CBSE Chemistry Experiment Purification of an Impure Sample of Benzoic Acid by the Process of Crystallisation

Benzoic acid is an organic compound which is used in dyes, perfumes and drugs. It has a wide range of applications. To get pure benzoic acid from crude sample, evaporation or crystallisation can be employed;  however, crystallisation is preferable to evaporation for this purpose. This is because some solid mixture particles disintegrate during evaporation, leaving behind impurities, whereas pure solid crystals are produced during crystallisation. 

Table of Content 

• Aim

• Apparatus Required

• Theory

• Filtration

• Crystallisation

• Procedure

• Observation

• Result

• Precautions

• Lab Manual Questions

• Viva Questions

• Practical Based Questions

• Result

Aim 

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

Materials Required

1. Burner

2. Beaker

3. Tripod Stand

4. Wire Gauze

5. Glass Rod

6. Filter Paper

7. Funnel

8. Test Tube

9. Cold Water

10. Crude Sample

11. Watch Glass

Theory

• Benzoic acid is an insoluble compound in water. It can be dissolved in boiling water.

• Impurities in the sample, which are insoluble in hot water, can be separated by filtration.

• Separating methods of crystallisation can be applied to get pure benzoic acid from impure samples.

Filtration

The process of filtering a liquid or gaseous fluid to remove solid particles while leaving the fluid behind in the filter media. The fluid that has been cleansed or the fluid that has had its solid particles removed might both be considered the desired product.

Crystallisation

There are two basic phases in the crystallisation process. The first is called nucleation and refers to the crystallisation of a supercooled liquid or a supersaturated solvent. Larger particle sizes and crystalline conditions are produced by the second process, often known as crystal growth.

Procedure

1. Preparation of Solution

• Using a tripod stand and wire gauze, place 150 millilitres of water in a 250-millilitre beaker and hold it there to boil. 

• Add minimal amounts of boiling water, just enough to dissolve the benzoic acid, while stirring slowly to another 250 ml beaker containing 2-3 grams of the crude benzoic acid sample.

•  If necessary, heating can be performed by closing with a watch glass.

Preparation of solution

2. Filtration of Solution

• Use a funnel and fluted filter paper to quickly filter the hot solution. 

• Insoluble impurities remain on the filter paper.

3. Cooling of Hot Solution

• Allow the filtered solution to naturally reach room temperature. 

• Place it in a bucket of cold water to cool it now.

4. Separation of Crystals

• Use a funnel and filter paper to divide the crystals according to their alteration. 

• With cold water, wash the crystals. 

• Transferring the crystals to another filter paper, gently pushing the crystals there to dry them.

5. Drying

• Put the crystals in a dry test tube and close the lid.

Observation

Result

From this crystallisation experiment, we separated crystals of benzoic acid from its impure sample and obtained opaque white crystals of benzoic acid.

Precautions

1. There should only be enough filtrate evaporated to start crystallisation.

2. Keep it from getting too hot or dry. A hot solution spill can be avoided by not heating the solution too much.

3. Without agitating it, the solution should be slowly chilled. Never allow it to cool down fast.

4. To transform crystals into solid form, wash them with liquid first.

Lab Manual Questions

1. How to find the crystallisation of the samples?

Ans: The easiest way to compute the % crystallisation if your sample has a sharp melting point is to make the ratio of the experimental heat of fusion to the theoretical value for 100% crystalline material.

2. Crystallisation works on which principle?

Ans: The limited solubility of a substance in a solvent at a particular temperature, pressure, etc. forms the basis of the crystallisation principle. A crystalline solid will emerge if these circumstances are altered to a situation where the solubility is reduced.

3. How are impurities taken out of crystals of benzoic acid?

Ans: The sample solution will contain dissolved benzoic acid as well as dissolved contaminants if the impure sample is dissolved in a small amount of hot solvent, in this example boiling water, then filtered to eliminate insoluble impurities.

4. What separating method is used for obtaining crystals from a sample?

Ans: The method of crystallisation that is converting solution into solid is used for obtaining crystal from impure samples.

