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Topic: Organic Chemistry
Level: OL/HL
Exam Focus: Section A Practical Skills
1. Experiment Snapshot
Feature
Details
Experiment
Recrystallisation of Benzoic Acid
Difficulty
⭐⭐⭐ Practical exam standard
Exam Importance
⭐⭐⭐⭐ Organic practicals are common Section A targets
Typical Marks
5–15 marks
2. Aim
To purify impure benzoic acid by recrystallisation.
3. Quick Theory
Recrystallisation purifies a solid by dissolving it in minimum hot solvent, removing insoluble impurities by hot filtration, then cooling to form pure crystals.
Examiner Tip: Link the practical step to its purpose. This is where students gain easy explanation marks.
4. Apparatus and Chemicals
Apparatus
Chemicals
Beaker, hot plate, funnel, filter paper, Buchner funnel if available, glass rod.
Impure benzoic acid, hot solvent such as water, cold solvent for washing.
Safety: Wear goggles. Take care with hot apparatus, organic liquids, bromine water, acids, alkalis and flammable substances.
5. Labelled Practical Diagram
6. Step-by-Step Procedure
Dissolve impure benzoic acid in the minimum amount of hot solvent.
Carry out hot filtration to remove insoluble impurities.
Allow the hot filtrate to cool slowly.
Collect the crystals by filtration.
Wash the crystals with a small amount of cold solvent.
Dry the crystals.
7. Observations / Results
Stage
Expected Observation
Hot solution
Benzoic acid dissolves.
Cooling
White crystals form.
Final product
Pure crystals are obtained after filtration and drying.
8. Sources of Error and Improvements
Source of Error
Improvement
Incorrect heating or poor timing.
Heat gently and follow the sequence carefully.
Product loss or contamination.
Use clean apparatus and correct collection/filtration technique.
Incorrect observation wording.
Use exact exam words such as colourless, precipitate, crystals, reflux, or melting range.
9. Student Common Mistakes
Using too much solvent, reducing the yield.
Filtering after the solution has cooled.
Washing crystals with hot solvent, causing them to dissolve.
Not drying crystals before weighing or melting point testing.
Cooling too quickly and trapping impurities.
10. Examiner Tips and Traps
Examiner Tip: Organic practicals often ask why a step is done, not just what was done.
Examiner Trap: Do not write vague observations like “changed colour” or “gas formed” without the exact result.
11. 🎮 Interactive Simulator
Try It Yourself
How much hot solvent should be used?
What should crystals be washed with?
Result Window
Ready
Choose the correct actions to complete the practical.
Score:0/0
12. Past Paper Style Questions
State the aim of this experiment. [2]
Describe the preparation method in correct order. [4]
State one safety precaution. [2]
State one observation and its conclusion. [3]
State one common source of error and one improvement. [4]
13. Mark Scheme
Aim correct [2].
Method in correct order with key apparatus/chemicals [4].
Safety point suitable for chemicals/heating [2].
Observation and conclusion linked correctly [3].
Valid error and improvement [4].
14. Quick Revision Card
Aim: To purify impure benzoic acid by recrystallisation.
Key theory: Recrystallisation purifies a solid by dissolving it in minimum hot solvent, removing insoluble impurities by hot filtration, then cooling to form pure crystals.
Most common mistake: Using too much solvent, reducing the yield.
Exam rule: Use exact observations and state the purpose of key steps.