Preparation of Benzoic Acid (OCR A Level Chemistry A): Revision Note

PAG 6.2: Preparation of benzoic acid

• Benzoic acid is a suitable substance for purification by recrystallisation as it

  • Readily available in most school labs

  • Dissolves in hot water, but not in cold water

  • Is relatively safe to handle in the laboratory

Method

• Add 1.0 g of impure benzoic acid to a small conical flask and place it on a hot plate

• Add the minimum amount of hot water (around 30–35 cm³) to dissolve the benzoic acid

• Once dissolved, carry out a hot filtration to remove insoluble impurities

  • Use a pre-heated funnel with fluted filter paper, resting on a hot conical flask

  • A short-stem funnel prevents premature solidification and blockages

• Allow the filtrate to cool and crystallise

  • If crystals don’t form, stir the mixture or scratch the inner glass surface with a glass rod

• Recover the solid product using Buchner filtration, then wash and dry it

Melting point suitability

• Benzoic acid is ideal for melting point testing because:

  • It does not decompose when heated

  • Its melting point is in a useful range (100–200 ^oC)

  • It melts sharply, allowing for precise measurement

Practical skills reminder

• This practical develops essential skills in the preparation and purification of a solid organic product, including:

  • Accurately measuring and dissolving a sample in the minimum volume of hot solvent

  • Performing hot filtration to remove insoluble impurities while avoiding premature crystallisation

  • Promoting crystal formation through controlled cooling and seeding techniques

  • Isolating the solid using Buchner filtration

  • Using melting point analysis to confirm purity and identity