Solubility and Saturation

Solubility is defined as the maximum amount of a solute that can dissolve in a given amount of solvent at a specific temperature and pressure. The degree to which a solute dissolves determines whether a solution is unsaturated, saturated, or supersaturated.

Unsaturated solution

An unsaturated solution contains less solute than the maximum amount it can dissolve at a given temperature. Additional solute can still be dissolved without any solid remaining undissolved.

Example: When 5 g of sugar is added to 100 g of water and dissolves completely, the solution is unsaturated.

Saturated solution

A saturated solution contains the maximum amount of solute that can dissolve at a particular temperature. Any further addition of solute will not dissolve and will instead settle at the bottom of the container.

Example: If 203.9 g of sugar is dissolved in 100 g of water at 20°C, the solution becomes saturated.

Supersaturated solution

A supersaturated solution holds even more solute than normal, usually by heating and then carefully cooling the solution. These solutions are unstable—adding a tiny crystal or shaking the solution can cause sudden crystallization.

Example: A supersaturated solution of sodium acetate crystallizes instantly when disturbed, releasing heat in the process.

Effect of Temperature on Solubility

Temperature plays a critical role in solubility, but its effect varies depending on the nature of the solute and solvent:

Solid Solutes in Liquid Solvents

Most solids (e.g., sugar, potassium nitrate, copper sulfate) exhibit increased solubility as temperature rises. Higher temperatures provide more energy to break solute-solute bonds, allowing more solute to dissolve.

Example: Heating a saturated sugar solution allows additional sugar to dissolve, increasing its solubility.

Exceptions (e.g., Ammonia, lithium carbonate, calcium hydroxide) show decreased solubility with rising temperature.

Example: Less calcium hydroxide dissolves in hot water compared to cold water.

Gaseous Solutes in Liquid Solvents

Gases (e.g., CO₂, SO₂) become less soluble as temperature increases. Increased kinetic energy causes gas molecules to escape the liquid phase more readily.

Example: Warm soda loses its fizz faster because CO₂ gas bubbles out of the solution.

Try This Activity

1. 1. 1. Mix 100g water with sugar until no more dissolves (saturated solution)
      2. Heat the solution and add more sugar – watch it dissolve!
      3. Let it cool slowly to create supersaturated solution