How to Make a Saturated Solution

It’s easy to learn how to make a saturated solution for growing crystals, cooking, or performing chemistry experiments. A saturated solution is a chemical solution that contains the maximum amount of dissolved solute for the solvent. Here is how to prepare a saturated solution and a look at how you know it’s saturated.

How to Make a Saturated Solution

Three ways of making a saturated solution are dissolving solute in solvent, evaporating solvent from an unsaturated solution, and seeding a supersaturated solution.

Dissolve a Solute in a Solvent

This is the usual method of making a saturated solution.

1. Stir solid solute into a liquid solvent.
2. Keep adding solid until it stops dissolving. This is when solid starts accumulating in the liquid. Usually, the solid is more dense than the liquid and sinks.
3. The liquid contents are a saturated solution.

Ideally, consult a solubility table for your solid in the desired solvent before you start. Otherwise, you run the risk of running out of solute before you reach saturation. Also, remember solubility depends on temperature. For example, only a tiny amount of table salt dissolves in cold water. Much more salt dissolves in hot water. Sometimes changing the temperature of an unsaturated solution makes it saturated. Usually, this means lowering the temperature. Pressure also affects solubility.

Remove Solvent

If you have an unsaturated solution, make it saturated by removing solvent. The easiest method is evaporation. Increase air circulation or temporarily raise the temperature of the solution and drive off solvent. Trickier methods involve removing excess solvent via chemical reactions.

For example, let’s say you are growing rock candy or sugar crystals. Sugar is very soluble in water, so maybe your crystals are not growing because you didn’t dissolve quite enough. You are out of sugar, so you can’t add more. What do you do? Put the solution in a warm location and turn on a fan. Evaporate some of the water (your solvent) and then chill the remaining solution.

Seed a Supersaturated Solution

Another common way of forming a saturated solution is crystallizing a supersaturated solution. A supersaturated solution contains more dissolved solute than the solvent holds. Since the solution is not at equilibrium, adding a seed initiates crystallization. The seed is a seed crystal or bit of solid solute. Sometimes stirring a supersaturated solution or offering it a rough surface is all you need. After crystals grow, the remaining liquid is a saturated solution.

Raising the temperature of a supersaturated solution often turns it into a saturated solution. Use caution, though, because a few solutions require chilling rather than warming.

Example: How to Make a Saturated Salt Solution

Now that you understand the basics, let’s apply the information. For example, how do you make a saturated salt solution?

1. Ideally, measure the temperature of the water and just add the amount of salt you need using a solubility table. A good practice is labeling the resulting solution with mass of salt, volume of water, and temperature.
2. Otherwise, just start stirring salt into water. When salt starts accumulating at the bottom of the container, you have a saturated solution.

Working With Liquid and Gaseous Solutes

Dissolving a solid solute in a liquid solvent is pretty straightforward. But, what if your solute is a liquid or a gas?

• A solution of a gas solute in a liquid solvent starts forming bubbles at saturation.
• A solution of a liquid solute in a liquid solvent reaches saturation when the two liquids start separating into layers.

There are two common situations where a solution never becomes saturated.

• Dissolving a gas in another gas does not form a saturated solution. Assuming the gases do not react, they freely mix in all proportions.
• Mixing two fully miscible liquids does not form a saturated solution. For example, water and ethanol mix in all proportions.

How to Tell If a Solution Is Saturated

How can you tell if a solution is saturated? If you see undissolved dissolved solid, the solution is saturated. However, if there is no visible solid, add more solute, stir, and see if it dissolves. If it does not dissolve, the solution is saturated. If it does dissolve, then the solution is not saturated.

A solution of a gas in a liquid reaches saturation when the dissolved gas starts forming bubbles. For example, when you crack open a carbonated drink, the supersaturated solution of carbon dioxide in liquid forms bubbles. The supersaturated solution transitions into a saturated solution.

References

• 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.
• IUPAC (1997). “Solution”. Compendium of Chemical Terminology (the “Gold Book’) (2nd ed.). Blackwell Scientific Publications. ISBN: 0-9678550-9-8. doi:10.1351/goldbook.S05746
• 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.