An Overview of Class 11 Chemistry Purification Of Chemical Substances By Crystallization Experiment
Chemical compounds contain various impurities; it is necessary to remove these impurities for their proper utilisation. Crystallisation is a method of purifying chemical substances by precipitating the chemicals as crystals from the solution.
Purification of chemical substances by crystallisation is used to harvest salt. Salt-containing water is vapourised to obtain the salt crystals. Purifying naturally occurring potash alum to obtain alum crystals is done by crystallisation.
Table of Content
Aim
Apparatus Required
Theory
Procedure
Observations
Result
Aim
To demonstrate the technique of purification of chemical substances by crystallisation of potash alum
Apparatus Required
Potash Alum Powder
Test Tube
Beaker
Glass Rod
Evaporation Dish
Crystallization Dish
Wire Gauge
Bunsen Burner
Theory
Purification of chemicals can be achieved by crystallization. Crystallization is based on solubility: compounds (solute) are soluble in hot solvents, when a hot solution of the compound is allowed to cool, the solubility decreases and the excess solute precipitates out and solidifies to form crystals of pure compound. The pure crystals can be filtered and separated from the impurities.
Impure Potash alum (Potassium Aluminium Sulfate) is dissolved in water, insolubles are separated by filtration and the solution is concentrated by heating to the crystallization point and subsequently cooled down. The crystals of the potash separate, and the impurities are left behind in the mother liquor.
Procedure
Take the impure potash alum in a clean beaker.
Dissolution of Potash Alum: Dissolve the alum powder using distilled water. Use a glass rod to stir the solution gently.
Heat the solution on a wire gauge at 60o-70o C and add on the impure samples until it is indissoluble
Take a circular filter paper and make a cone and fit it well in a funnel; spray water on the paper cone and paste it well on the funnel wall using a thumb
Place the funnel on a funnel stand and keep an evaporating dish underneath to collect the filtrate
Filtration of insoluble impurities: Hold a glass rod on the funnel in a slanting position and pour the solution. The insoluble impurities are filtered by filter paper, and the filtrate is collected on the evaporation dish.
Heat the filtrate collected in the evaporation dish gently, and stir the solution for uniform evaporation.
Preparation of supersaturated solution: Reduce the volume of the solution to half, take a drop of the solution at this point on a glass rod, and cool it.
The formation of thin potash alum crystals indicates the solution has reached the crystallisation point. Stop the heating after this.
Crystallization: Pour the concentrated solution onto a crystallisation dish, cover it and leave it undisturbed to cool. As the concentrated solution cools down, solid crystals form and separate.
Give some time after the crystal formation takes place, and following that, decant the mother liquor
Wash the crystals with alcohol and cold water.
Dry the crystal using a desiccator or by placing the crystals on a porous plate.
Observations
Result
Large crystals of potash alum were obtained after cooling down the supersaturated solution. Transparent solid potash alum crystals formed as the concentrated solution cooled. The crystals were collected separately, leaving behind the dissolved impurities in the mother liquor.
Precautions
Do not boil the solution
Do not heat the filtrate to dryness
Do not pour the hot solution into the filter paper immediately after heating
Carefully hold the glass during pouring to prevent rupture of the filter paper
Lab Manual Questions
1. What is mother liquor?
Ans: Mother liquor is the remaining solution after the crystals separate out.
2. What is Potash Alum?
Ans: Potash alum is Potassium Aluminium Sulfate KAl(SO4)2·12H2O
3. When does the crystallisation begin?
Ans: First crystallisation starts when the concentrated solution starts cooling.
4. How is the filtrate concentrated?
Ans: The filtrate is concentrated by heating it and reducing its volume.
Viva Questions
1. What are alums?
Ans. Alums are double salts containing sulphate ions and have similar compositions and properties. Some examples of alums are sodium alum, ammonium alum, and chrome alum.
2. What are the methods used in water purification?
Ans. The methods used for water purification are:-
Physical processes, such as filtration, sedimentation, or distillation
Biological processes, such as sand filters, active carbon
Chemical processes, such as flocculation, chlorination, and the use of ultraviolet light.
