To purify impure sample of benzoic acid by the process of crystallisation.  

Chemistry Lab ManualNCERT Solutions Class 11 Chemistry Sample Papers

Theory

Benzoic acid is a crystalline solid that has moderate solubility in hot water and low solubility in cold water. Its structure is :

purify-impure-sample-benzoic-acid-process-crystallisation

Benzoic acid is recrystallised by dissolving it in hot water.

Requirements

Crude sample of benzoic acid, 250 ml beakers (two), funnel, a policeman and a trough.

Procedure

1. Preparation of Solution. Take about 150 ml of water in a 250 ml beaker and keep it for boiling using tripod stand and wire gauze. In another 250 ml beaker take 2-3 gm of the crude sample of benzoic acid and add gradually with stirring minimum quantity of boiling water just sufficient to dissolve benzoic acid. Heating can be done if required.
2. Filtration of the Solution. Filter the hot solution immediately using fluted filter paper placed in a funnel. Insoluble impurities are left on the filter paper.
3. Cooling the Hot Saturated Solution. Let the filtered solution come to room temperature by itself. Now cool it by placing in cold water trough.
4. Separation of Crystals and Drying. Separate the crystals by Alteration using fun-nel and filter paper. Wash the crystals with cold water. Transfer the crystals on another filter paper and dry them by pressing gently between the folds of a filter paper. Transfer the crystals to a dry test tube and cork it.
The crystals of benzoic acid are opaque white.