What Is an Unsaturated Solution?

An unsaturated solution is a chemical solution in which the solute concentration is lower than its equilibrium solubility. All of the solute dissolves in the solvent.

When a solute (often a solid) is added to a solvent (often a liquid), two processes occur simultaneously. Dissolution is the dissolving of the solute into the solvent. Crystallization is the opposite process, where the reaction deposits solute. In an unsaturated solution, the rate of dissolution is much greater than the rate of crystallization.

Examples of Unsaturated Solutions

• Adding a spoonful of sugar to a cup of hot coffee produces an unsaturated sugar solution.
• Vinegar is an unsaturated solution of acetic acid in water.
• Mist is an unsaturated (but close to saturated) solution of water vapor in air.
• 0.01 M HCl is an unsaturated solution of hydrochloric acid in water.

Types of Saturation

There are three levels of saturation in a solution:

1. In an unsaturated solution, there is less solute than the amount that can dissolve, so it all goes into solution. No undissolved material remains.
2. A saturated solution contains more solute per volume of solvent than an unsaturated solution. The solute has dissolved until no more can, leaving undissolved matter in the solution. Usually, the undissolved material is denser than the solution and sinks to the bottom of the container.
3. In a supersaturated solution, there is more dissolved solute than in a saturated solution. The solute can easily fall out of solution by crystallization or precipitation. Special conditions may be needed to supersaturate a solution. It helps to heat a solution to increase solubility so more solute can be added. A container free of scratches also helps keep solute from falling out of solution. If any undissolved material remains in a supersaturated solution, it can act as nucleation sites for crystal growth.