CBSE Chemistry Experiment Preparation of Crystals of Potash Alum (K2SO4.Al2(SO4)3.24H2O) from the Crude Sample

The potassium sulphate dodecahydrate is chemically known as potash alum. Evaporation or crystallisation can be used for separation, but crystallisation is a better method of separating the mixture than evaporation. This is because, on evaporation, some solid particles in the mixture get decomposed leaving behind impurities whereas, in crystallisation, pure crystals of solids are obtained.

Pure Potash alum can be used for water purification and fire textiles. It retains a variety of extremely complex species, such as aluminium polymers, depending on the many circumstances, such as dosage, pH, and the pace and mixing of the hydroxide supplement.

Table of Content

• Aim

• Apparatus Required

• Theory

• Procedure

• Observations

• Result

• Precautions

• Lab Manual Questions

• Viva Questions

• Practical Based Questions

Aim

To prepare crystals of Potash alum from the crude sample.

Apparatus Required

1. Crude Sample of Potash Alum

2. A China Dish

3. A Funnel

4. An Evaporating Dish

5. A Glass Rod

6. Beaker (400 ml)

7. Water

8. Alcohol

Theory

• Potash alum is chemically known as potassium sulphate, which is very soluble in water. 

• Potash alum dissolved in water along with solid impurities which are insoluble in water can be separated by filtration.

• After concentrating the filtrate, alum crystals that are pure separate as they cool. The mother liquor retains the soluble contaminants.

Filtration

The technique of removing solid particles from a liquid or gaseous fluid by passing the fluid through a filter media while keeping the solid particles behind. The desired product could be either the fluid that has been cleared or the fluid that has had its solid particles removed.

Concentration

A solute and a solvent are separated during a concentration procedure.

Crystallisation

Crystallisation takes place in two main stages. The first is nucleation, which is the formation of crystals from a supercooled liquid or a supersaturated solvent. The second process, known as crystal growth, results in an increase in particle size and a crystallised state.

Procedure

1. Preparation of Alum Solution

• A 400 ml beaker is required. Add 25–30 ml of water and 5–6 grams of the crude potash alum sample to it.

• To clarify the solution, shake the beaker's contents.

• The sample's total amount of alum should be warmed up to dissolve it.

2. Filtration of the Solution Followed By Concentration of Filtrate to Crystallisation Point

• Filter the mixture, then put the filtrate in a China bowl.

• The solution should be reduced to about one-third of its original volume in the China dish by heating it over a sand bath or wire gauge.

• With a glass rod, the solution is thoroughly swirled while it warms up to prevent the development of a crust on the dish's edge. 

• Remove a drop of the solution from the glass rod's end and blow it out to chill it.

• The glass rod's appearance of a thin crust indicates that the crystallisation point has been reached. At this point, turn off the heat by turning off the burner.

•  Place the heated, saturated solution in a dish for crystallisation.

Filtration of the solution

3. Cooling Hot Solution

• Place the dish containing the hot, saturated solution atop a beaker of fully filled water and let it slowly cool for a while. 

• Alum crystals that are clear, colourless and octahedral start to break apart.

• The crystallisation is finished in around 30 minutes.

4. Separation of Crystals

• Carefully pour off the mother liquor.

• Cleanse the crystal alum in a cool alcohol and water solution. 

• Remove the crystals from the solution-soaked filter paper.

Filtrate to China dish

5. Drying

• Transferring the crystals to another filter paper, dry them by gently pushing in between the filter paper’s folds.

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

Observations

Result

From this crystal alum experiment, we separated crystals of alum from its impure sample and obtained transparent colourless crystals of potash alum.

Precautions

• Only the amount of filtrate needed to cause crystallisation should be evaporated.

• Never let it get too hot or dry. Avoid heating the solution too much, so spilling of hot solution can prevent it.

• The solution should be gently chilled without being disturbed. Never let it cool down quickly.

• Wash crystals in liquid to get crystals in solid form.

Lab Manual Questions

1. Why are the alum crystals washed with an alcohol-water mixture rather than deionized water?

Ans: A vacuum filter is used to separate the alum crystals from the solution, and they are then cleaned with a solution of alcohol and water. The crystals are not dissolved by this washing liquid; it only eliminates any contaminants from them. Since alcohol is more volatile than water, it also helps to dry the crystals quickly.

2. How to calculate the crystallisation of a sample?

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.