3. What is crystallisation?
Ans: Crystallization is the process where the insoluble chemical compounds solidify and form crystals when a hot saturated solution of the compound is slowly cooled down.
4. What are crystals?
Ans: Crystals are solids with well defined-geometry and shapes.
5. What is solubility?
Ans: Solubility is the amount of solute which can be dissolved in a solvent to make a saturated solution.
6. Give use of the crystallization method?
Ans: Crystallization is used to purify chemical substances from impurities. Chemical compounds form pure crystals when they precipitate out of the hot saturated solution upon cooling.
7. Name a few uses of Potash Alum.
Ans: Potash Alum is used for the purification of drinking water and in flame retardant. It is also found in baking powder.
8. What is a saturated solution?
Ans: A saturated solution is a solution which can no more dissolve solutes at a given temperature.
9. Why is a hot saturated solution used for crystallisation?
Ans: Heating increases the solubility of a compound, so a hot solution will dissolve more of the given chemical substance and be saturated with a higher concentration of the solute.
10. Why do you cool the solution slowly for crystallisation?
Ans: Saturated solutions are cooled slowly to allow the crystals to grow bigger. It helps in better separation and formation of proper geometry of the crystals.
Practical-Based Questions
What is the nature of Potash Alum
Acidic
Basic
Amphoteric
Neutral
Ans. It is acidic
Large crystals are obtained by
Centrifugation
Crystallization
Sedimentation
Distillation
Ans: Crystallization
What is Potash Alum?
Sodium sulfate
Potassium chloride
Potassium aluminium sulfate
Potassium sulfate
Ans: Potassium aluminium sulfate
Which is the natural mineral source of Potash Alum?
Kalinite
Alunite
Leucite
All of the above
Ans: All of the above
What is the colour of the Potash Alum?
Black
Blue
White
Pink
Ans: White
What is the colour of Potash crystals?
Transparent (Colourless)
Blue
White
Pink
Ans: Transparent (Colourless)
What is the geometry of Potash Alum crystals?
Cubic
Octahedral
Orthorhombic
Triclinic
Ans: Octahedral
A crystal is?
A solid
A solution
A solid with definite geometry
A dilute solution
Ans: A solid with definite geometry
A crystal can be dried by
Paper
Desiccator
Gentle heating
Leaving in the open
Ans: Desiccator
Crystallization is used for
Solidify chemical compound
The precipitate, a solid compound
Purify, a chemical substance
Separate the impurities
Ans: Purify, a chemical substance
Conclusion
Large crystals of the pure substance are obtained by crystallization. The super-saturated solution of Potash alum formed pure crystals when cooled down slowly. The experiment demonstrated the successful purification of potash alum by the process of crystallization.
FAQs on Class 11 Chemistry Purification Of Chemical Substances By Crystallization Experiment
1. For a 5-mark question in the Class 11 exam, what are the essential steps to purify an impure sample of copper sulphate (CuSO₄) using crystallization?
As per the CBSE 2025-26 practical syllabus, the purification of an impure copper sulphate sample by crystallization involves these key steps:
- Preparation of Solution: Dissolve the impure sample in a minimum amount of distilled water in a beaker, stirring constantly.
- Filtration of Hot Solution: Filter the hot solution to remove any insoluble impurities. This step is crucial and must be done quickly to prevent premature crystallization on the filter paper.
- Concentration: Gently heat the filtrate in a china dish over a water bath to evaporate the water until the point of crystallization is reached. This is checked by dipping a glass rod and blowing on it; a thin crust of crystals should form.
- Slow Cooling: Cover the dish with a watch glass and allow it to cool slowly and undisturbed. Slow cooling ensures the formation of large, well-defined crystals.
- Separation and Drying: Separate the crystals (mother liquor) from the liquid (filtrate) by decantation or filtration. Wash the crystals with a small amount of cold water or ethanol to remove adhering impurities and then dry them between folds of filter paper.
