In chemistry, an unsaturated solution is a chemical solution containing less than the maximum amount of solute that can be dissolved. The solute dissolves completely, leaving no undissolved material at the bottom of the container.
Unsaturated, Saturated, and Supersaturated
As solute concentration increases, a solution goes from unsaturated to saturated to supersaturated.
|Type of Saturation||Definition|
|Unsaturated Solution||A solution in which the solute completely dissolves. More solute may be added and dissolved. Concentration is lower than a saturated solution.|
|Saturated Solution||A solution in which no more solute can dissolve. At the saturation point, all of the solute is dissolved, but adding more solute will leave some undissolved.|
|Supersaturated Solution||A solution that contains more solute than a saturated solution. Usually, this results in undissolved material that tends to crystallize. Sometimes a supersaturated solution contains dissolved solute that exceeds the normal solubility.|
Saturation and Solubility
The amount of solute that will dissolve in a solvent is its solubility. Solubility depends on the solvent. For example, salt dissolves in water, but not in oil. Solubility of solids in water usually increases with temperature. For example, you can dissolve more sugar or salt in hot water than in cold water. Solubility also depends on pressure, although it is less of a factor and is often discounted in everyday calculations.
Because solubility depends on temperature, a solution that is unsaturated at a higher temperature may become saturated or even supersaturated at a lower temperature. Scientists and cooks commonly use heat to prepare unsaturated solutions when the solute wouldn’t fully dissolve at a lower temperature. To determine whether a certain amount of solute will form an unsaturated or saturated solution (or even dissolve at all), you can consult a solubility table.
It’s not always possible to tell unsaturated, saturated, and supersaturated solutions apart by simple visual inspection. In some cases, all three types of solutions may be free of undissolved material. Careful temperature control can produce a supersaturated solution with no undissolved material. This is a supercooled solution. Disturbing a supercooled solution upsets the equilibrium and initiates crystallization. The hot ice demonstration works on this principle.
Example of Saturated and Unsaturated Solutions
Stirring sugar or salt into water forms an unsaturated, saturated, or supersaturated solution, depending on how much sugar or salt (the solute) you add to the solvent (water). When you add a small amount of solute, all of it dissolves, forming an unsaturated solution. If you continue adding solute, you’ll reach a point where no more will dissolve. This is a saturated solution. Adding even more solute forms a supersaturated solution.
On a molecular level, when you add salt (NaCl) to water the ionic crystals dissociate into Na+ and Cl– ions. These ions and the water molecules have kinetic energy, so sometimes the ions bounce into each other and reform NaCl. The process of dissolved solute returning to its solid state is called recrystallization. In an unsaturated solution, recrystallized salt dissolves again. When you add more salt, the concentration of ions increases. Eventually a point comes where dissolution and recrystallization occur at the same rate. This equilibrium can be written as a chemical equation.
NaCl(s) ⇆ NaCl(aq)
NaCl(s) ⇆ Na+(aq) + Cl–(aq)
- 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
- Ran, Y.; N. Jain; S.H. Yalkowsky (2001). “Prediction of Aqueous Solubility of Organic Compounds by the General Solubility Equation (GSE)”. Journal of Chemical Information and Modeling. 41 (5): 1208–1217. doi:10.1021/ci010287z
Do some chemicals’ solubility _decrease_ with higher temperature? I am thinking carbon dioxide in water. When you open a warm soda it fizzes much more than when you open a cold one. What determines which way solubility changes with temperature? Thanks. (You do a great job day after day in selecting topics and explaining them well. Much appreciated.) -Bill
Of course, you’re absolutely correct! Gas solubility in water generally does decrease as temperature increases (while gas solubility in organic solvents typically increases with temperature). Also, the solubility of a few salts decreases with temperature (e.g., Ce2(SO4)3 9H2O). In some cases, solubility increases up to a point and then decreases (e.g., Na2SO4). For some solutes, solubility is practically independent of temperature. If dissolution is exothermic, the heat can shift the equilibrium, accounting for the variation in behavior of some substances.
Maybe you can do an article on why sometimes solubility goes one way and sometimes the other. What is the underlying entropy or enthalpy or whatever that causes the behavior.
Thanks again for your series. As I said—it is wonderful,
(I am a statistician. (MA, Berkeley. PhD, Yale, a few decades back. Recently retired.). If at any point you do a statistics or probability piece and want it previewed let me know. I am happy to offer suggestions for you to consider.)