3. What is the need of heating crude samples?

Ans: On heating potash alum, it loses water of crystallisation. It loses water molecules and gets converted into anhydrous potassium aluminium sulphate.

4. What is the principle behind crystallisation?

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.

Viva Questions

1. What is the purpose of crystallisation in Labs?

Ans: Crystallisation is mostly used in the organic chemistry lab to purify impure solids.

2. What is meant by the saturated solution?

Ans: A solution is said to be saturated when it has dissolved all the solute it can.

3. What is the chemical name of the potash alum?

Ans: Aluminium sulphate

4. What are the characteristics of crystals?

Ans: They are transparent solids with definite pattern arrangements of atoms within them.

5. What are the uses of potash alum in daily life?

Ans: Potash alum is a natural substance used for water purification, fireproof textiles and in baking powders.

6. What is the pH of the potash alum?

Ans: The pH of the Potash alum is around 3.5.

7. What are the three types of crystallisation?

Ans: Evaporative crystallisation, cooling crystallisation from solution or the melt and reactive crystallisation or precipitation.

8. Why doesn't the hot, saturated solution cool down immediately? 

Ans: Crystals enlarge as the saturated fluid slowly cools. Instead of providing a massy substance with improper geometry, it aids in their better separation into units.

9. What is meant by solubility?

Ans: The ability of a substance, the solute, to form a solution with another substance, the solvent.

10. What is meant by crude substance?

Ans: Crude substances are in a natural or unrefined state and have not yet been used in manufacturing processes.

Practical-Based Questions

1. Which filter paper is used in laboratories, mainly for filtration?

1. Whatman  41

2. Whatman 1

3. Whatman 42

4. None of the above

Ans: Whatman 41 filter paper is commonly used for quantitative and qualitative analysis in laboratories.

2. What is the colour of the crystals of the potash alum?

1. White

2. Colourless

3. Green

4. Red

Ans: Potash alum is a colourless crystal.

3. Select common laboratory solvents for crystallisation.

1. Water

2. Ethanol

3. Acetone

4. All f the above

Ans: Water, ethanol and acetone are common solvents used for crystallisation.

4. What is the product obtained when potash alum reacts with an excess of NaOH?

1. Clear solution

2. A white precipitate

3. A bluish precipitate

4. A yellowish precipitate

Ans: Reaction of NaOH with potash alum results in the formation of water-soluble sodium aluminate.

5. What is the chemical formula of a potash alum?

1.  K2SO4.Al2(SO4)3.24H2O

2.  K2SO4.Al2(SO4)3.12H2O

3.  KSO4.Al2(SO4)3.24H2O

4.  K2SO4.Al(SO4)3.24H2O

Ans: The chemical formula of potash alum is  K2SO4.Al2(SO4)3.24H2O.

6. Choose the order of preparation of potash alum.

1. Preparation of solution-cooling-filtration-separation-drying

2. Preparation of solution-filtration-cooling-separation-drying

3. Preparation of solution-separation-filtration-cooling-drying

4. Drying-preparation of solution-cooling-separation-filtration

Ans: Preparation of potash alum from crude follows the order: Preparation of solution-filtration-cooling-separation-drying.

7. What are the metals present in the potash alum?

1. Aluminium, Iron

2. Potassium, Iron

3. Aluminium, Phosphorus

4. Potassium, Aluminium

Ans:  Potassium and aluminium are the metals present in potash alum.

8. Potash alum turns blue litmus paper red. It is  ________ in nature.

1. Basic

2. Acidic

3. Neutral

4. None of the above

Ans:  Acid turns blue litmus to red. This implies potash alum is acidic in nature.

9. What technique is used to isolate the alum after it was recrystallized?

1. Gravity filtration

2. Centrifugal filtration

3. Vacuum filtration

4. Hot filtration

Ans: Vacuum filtration is used to isolate the alum after it was recrystallized.

10.  What property of potash alum makes it useful for the purification of water?

1. Solubility in water

2. Removal of hardness

3. Melting point

4. None of the above

Ans:  Potash alum help in the removal of hardness from water.

Conclusion

From the above crystal alum experiment, we concluded that pure potash alum can be separated from crude samples through crystallisation. Evaporation is also a method to separate pure samples, but it is not as effective in separating the mixture as crystallisation. This is because some solid particles in a mixture dissolve during evaporation, leaving impurities behind, but pure solid crystals are produced during crystallisation.