Solubility Definition (Chemistry)

Understand What Solubility Means

Solubility is a measure of how well one substance dissolves into another.
Solubility is a measure of how well one substance dissolves into another. ilbusca / Getty Images

Solubility Definition

Solubility is defined as the maximum quantity of a substance that may be dissolved in another. It is the maximum amount of solute that may be dissolved in a solvent at equilibrium, which produces a saturated solution. When certain conditions are met, additional solute may be dissolved beyond the equilibrium solubility point, which produces a supersaturated solution. Beyond saturation or supersaturation, adding more solute does not increase the concentration of the solution.

Instead, the excess solute starts to precipitate out of solution.

The process of dissolving is termed dissolution. Solubility is not the same property of matter as rate of solution, which describes how quickly a solute dissolves in a solvent. Neither is solubility the same as the ability of a substance to dissolve another as a result of a chemical reaction. For example, zinc metal "dissolves" in hydrochloric acid through a displacement reaction that results in zinc ions in solution and the release of hydrogen gas. Zinc ions are soluble in acid. The reaction is not a matter of the solubility of zinc.

In familiar cases, a solute is a solid (e.g., sugar, salt) and a solvent is a liquid (e.g., water, chloroform), but the solute or solvent may be a gas, liquid, or solid. The solvent may either be a pure substance or a mixture.

The term insoluble implies a solute is poorly soluble in a solvent. In very few cases is it true no solute dissolves.

Generally, an insoluble solute still dissolves a little. While there is no hard-and-fast limit that defines a substance as insoluble, it's common to apply a threshold where a solute is insoluble is less than 0.1 gram dissolves per 100 milliliters of solvent.

Miscibility and Solubility

If a substance is soluble at all proportions in a specific solvent, it is call miscible in it or possesses the property called miscibility.

For example, ethanol and water are completely miscible with each other. On the other hand, oil and water do not mix or dissolve in each other. Oil and water are considered to be immiscible.

Solubility in Action

How a solute dissolves depends on the types of chemical bonds in the solute and solvent. For example, when ethanol dissolves in water, it maintains it molecular identity as ethanol, but new hydrogen bonds form between ethanol and water molecules. For this reason, mixing ethanol and water produces a solution with a smaller volume than you would get from adding together the starting volumes of ethanol and water.

When sodium chloride (NaCl) or other ionic compound dissolves in water, the compound dissociates into its ions. The ions become solvated or surrounded by a layer of water molecules.

Solubility involves dynamic equilibrium, involving opposing processes of precipitation and dissolution. Equilibrium is reached when these processes occur at a constant rate.

Units of Solubility

Solubility charts and tables list the solubility of various compounds, solvents, temperature, and other conditions. The IUPAC defines solubility in terms of a proportion of solute to solvent. Allowable units of concentration include molarity, molality, mass per volume, mole ratio, mole fraction, and so on.

Factors That Affect Solubility

Solubility may be influences by the presence of other chemical species in a solution, the phases of the solute and solvent, temperature, pressure, solute particle size, and polarity.