Viva Questions

1. What is meant by crystallisation?

Ans: Crystals are the names given to substances when they exist in clearly defined geometrical shapes. These are created when a heated, salt-saturated fluid is allowed to cool slowly without being disturbed. Crystallisation is the name given to this process.

2. What is meant by filtration?

Ans: Insoluble components are separated by putting the solution through filter paper.

3. Which method is used to purify the impure solid?

Ans: Some impure solids are purified by Crystallisation. 

4. What are the characteristics of a crystal?

Ans: A crystal has a definite shape and geometry. They are rigid and incompressible.

5. Is benzoic acid soluble in cold water?

Ans: No, benzoic acid is soluble in hot or boiling water.

6. What is the molecular formula of benzoic acid?

Ans: The chemical formula of benzoic acid is C6H5COOH.

7. What is the boiling point of benzoic acid?

Ans: The boiling point of benzoic acid is 249.2 °C.

8. What are the uses of benzoic acid?

Ans: Food and beverages frequently use benzoic acid (BA) as an antibacterial preservative, particularly carbonated drinks.

9. What is meant by the concentrated solution?

Ans: A concentrated solution contains a lot more solute than it could dissolve.

10. What is the principle of crystallisation?

Ans: The limited solubility of a substance in a solvent at a particular temperature, pressure, etc. forms the basis of the crystallisation principle.

Practical-Based Questions

1. Benzoic acid can be prepared by

1. Oxidation of benzyl alcohol

2. Reduction of Benzyl Alcohol

3. Using Benzyl Alcohol as a grignard reagent

4. None of the above

Ans: Benzoic acid can also be prepared by the oxidation of benzyl alcohol with acidic or alkaline KMnO4 or acidified K2Cr2O7.

2. The common solvent used for crystallisation of Benzoic Acid is

1. Water

2. Ethanol

3. Acetone

4. All of the above

Ans: Water is used as the common solvent for crystallisation of Benzoic Acid.

3. Benzoic acid is soluble in ____.

1. Cold water

2. Hot water

3. Acids

4. None of the above

Ans:  Benzoic acid is nearly insoluble in cold water, but soluble in hot water.

4. What is the melting point of benzoic acid?

1. 249.2 °C

2. 122.3 °C

3. 112.3 °C

4. 123.5 °C

Ans:  The melting point of benzoic acid is 122.3 °C.

5. Choose the separation method among the following.

1. Evaporation

2. Filtration

3. Decantation

4. All of the above

Ans: Evaporation, filtration and decantation are different types of separating methods.

6. Crystallisation is an example of _____.

1. Physical change

2. Chemical change

3. Both a & b

4. None of the above

Ans: In crystallisation, the liquid state is converted into a solid, hence it is a physical change. No new products are formed, so not a chemical change.

7. How to separate impure benzoic acid

1. Sublimation

2. Sedimentation

3. Filtration

4. None of the above

Ans: Impure benzoic acid can be purified by sublimation

8. Choose the weak acids.

1. Benzoic acid

2. Hydrochloric acid

3. Acetic acid

4. Both a & c

Ans:  Both Acetic acid and benzoic acid are weak organic acids.

9. Choose the correct order of separation of benzoic acid from its sample.

1. Preparation of solution-Filtration-Cooling-Separation of crystal-Drying

2. Preparation of solution-Separation of crystal-Drying-Filtration-Cooling

3. Separation of crystal-Drying-Filtration-Cooling-Preparation of solution

4. Separation of crystal-Cooling-Preparation of solution-Drying-Filtration

Ans: Correct order of separation of benzoic acid from its sample is the preparation of solution-filtration-cooling-separation of crystal-drying.

10. Benzoic acid is an ____.

1. Mineral acid

2. Aromatic carboxylic acid

3. Aliphatic carboxylic acid

4. Both a & c

Ans: Benzoic acid is an aromatic carboxylic acid.

Conclusion

From the crystallisation experiment described above, we can conclude that crystallisation can be used to differentiate pure benzoic acid from impure samples. Although evaporation can also be used to separate pure samples, crystallisation is more efficient at doing so. This is because pure solid crystals are created during crystallisation, whereas some solid particles in a mixture melt during evaporation, leaving impurities behind. Crystallisation is an example of a chemical change.