, leaving a saturated solution.
- In some cases, lowering the temperature of an unsaturated solution reduces the solute solubility enough to form a saturated solution.
What Won’t Make a Saturated Solution
There are two situations where a solute and solvent can’t form a saturated solution.
- Immiscible chemicals don’t form solutions, saturated or otherwise. For example, you can’t make a solution of oil and water because they won’t mix. Similarly, you can’t make a solution of salt and paper. Neither chemical dissolves in the other.
- Likewise, fully miscible solutions don’t form saturated solutions because, by definition, they combine in all proportions. For example, ethanol and water freely mix. There is no saturation point.
Basically, to form an unsaturated, saturated, and supersaturated solution you need a solute that is at least partially soluble in the solvent.
- Hefter, G.T.; Tomkins, R.P.T (eds.) (2003). The Experimental Determination of Solubilities. Wiley-Blackwell. ISBN 978-0-471-49708-0.
- Hill, J. W.; Petrucci, R. H.; et al. (2004) General Chemistry (4th ed.). Pearson. ISBN: 978-0131402836
- Hülya Demir, Cengiz Özmetin, M.Muhtar Kocakerim, Sinan Yapıcı, Mehmet Çopur. Determination of a semi empirical kinetic model for dissolution of metallic copper particles in HNO3 solutions. Chemical Engineering and Processing: Process Intensification 2004, 43 (8) , 1095-1100. doi:10.1016/j.cep.2003.11.002
- Petrucci, R.H.; Herring, F.G.; Madura, J.D.; Bissonnette, C. (2010). General Chemistry: Principles and Modern Applications (10th ed.). Pearson Prentice Hall. ISBN: 978-0132064521.