Wait, What? Cooling a saturated solution does not force all of its dissolved solute to crystallise.

Some solute normally remains dissolved in the cold mother liquor. Some solution clings to the crystals. Some solvent may evaporate while heating. Some crystals may be lost during transfer. Crystallisation is therefore an equilibrium-and-separation problem, not simply “cool it and collect everything.”

Solubility is conditional

Solubility is the maximum amount of solute that can dissolve in a specified amount of solvent under specified conditions, especially temperature. For many solid solutes in water, solubility increases with temperature, though the size and even direction of the effect depend on the substance.

A saturated solution is at the solubility limit under those conditions. It is not a solution containing “a lot” of solute in an informal sense.

Why crystallisation works

If a hot saturated solution contains more dissolved solute than can remain dissolved at a lower temperature, cooling creates supersaturation or approaches it. Nucleation and crystal growth can then transfer solute from solution into an ordered solid phase.

The useful yield depends on the difference in solubility between the hot and cold conditions, not simply on the hot solubility alone.

Quantitative window: predicting theoretical crystal mass

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 100 g water holds a saturated solution at 80 °C, it contains 80 g dissolved solute.

At 20 °C, about 30 g can remain dissolved. The ideal crystal mass is therefore:

80 − 30 = 50 g

This assumes no solvent evaporates, no solute is lost mechanically and equilibrium is reached. Real recovery can differ.

Evaporation changes both concentration and the denominator

If water evaporates while making the hot solution, there is less solvent than you think. That can increase crystallisation yield, but it also means a calculation based on the original water mass is wrong. In controlled measurements, cover vessels where appropriate and record masses rather than assuming solvent quantity stayed constant.

Why rapid cooling is not always best

Fast cooling can create many nucleation sites and small crystals. Slow cooling often allows fewer crystals to grow larger and can reduce trapping of mother liquor or impurities. If maximum recovery is the only goal, colder conditions may help; if crystal quality and purity matter, the optimum procedure may differ.

Seeding and scratching: nucleation is a kinetic barrier

A supersaturated solution can sometimes remain clear because crystal nucleation has not begun. Adding a tiny seed crystal or scratching the glass can provide a nucleation site. The absence of immediate crystals therefore does not prove the solution is unsaturated.

Mother liquor is evidence, not waste

The liquid remaining after crystals form is called mother liquor. It still contains dissolved solute and often a higher proportion of soluble impurities. Washing crystals with a small amount of cold solvent can remove adhering mother liquor while limiting dissolution of the product.

Using warm wash solvent may dissolve a substantial amount of the crystals you just made.

Dry mass means actually dry

Wet crystals can appear to give an excellent yield because adhering solvent contributes mass. A strong method dries crystals and, for quantitative work, checks for approximately constant mass. “Looks dry” is weaker evidence than repeated mass measurements that stop changing significantly.

Yield and purity can pull in opposite directions

Recovering every last solid particle can increase apparent yield while carrying more impurities or mother liquor. Recrystallisation deliberately sacrifices some product because desired material remains dissolved, but the recovered crystals can be purer.

This is an important Chemistry trade-off: maximum mass is not automatically maximum quality.

Observation versus inference

Observation: “Colourless crystals appeared as the solution cooled from 70 °C to room temperature.”

Inference: “The solution became sufficiently supersaturated for nucleation and crystal growth.”

Further inference: “The solute is less soluble under the final conditions than under the hot preparation conditions.”

Overclaim: “All dissolved solute crystallised.” That requires a mass balance and is usually false.

Failure modes

• Too much solvent: much product remains dissolved after cooling.
• Uncontrolled evaporation: solvent mass and calculated solubility are wrong.
• Warm washing: product redissolves.
• Insufficient drying: yield is artificially high.
• Transfer losses: crystals remain on glassware or filter material.
• Assuming no crystals means unsaturated: nucleation may simply be delayed.

Unfamiliar transfer: choosing a recrystallisation solvent

A useful recrystallisation solvent often dissolves the desired compound much better when hot than cold. A solvent in which the compound is highly soluble at both temperatures gives poor recovery; one in which it is barely soluble even when hot makes dissolution difficult. The same solubility-difference reasoning transfers directly.

Secondary → JC → deeper Chemistry

Secondary: prepare saturated solutions, interpret solubility curves, crystallise by cooling or controlled evaporation and separate crystals by filtration.

JC: calculate theoretical recovery, reason about supersaturation and nucleation, distinguish yield from purity and evaluate mother-liquor losses.

Deeper Chemistry: crystallisation extends to phase equilibria, nucleation kinetics, polymorphism, solvent selection, industrial crystallisers and crystal-purity engineering.

Checkpoint

Two students start with identical hot saturated solutions. Student A cools slowly and obtains 12.0 g dry crystals. Student B obtains 14.5 g crystals but weighs them while visibly wet. Who has the higher trustworthy yield?

Answer key and WHY reasoning

Student A currently has the more trustworthy measurement. Student B’s mass includes unknown adhering solvent, so 14.5 g is not yet a valid dry-product mass. Dry B’s crystals to an appropriate endpoint before comparing yields.

How to study this practical

Track every gram: solute initially dissolved, solvent present, solute remaining in mother liquor, recovered dry crystals and transfer losses. Then ask what temperature changed and why. This mass-balance view turns crystallisation from recipe memorisation into chemical reasoning.

Evidence boundaries

A school crystallisation experiment demonstrates temperature-dependent solubility and separation under the chosen solvent conditions. Crystal appearance alone does not establish chemical purity, exact composition or polymorphic identity.

Authoritative next steps

• Royal Society of Chemistry: practical chemistry experiments
• IUPAC Gold Book: chemistry terminology
• SEAB O-Level syllabus directory

Teaching Guide

Give students a solubility table and make them predict the theoretical crystal recovery before touching apparatus. Afterwards, require a mass-balance explanation for the difference between theoretical and dry recovered mass. This makes every “loss” chemically locatable.