CBSE Chemistry Experiment Purification of Chemical Substances by Crystallization

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

1. Potash Alum Powder

2. Test Tube

3. Beaker

4. Glass Rod

5. Evaporation Dish

6. Crystallization Dish

7. Wire Gauge

8. Bunsen Burner

Theory

1. 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.

2. 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

1. Take the impure potash alum in a clean beaker.

2. Dissolution of Potash Alum: Dissolve the alum powder using distilled water. Use a glass rod to stir the solution gently.

3. Heat the solution on a wire gauge at 60o-70o C and add on the impure samples until it is indissoluble

4. 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

5. Place the funnel on a funnel stand and keep an evaporating dish underneath to collect the filtrate

6. 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.

7. Heat the filtrate collected in the evaporation dish gently, and stir the solution for uniform evaporation. 

8. 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. 

9. The formation of thin potash alum crystals indicates the solution has reached the crystallisation point. Stop the heating after this.

10. 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.

11. Give some time after the crystal formation takes place, and following that, decant the mother liquor

12. Wash the crystals with alcohol and cold water.

13. 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

1. Do not boil the solution

2. Do not heat the filtrate to dryness

3. Do not pour the hot solution into the filter paper immediately after heating

4. 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:- 

1. Physical processes, such as filtration, sedimentation, or distillation

2. Biological processes, such as sand filters, active carbon

3. 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

1. What is the nature of Potash Alum 

1. Acidic

2. Basic

3. Amphoteric

4. Neutral

Ans. It is acidic

2. Large crystals are obtained by 

1. Centrifugation

2. Crystallization

3. Sedimentation

4. Distillation

Ans: Crystallization

3. What is Potash Alum?

1. Sodium sulfate

2. Potassium chloride

3. Potassium aluminium sulfate

4. Potassium sulfate

Ans: Potassium aluminium sulfate

4. Which is the natural mineral source of Potash Alum?

1. Kalinite

2. Alunite

3. Leucite

4. All of the above

Ans: All of the above

5. What is the colour of the Potash Alum?

1. Black 

2. Blue 

3. White

4. Pink

Ans: White

6. What is the colour of Potash crystals?

1. Transparent (Colourless)

2. Blue 

3. White

4. Pink

Ans: Transparent (Colourless)

7. What is the geometry of Potash Alum crystals?

1. Cubic

2. Octahedral

3. Orthorhombic

4. Triclinic

Ans: Octahedral

8. A crystal is?

1. A solid

2. A solution

3. A solid with definite geometry

4. A dilute solution

Ans: A solid with definite geometry

9. A crystal can be dried by 

1. Paper

2. Desiccator

3. Gentle heating

4. Leaving in the open

Ans: Desiccator

10. Crystallization is used for

1. Solidify chemical compound

2. The precipitate, a solid compound

3. Purify, a chemical substance

4. 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.