How Crystallisation Purifies a Solid
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
A dissolved solid can seem impossible to recover because its particles are spread throughout a liquid. Crystallisation uses changes in solubility to bring those particles back together in an ordered solid. The method is especially useful when you want a solid product rather than the solvent.
A solution can become saturated
When a soluble solid is added to a solvent, its particles separate and spread through the liquid. There is a limit to how much can dissolve under a particular set of conditions. Once that limit is reached, the solution is saturated.
For many solid solutes, more can dissolve at a higher temperature than at a lower one. Heating can therefore allow a concentrated solution to contain a large amount of dissolved material. As the solution later cools, the solvent may no longer be able to keep all of that solute dissolved.
The excess solute begins to leave the solution and form a solid crystal structure. This change is physical rather than the creation of a new chemical substance.
Slow crystal growth can improve purity
In a typical purification, an impure solid is dissolved in a suitable hot solvent. Insoluble impurities can be removed by filtration while the solution remains warm. The filtered solution is then allowed to cool so the desired substance crystallises.
As crystals form, particles of the desired substance fit into an ordered lattice. Many dissolved impurities do not fit easily into that structure and remain in the solution around the crystals, called the mother liquor.
Cooling too quickly can trap more impurities or produce very small crystals. Slow cooling often gives particles more time to arrange themselves properly. After crystallisation, the solid is filtered, washed with a small amount of cold solvent and dried.
Good technique protects the yield
Using too much solvent can reduce the amount of solid recovered because more product remains dissolved after cooling. Using too little may prevent the sample from dissolving properly. The aim is to use enough hot solvent to dissolve the desired substance without leaving a large unnecessary volume.
A careful sequence is:
- •Choose a solvent in which solubility changes usefully with temperature.
- •Dissolve the solid using a small amount of hot solvent.
- •Remove insoluble material while the solution is hot.
- •Cool the solution so crystals can form.
- •Filter, wash and dry the crystals.
Purity and yield can pull in different directions. Washing crystals thoroughly can remove more contaminated solution, but some product may dissolve and be lost. Good technique aims for a useful balance, producing crystals that are clean without throwing away more material than necessary.
The takeaway
Crystallisation purifies a solid by taking advantage of changes in solubility. The substance dissolves under one set of conditions and forms an ordered solid again as the solution cools or becomes more concentrated. Control the solvent amount, temperature and cooling rate, and you can separate useful crystals from many unwanted impurities.