Saturated, Unsaturated, and Supersaturated Solutions

A saturated solution holds the maximum amount of dissolved solute at a given temperature, an unsaturated solution holds less than that maximum, and a supersaturated solution temporarily holds more than the maximum. In brief: at 20 °C, 100 g of water dissolves up to 35.9 g of NaCl, so 30 g of salt gives an unsaturated solution, 35.9 g gives a saturated one, and any extra salt stays undissolved.

These three terms describe how much solute is dissolved compared with the amount that the solvent can hold. They are important for crystallization, the purification of chemicals, cooking and weather, where clouds and ice form from supersaturated air. This article defines the terms, explains the dynamic equilibrium in a saturated solution, and shows how to classify a solution.

What do saturated, unsaturated and supersaturated mean?

The solubility is the maximum amount of solute that dissolves in a given amount of solvent at a given temperature. It is often given in grams per 100 g of water.

What happens in a saturated solution?

When excess solid is in contact with its saturated solution, nothing seems to happen, but the system is in dynamic equilibrium. Particles continue to leave the crystal at the same rate as dissolved particles return to it:

NaCl(s) ⇌ Na+(aq) + Cl−(aq)

This explains why the amount of undissolved solid stays the same, even though the solid is constantly exchanging particles with the solution. The concentration of the saturated solution at a given temperature is a constant, which is the basis of the solubility product constant, K_sp.

How can you tell which type a solution is?

A simple laboratory test is to add one small crystal of the solute to the solution and observe what happens.

• The crystal dissolves: the solution is unsaturated.
• The crystal stays the same size: the solution is saturated.
• The crystal grows or a large amount of solid forms: the solution is supersaturated.

Alternatively, compare the amount of dissolved solute with the solubility value from a table or a solubility curve at the same temperature.

How is a supersaturated solution made?

The solubility of most solids increases with temperature. If a saturated solution is prepared at high temperature and cooled slowly without disturbance and without dust or seed crystals, the excess solute may stay dissolved even though the solubility has fallen. The solution is then supersaturated, which is a metastable state. The system can lower its energy by crystallizing, but this needs a nucleus on which the first crystals can form.

A tiny seed crystal, a scratch on the glass or a shock is enough to start crystallization, which then spreads through the solution within seconds. Sodium acetate (CH₃COONa) is the standard demonstration: when a supersaturated solution of it crystallizes, it releases heat, which is used in reusable hand warmers. The metal disk inside the pack is flexed to start the process, and the pack is reset by heating it in hot water to redissolve the crystals.

Worked examples

Example 1: classifying solutions of sodium chloride

The solubility of NaCl is 35.9 g per 100 g of water at 20 °C. Classify each mixture of salt in 100 g of water at 20 °C.

• 20.0 g NaCl: 20.0 < 35.9, so the solution is unsaturated.
• 35.9 g NaCl: the solution is exactly saturated.
• 40.0 g NaCl: the solution is saturated and 40.0 − 35.9 = 4.1 g remains undissolved.

Example 2: how much solid remains

How much NaCl dissolves in 250 g of water at 20 °C? The solubility scales with the mass of water: 35.9 × 250 / 100 = 89.8 g. If 100 g of salt is added, 100 − 89.8 = 10.2 g does not dissolve.

Example 3: a supersaturated solution

A solution has 45 g of a salt dissolved in 100 g of water at 10 °C, while the solubility at this temperature is 33 g per 100 g. The solution contains 12 g more than the maximum, so it is supersaturated. On seeding, up to 12 g of the solid crystallizes.

Where are these solutions used?

• Crystallization and purification: a hot, nearly saturated solution is cooled so that pure crystals form, while impurities stay in solution.
• Rock candy and sugar syrup: sugar crystals grow on a string in a supersaturated sugar solution.
• Weather: air that is supersaturated with water vapor forms clouds when particles of dust or silver iodide provide nuclei, which is the principle of cloud seeding.
• Geology: mineral crystals grow from hot solutions in rock cavities as they cool.
• Medicine: drug formulations must avoid unwanted crystallization of the drug from solutions in storage.

How did the idea of saturation develop?

The preparation of salts by evaporation and crystallization is ancient. The scientific study of solubility as a function of temperature began in the 18th and 19th centuries. The idea that a saturated solution is in equilibrium with the solid developed together with chemical thermodynamics in the second half of the 19th century. Wilhelm Ostwald described in 1897 the metastable zone of supersaturation, within which crystals do not form spontaneously but may grow once started.

Which mistakes are common?

• Confusing saturated with concentrated: a saturated solution of a poorly soluble substance, such as CaCO₃, is very dilute.
• Forgetting the temperature: the same solution can be unsaturated when hot and supersaturated when cooled.
• Believing that a saturated solution contains undissolved solid: it can be saturated without any solid, so long as the dissolved amount equals the solubility.
• Assuming supersaturated solutions are unstable at once: they can remain unchanged for days without a nucleus.
• Using the solubility for the wrong amount of solvent: scale the solubility with the mass of solvent.