Wait, What? Cooling a solution can make a solid appear even though no new substance has been made.

That is the heart of crystallisation. The solute was already present in the solution. Cooling changes how much of it can remain dissolved. Once the solution reaches saturation and then becomes supersaturated, excess solute leaves the solution and enters an ordered solid phase.

The practical question

The experimental job is to connect temperature, solvent amount and dissolved solute mass. A solubility curve gives the maximum amount of solute that can dissolve in a stated amount of solvent at a given temperature.

The Royal Society of Chemistry’s temperature–solubility experiment asks students to heat a known solute–water mixture, cool it and record the temperature at which crystals first appear, then repeat at different solvent amounts. That structure turns crystallisation into a quantitative relationship rather than a visual trick. See the RSC experiment.

Saturated does not mean “full of solid”

A saturated solution contains the maximum dissolved solute possible under the stated conditions. It may look completely clear. Visible undissolved solid is not required for the solution itself to be saturated, although excess solid can coexist with it at equilibrium.

Why temperature matters

For many solids in water, solubility increases with temperature, though the exact relationship depends on substance and solvent. A hot saturated solution may therefore hold more dissolved solute than the same amount of solvent can retain after cooling.

The difference between hot and cold solubility sets the theoretical crystallisation yield.

Quantitative window: predicting crystal yield

Suppose a solute has solubility 80 g per 100 g water at 80 °C and 30 g per 100 g water at 20 °C. If 80 g is dissolved in 100 g water at 80 °C and the solution cools to 20 °C, the ideal mass that crystallises is:

80 − 30 = 50 g

That is a model prediction. The recovered mass may be lower because some solute remains in the mother liquor, crystals may be lost during transfer, or the solution may not fully reach equilibrium.

Supersaturation and nucleation

A solution can sometimes cool below the expected crystallisation point without crystals appearing immediately. It is then supersaturated: more solute remains dissolved than the equilibrium solubility predicts.

Crystal formation needs nucleation. Dust, scratches, a seed crystal or disturbance can provide a site for ordered solid to begin forming. This is why “first crystal temperature” can depend on apparatus cleanliness and handling as well as equilibrium solubility.

Cooling rate changes crystal size

Slow cooling often allows fewer nuclei to grow for longer, producing larger crystals. Faster cooling can create many nuclei and smaller crystals. The RSC explicitly uses cooling rate to teach crystal-size differences. See the RSC crystal lesson.

Purification by recrystallisation

Recrystallisation exploits differing solubilities. The desired compound is dissolved in a minimum amount of hot solvent. Insoluble impurities can be removed by hot filtration. On cooling, the desired substance crystallises while many soluble impurities remain in the mother liquor.

Too much solvent reduces recovery because more desired product remains dissolved after cooling. Too little solvent may leave the desired substance undissolved before filtration. The RSC highlights both failure modes in its impure-solid purification practical. See the RSC practical.

Why washing crystals is a trade-off

Washing removes mother liquor that may contain dissolved impurities. But the wash solvent can also dissolve some product. Use a small amount of cold solvent so impurity removal is improved while product loss is limited.

Observation versus inference

Observation: “Crystals first appeared at 42 °C.”

Inference: “The solution had reached conditions where crystal nucleation became favourable.”

Stronger inference: “This temperature estimates the saturation boundary for this concentration, but nucleation delay may shift the observed first-crystal temperature below the equilibrium value.”

Failure modes

• Too much solvent: poor crystal yield after cooling.
• Too little solvent: desired compound may be lost during hot filtration.
• Fast cooling: many small crystals can trap mother liquor.
• Inconsistent first-crystal judgement: nucleation is not perfectly reproducible.
• Crystals weighed wet: apparent yield becomes too high.
• Warm wash solvent: excessive product dissolves.

Unfamiliar transfer: pharmaceutical purification

The same logic appears in organic synthesis. A reaction product can be purified by choosing a solvent in which it is much more soluble hot than cold. The transferable skill is to reason about solubility contrast, not to memorise one salt.

Secondary → JC → deeper Chemistry

Secondary: interpret saturation, solubility curves and cooling crystallisation.

JC: calculate theoretical yield, analyse supersaturation and nucleation, use recrystallisation for purification and evaluate solvent choice.

Deeper Chemistry: crystallisation science extends to phase equilibria, metastability, nucleation kinetics, polymorphism and industrial crystalliser design.

Checkpoint

A student dissolves a product in 100 cm³ hot solvent when only 30 cm³ was needed. After cooling, very few crystals form. What went wrong?

Answer key and WHY reasoning

Too much solvent was used. Even after cooling, a larger amount of product can remain dissolved in the mother liquor, so recovery falls. The solution may never become sufficiently supersaturated for much crystallisation.

How to study this practical

Practise reading solubility curves in both directions: temperature → maximum dissolved mass, and dissolved mass → crystallisation temperature. Then connect every procedural step to either yield, purity or evidence quality.

Evidence boundaries

A school solubility curve applies to the stated solute–solvent system and concentration units. Different solvents, hydrates, impurities and polymorphs can change crystallisation behaviour. First-crystal temperature is also an operational measurement influenced by nucleation.

Authoritative next steps

• Royal Society of Chemistry: effect of temperature on solubility
• Royal Society of Chemistry: purifying an impure solid
• RSC: evaporation, filtration and crystallisation

Teaching Guide

Give students the same product with two different solvent volumes and ask them to predict both yield and purity before doing any calculation. Then ask why “crystals appeared” is not identical to “equilibrium solubility was reached exactly here.” This builds chemical judgement rather than recipe-following.