Class 11 Chemistry Lab Manual – Purification of Chemical Substances by Crystallisation

Crystallisation is a fundamental technique in chemistry used to purify solid substances. In Class 11 Chemistry, students learn how to remove impurities from a solid by dissolving it in a suitable solvent and allowing pure crystals to form. This experiment introduces essential skills in solubility, heating, filtration, and careful observation.

As per the curriculum prescribed by the Central Board of Secondary Education, practical work like crystallisation is a critical component of Class 11 Chemistry. Students are evaluated based on their understanding of the method, proper handling of apparatus, safety practices, and systematic recording of results. Studentbro.in provides detailed guidance to ensure students perform this experiment effectively.

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Importance of Crystallisation

Crystallisation is important because it:

• Purifies solid substances from impurities

• Demonstrates solubility differences at different temperatures

• Teaches controlled heating and slow cooling techniques

• Helps in obtaining pure, well-formed crystals for analysis

• Is widely used in chemical laboratories and industries

Learning crystallisation strengthens practical understanding of physical chemistry principles.

Objective of the Experiment

The main objectives include:

• To purify a given impure solid using crystallisation

• To understand solubility and the effect of temperature on dissolution

• To observe crystal formation

• To learn proper filtration and handling techniques

• To record observations and results systematically

These objectives help students develop essential laboratory skills.

Theory Behind Crystallisation

Crystallisation is based on the principle that solubility of a solid in a solvent increases with temperature. An impure solid is dissolved in a suitable solvent at high temperature to form a saturated solution. On slow cooling, pure crystals are formed as the solubility decreases. Impurities remain dissolved in the solution, called the mother liquor.

This method is widely used in laboratories to purify chemicals such as alum, copper sulfate, and sodium chloride. Crystallisation demonstrates concepts like solubility, saturation, and controlled phase change.

Apparatus and Materials Required

• Beaker

• Conical flask

• Glass rod

• Funnel

• Filter paper

• Tripod stand

• Burner or heating source

• Spatula

• Sample of impure solid

• Suitable solvent

Proper handling of heating equipment and glassware is essential to avoid accidents.

Procedure of the Experiment

The general steps are:

1. Weigh a small quantity of impure solid.

2. Place it in a beaker and add a minimum quantity of hot solvent to dissolve the solid.

3. Stir the mixture gently to aid dissolution.

4. Filter the hot solution to remove insoluble impurities.

5. Allow the filtrate to cool slowly at room temperature to form crystals.

6. Collect the crystals by filtration and dry them carefully.

7. Observe the formation, size, and purity of the crystals.

Slow cooling ensures the formation of well-shaped crystals and better purity.

Observations to Record

Students should note:

• Quantity of impure solid

• Type and volume of solvent used

• Temperature of the solution

• Time taken for crystal formation

• Appearance and size of crystals

Accurate and systematic observations are essential for practical evaluation.

Result of the Experiment

The impure solid is successfully purified, and pure crystals are obtained. The effectiveness of purification can be observed by the sharpness and clarity of crystals.

Precautions to Follow

• Use a suitable solvent for the solid

• Avoid overheating the solution

• Stir gently to prevent splashing

• Allow slow cooling without disturbance

• Use clean and dry apparatus

• Handle hot glassware with care

• Dispose of mother liquor safely

Following these precautions ensures safety and accurate results.

Common Errors to Avoid

• Using excessive solvent

• Heating too rapidly

• Disturbing the solution while cooling

• Impure filtration apparatus

• Incomplete drying of crystals

Avoiding these mistakes improves reliability and quality of results.

Viva Questions for Crystallisation Experiment

Students may be asked:

• What is crystallisation?

• Why should the solution be cooled slowly?

• How does solubility change with temperature?

• Why are impurities left in the mother liquor?

• How do you select a suitable solvent?

• What precautions are important during crystallisation?

Preparation of these questions ensures confidence during viva exams.

Applications of Crystallisation

Crystallisation is widely used in:

• Purification of salts and chemicals

• Pharmaceutical manufacturing

• Laboratory preparation of pure substances

• Food industry for sugar and salt purification

• Chemical research and quality control

This technique connects theoretical knowledge with real-life chemical applications.

Role in Practical Examination

In practical exams, students are assessed based on:

• Correct apparatus setup

• Accuracy of procedure

• Observation recording

• Purity and quality of crystals obtained

• Viva performance

Mastering crystallisation ensures better performance in practical assessments.

Benefits of Learning This Technique

• Develops observation and patience

• Enhances practical handling skills

• Strengthens understanding of solubility and phase changes

• Improves systematic recording and analysis

• Builds confidence in laboratory work

Crystallisation provides a foundation for advanced purification techniques in chemistry.

How Studentbro.in Helps Students

Studentbro.in offers:

• Step-by-step explanation of crystallisation procedure

• Detailed theory and concept understanding

• Observation and result format

• Safety and precaution guidelines

• Important viva questions

• Exam-oriented practical preparation tips

This ensures students are fully prepared for chemistry laboratory sessions.

Conclusion

The Purification of Chemical Substances by Crystallisation experiment is an essential practical in Class 11 Chemistry. It teaches students how to purify solids, understand solubility, and observe crystal formation carefully. By following a systematic procedure and recording observations accurately, students can successfully perform this experiment and gain a strong foundation in laboratory techniques.

With structured guidance from Studentbro.in, students can confidently conduct crystallisation experiments, maintain well-organized practical records, and excel in Class 11 Chemistry practical examinations.