2. What is the fundamental principle behind the purification of substances by crystallization, and why is it more effective than simple filtration?
The fundamental principle of crystallization is the difference in solubility of the compound and its impurities in a suitable solvent. The impure substance is dissolved in a solvent at a higher temperature to form a saturated solution. Upon cooling, the pure substance, being less soluble at the lower temperature, crystallises out, while the more soluble impurities remain dissolved in the solvent (mother liquor).
Crystallization is more effective than filtration because filtration only removes insoluble impurities. Crystallization, on the other hand, is designed to separate the desired compound from soluble impurities, which is a more common and challenging purification task.
3. Why is it a critical mistake to heat the solution to complete dryness during a crystallization experiment? What are the expected consequences?
Heating a solution to complete dryness is a major procedural error in crystallization for two main reasons:
- Contamination: Evaporating the solvent completely will cause both the pure substance and the soluble impurities to solidify together. This defeats the entire purpose of purification, as the final product will be just as impure as the starting material.
- Poor Crystal Quality: Rapid evaporation to dryness does not allow for the slow, ordered arrangement of particles into a proper crystal lattice. This results in the formation of a powder or very small, poorly-formed crystals instead of the distinct, geometric shapes expected from a successful crystallization.
4. How would you choose an ideal solvent for purifying a given compound by crystallization? This is a frequently asked conceptual question.
Choosing an ideal solvent is crucial for a successful crystallization. The following characteristics are considered important:
- The compound to be purified should be highly soluble at a high temperature but only sparingly soluble at a low temperature in the chosen solvent.
- The solvent should not react chemically with the substance being purified.
- The impurities present should either be completely insoluble in the solvent (so they can be filtered off) or highly soluble (so they remain in the mother liquor after cooling).
- The solvent should be volatile enough to be easily removed from the final purified crystals but not so volatile that it evaporates too quickly during filtration.
5. What might be the reason if crystals do not form even after a hot, saturated solution is cooled? What steps can be taken to induce crystallization?
If crystals fail to form upon cooling, it is likely due to the formation of a supersaturated solution, where the solute remains dissolved even below its normal saturation temperature. To induce crystallization, you can try the following methods:
- Seeding: Add a tiny, pure crystal of the same substance to the solution. This 'seed' acts as a nucleation site, providing a template for other molecules to deposit onto and begin crystal growth.
- Scratching: Scratch the inside surface of the beaker with a glass rod. The microscopic scratches provide rough surfaces that can act as nucleation points for crystallization to begin.
- Further Cooling: Place the beaker in an ice bath to lower the temperature further, which decreases the solubility and can force the substance to crystallize out.
6. For an exam, differentiate between crystallization and simple evaporation as methods of purification.
The key differences between crystallization and evaporation are important for Class 11 Chemistry:
- Purpose: Crystallization is used to separate a pure solid from a solution containing soluble impurities. Evaporation is used to recover a non-volatile solid solute from a solution where the solvent is volatile, and it does not separate soluble impurities.
- Process: Crystallization involves cooling a concentrated solution to form crystals. Evaporation involves heating a solution to drive off the solvent completely.
- Product: Crystallization yields a pure, solid substance with a definite geometric shape (crystals). Evaporation yields the solid solute along with any soluble impurities that were present, often as a powder or amorphous solid.
7. What important precautions must a student take while performing the crystallization of benzoic acid in the school laboratory?
When performing the crystallization of benzoic acid using water as a solvent, the following precautions are important for both safety and accuracy:
- Use the minimum amount of hot water to dissolve the benzoic acid to ensure the solution is saturated and yields maximum crystals upon cooling.
- Filter the solution while it is still hot to prevent benzoic acid from crystallizing prematurely on the filter paper.
- Do not overheat the solution to dryness, as this will defeat the purpose of removing soluble impurities.
- Allow the solution to cool slowly and without disturbance to obtain well-defined, needle-like crystals. Sudden cooling results in small, impure crystals.
- Wash the final crystals with a very small amount of cold distilled water to remove any adhering mother liquor.
