Upon completion of this topic, learners will be able to:

• identify the types of solution.
• define solubility and describe how to determine whether a solution is saturated, unsaturated, or supersaturated.
• describe the role of equilibrium in the solution process and its relationship to the solubility of a solute.

A solution does not always involve water. Solutions may exist as gases, liquids, or solids. Some possible solute-solvent combinations of gases, liquids, and solids in solutions are summarized.

Saturated solutions

When a small amount of solute is added to a solvent, the solute dissolves very easily in the solvent. If the addition of solute continued stage is reached when the solvent cannot dissolve any more solute. At this stage, further added solute remains undissolved and it settles down at the bottom of the container.

A solution containing maximum amount of solute at a given temperature is called a saturated solution. A saturated solution is one in which undissolved solute is in equilibrium with dissolved solute.

Unsaturated solutions

If a solution contains less solute than the amount needed to form a saturated solution, the solution is known as unsaturated. Thus, a solution containing 10.0 g of NaCl per 100 mL of water at 0°C is unsaturated because it has the capacity to dissolve more solute.

What happens if a solute is added to an unsaturated solution?

Supersaturated solutions

When saturated solutions are heated, they develop further capacity to dissolve more solute. A solution that contains more solute than a saturated solution is known as supersaturated solution. Because the solute in a supersaturated solution is present in an amount higher than the equilibrium concentration, supersaturated solutions are unstable. The addition of a small crystal of the solute (a seed crystal) provides a template for crystallization of the excess solute, leading to a saturated solution in contact with excess solid.