Recrystallization Virtual Lab: Purity and Melting Point

This recrystallization virtual lab matches the AQA A-level Chemistry required practical on preparing a pure organic solid and testing its purity, and the purification and melting-point work in AP Chemistry and IB Chemistry. Students dissolve crude benzoic acid in the minimum volume of near-boiling water, filter it hot, cool it, collect the crystals by vacuum filtration and dry them to constant mass. Then they measure melting ranges to show the product is purer than the crude solid, and repeat with fast cooling to see why chemists cool slowly. One period gives a percentage recovery, four melting ranges and a clear purity argument.

Curriculum links

• AQA A-level Chemistry, required practical 10: preparation of a pure organic solid and a test of its purity (specification 3.3, organic analysis and synthesis).
• AQA GCSE Chemistry 4.8.1.1: pure substances melt and boil at a specific temperature; impurities change the melting point.
• AP Chemistry Unit 3 (intermolecular forces and solubility) and IB Chemistry Tool 1 (experimental techniques: recrystallization and melting point).

Simulic is not affiliated with or endorsed by the College Board, AQA, the IB or any exam board.

Before the lab (5 min)

Ask students to commit to a prediction, on paper or as question 1 of the class link:

"Crude benzoic acid melts over a range of temperatures. After recrystallization, how will the melting range of your product compare?"

Most students expect "the same, it's the same compound". Others know purity matters but can't say whether the range moves up or down. Leave both views on the board.

Method in the simulation

Keep the defaults on the Recrystallization tab: Benzoic acid, crude sample 2.0 g, cooling rate 2 °C/min.

1. Press +5 mL hot water until only a little white solid is left, then +1 mL until it just dissolves. Record this minimum volume. The dark specks are insoluble impurities; more water will not dissolve them.
2. Press Hot filtration, then Cool to 20 °C. Note the temperature where crystals start to form.
3. Press Vacuum filtration, then Wash: 5 mL ice-cold water once.
4. Press Dry 10 min & weigh until two readings agree within 0.01 g, then Record result. Percentage recovery = mass of product ÷ 2.00 g × 100.
5. Press New run, set the cooling rate to 20 °C/min and repeat steps 1 to 4 with the same volume of water.
6. Open the Melting point & ID tab. For each sample (the crude solid, the product of run 1, the product of run 2), heat at 20 °C/min to about 100 °C, then switch to 1 °C/min. Press Melting starts at the first drop of liquid and Fully melted when the liquid is clear.

Run	Water (mL)	Cooling rate (°C/min)	Final mass (g)	Recovery (%)	Crystals	Melting range (°C)
Crude solid	—	—	2.00	—	—
1		2
2		20

Expected results

All values come from the simulation.

• Minimum volume: 30 mL leaves solid undissolved; 32 mL dissolves it. Crystals start to form at about 99 °C. At this volume about 0.06 g crystallizes in the funnel during hot filtration, and about 0.09 g stays dissolved in the mother liquor at 20 °C.
• Run 1 (2 °C/min): balance readings 1.90 → 1.72 → 1.67 → 1.65 → 1.65 g. Recovery 82.5%, crystals 1.8 mm and white.
• Run 2 (20 °C/min): 1.68 g, recovery 84.0%, crystals only 0.6 mm and cream colored.

Melting ranges read at 1 °C/min (the sample lags behind the thermometer, so readings come out about 0.4 °C high):

Sample	Melting starts (°C)	Fully melted (°C)
Crude benzoic acid	108.8	117.7
Product of run 1 (slow cooling)	121.6	122.6
Product of run 2 (fast cooling)	118.2	121.3
Pure reference (literature 122.4 °C)	122.1	122.7

The product of run 1 finishes melting about 4.9 °C higher than the crude solid, and its range shrinks from about 9 °C to 1 °C. Run 2 recovered more mass but is less pure: fast cooling trapped impurities inside small crystals.

Questions for students

1. (Prediction, asked again after the lab) How will the melting range of your product compare with that of the crude solid?
2. When you compare the melting ranges of two samples, what must be the same?
3. What percentage recovery did run 1 give?
4. How many degrees higher is the end of melting for the product of run 1 than for the crude solid?
5. Run 2 used fast cooling. Explain its crystals and melting range.

Answers for teachers: (1) Higher and narrower. (2) The heating rate near the melting point (1 to 2 °C/min). (3) 82.5% (accept 81 to 84%). (4) About 4.9 °C (accept 3.9 to 5.9 °C). (5) Fast cooling forms many small crystals quickly, and they trap mother liquor and soluble impurities. Impurities lower the melting point and widen the range (118.2 to 121.3 °C). The extra mass is impurity, not extra benzoic acid.

Common misconceptions

• "Higher recovery means a better product." Run 2 has the higher recovery and the lower purity.
• "More hot water dissolves the impurities, so use plenty." Extra water keeps more product in the mother liquor: 60 mL gave 81.5% instead of 82.5%. With benzoic acid the loss is small; it is larger for more soluble compounds.
• "An impure solid melts higher." Impurities lower the melting point and widen the range.

Extension

• Wash with hot water instead of ice-cold: recovery drops to 75%. Ask why the wash must be cold.
• Use New unknown X on the Melting point & ID tab and identify it from its melting point, density, TLC and a mixed melting point.

FAQ

Why is the recovery not 100%?

The crude solid is only 90% benzoic acid. Some product also stays dissolved in the cold mother liquor, some crystallizes in the funnel, and a little dissolves in the wash.

Why do my melting points differ from my partner's by a few tenths?

The readings depend on when each student presses the buttons and on the heating rate. At 5 °C/min or faster the sim warns that readings are too high.

Can students try other compounds?

Yes. Salicylic acid and acetanilide are in the Compound menu, each with its own solubility curve and melting point.

Related simulations and guides

Organic compound separation – distillation, extraction, TLC and paper chromatography Separating mixtures: decanting, filtration, evaporation, separating funnel, distillation

For a crystallization practical at GCSE level, see the making salts virtual lab. For more ideas, see interactive chemistry lesson ideas.