Recrystallization of Benzoic Acid

Purifying a solid by dissolving in hot solvent and cooling

Crystallization Intermediate (High School) 45 min ~$10.00

Objective

Purify impure benzoic acid by recrystallization from hot water, demonstrating the standard purification technique for organic solids.

Background

Recrystallization exploits the difference in solubility of a substance at different temperatures. Benzoic acid is very soluble in hot water but nearly insoluble in cold water. By dissolving impure benzoic acid in hot water and cooling slowly, pure crystals form while impurities remain in solution.

Safety Warnings

  • Handle hot water and hot glassware with care
  • Benzoic acid dust can irritate airways
  • Use heat-resistant gloves when handling hot flasks

Required PPE

goggles lab_coat

Materials

  • Impure benzoic acid (3 g)
    Mixed with charcoal or sand
  • Distilled water (200 mL)
  • Activated charcoal (0.5 g)
    Optional, for decolorization

Equipment

250 mL Erlenmeyer flask Hot plate Buchner funnel Filter flask Vacuum pump Melting point apparatus Ice bath Watch glass

Procedure

1

Weigh 3 g of impure benzoic acid and record the mass. Note the appearance (color, particle size).

3 min
2

Add the impure sample to 100 mL of distilled water in the Erlenmeyer flask. Heat to boiling while stirring.

8 min Handle hot flask with care
3

If the solution is colored, add 0.5 g activated charcoal, swirl, and filter hot through fluted filter paper.

5 min
4

Allow the clear filtrate to cool slowly to room temperature. Do not disturb.

15 min
5

Cool further in an ice bath to maximize crystal yield.

5 min
6

Collect the crystals by vacuum filtration. Wash with a small amount of ice-cold water.

5 min
7

Dry the crystals on a watch glass. Weigh and calculate percent recovery.

3 min
8

Determine the melting point (pure benzoic acid: 122°C) and compare with the impure sample.

5 min

Expected Results

White, needle-like crystals with a sharp melting point at 121-123°C. Typical recovery is 60-80%. The purified crystals should be whiter and have a sharper melting point than the crude material.

Cleanup

Dispose of filtrate down the drain (benzoic acid is food-grade). Rinse glassware. Dispose of charcoal in solid waste.

Details

Category
Crystallization
Difficulty
Intermediate (High School)
Duration
45 min
Est. Cost
$10.00
Steps
8
Materials
3

More Experiments

Alum Crystal Growing 30 min Bismuth Rainbow Crystal Growing 45 min Crystallization of Salol (Phenyl Salicylate) 30 min Fractional Crystallization of Salt and Sand 35 min Growing Copper Sulfate Crystals 30 min Sugar Crystal Rock Candy 25 min Supersaturation and Instant Crystallization 35 min

Frequently Asked Questions

What is the objective of Recrystallization of Benzoic Acid?
Purify impure benzoic acid by recrystallization from hot water, demonstrating the standard purification technique for organic solids.
How difficult is Recrystallization of Benzoic Acid?
This experiment is rated as Intermediate (High School). It takes approximately 45 minutes to complete.
What safety precautions are needed for Recrystallization of Benzoic Acid?
Key safety precautions include: Handle hot water and hot glassware with care; Benzoic acid dust can irritate airways; Use heat-resistant gloves when handling hot flasks.
What materials are needed for Recrystallization of Benzoic Acid?
The main materials required are: Impure benzoic acid, Distilled water, Activated charcoal.
What results should I expect from Recrystallization of Benzoic Acid?
White, needle-like crystals with a sharp melting point at 121-123°C. Typical recovery is 60-80%. The purified crystals should be whiter and have a sharper melting point than the crude material.