Recrystallization and melting point – purifying and identifying organic solids
ChemistryApplied & Practical ChemistryAges 16–17
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Sign in to playA virtual purification lab. Recrystallize crude benzoic acid, salicylic acid or acetanilide from the minimum volume of hot water, with hot filtration, controlled cooling, vacuum filtration, washing and drying to constant mass, then work out the percentage recovery. Test purity with a melting-point apparatus, identify an unknown from its melting point, density and TLC, purify a liquid product with a separating funnel and drying agent, and weigh out formulations.
Lesson: Purification of organic solids by recrystallization; melting point as a test of purity; formulations
What it shows
Recrystallization purifies a solid because most compounds are far more soluble in hot solvent than in cold. Dissolving the crude solid in the minimum of boiling water and then cooling it brings the compound out as crystals, while soluble impurities stay in the mother liquor and insoluble ones are removed by hot filtration. Slow cooling grows larger, purer crystals. Purity is checked by melting point: a pure compound melts sharply at its known temperature, whereas impurities lower the melting point and widen the range. The same ideas apply to purifying liquids and to formulations made to a precise recipe.
How to use
In Recrystallization, add hot water in portions until the solid just dissolves, use Hot filtration, choose a Cooling rate and press Cool to 20 °C. Collect the crystals by Vacuum filtration, wash, dry and weigh until the mass is constant, then press Record result. In Melting point & ID, heat a capillary and press Melting starts and Fully melted. The other tabs cover a liquid product and formulations.
Parameters you can change
- Part of the practical Recrystallization, Melting point and identification, Purifying a liquid product, Formulations
- Compound to purify Benzoic acid, Salicylic acid, Acetanilide (N-phenylethanamide)
- Cooling rate 0.5–20 °C/min
- Mass of crude sample 1–5 g
Questions to explore
- Why does using much more hot water than the minimum lower the percentage recovery?
- How do the melting ranges of the crude solid and your product show which one is purer?
- Why is the organic layer of 2-chloro-2-methylpropane the upper layer in the separating funnel?
Teaching ideas
All guides- Recrystallization Virtual Lab: Purity and Melting PointA recrystallization virtual lab: purify crude benzoic acid, calculate the percentage recovery and test purity by melting point, then try fast cooling.
- Paper Chromatography Virtual Lab: Food Colors and RfA paper chromatography virtual lab: separate candy food colors, measure spots with a ruler, calculate Rf values and identify the dyes from reference lanes.
