Chemistry Experiment- To Prepare a Pure Sample of Potash Alum
In medicines, potassium alum is primarily used as an astringent (or styptic) and antiseptic. Styptic pencils are rods made of potassium alum or aluminium sulphate that are used topically to reduce bleeding in minor cuts and abrasions, nosebleeds, and haemorrhoids and relieve pain from stings and bites. After shaving, potassium alum blocks are rubbed over wet skin.
Potassium alum is also used topically to treat pimples and acne, as well as to cauterise aphthous ulcers in the mouth and canker sores because it has a drying effect on the area and reduces irritation. It has been used to stop bleeding in cases of hemorrhagic cystitis and treat hyperhidrosis in some countries.
Table of Contents
Aim of the Experiment
Apparatus Required
Theory
Procedure
Observations
Result
Precautions
Aim of the Experiment
To prepare potash alum and submit it for inspection.
Apparatus Required
Two Beakers
China-Dish
Funnel
Funnel-Stand
Glass-Rod
Wash-Bottle
Tripod Stand
Wire-Gauze
Potassium Sulphate
Aluminium Sulphate
Dilute Sulphuric Acid
Theory
Potash alum is made by dissolving an equimolar mixture of hydrated aluminium sulphate and potassium sulphate in a small amount of water containing a little sulphuric acid and then crystallising the resulting solution, which produces octahedral crystals of potash alum.
Procedure
Take a beaker with a capacity of 250 mL. After washing, add 2.5 g potassium sulphate crystals to it. Pour in about 20 mL of water. Stir the crystals to dissolve them. If necessary, warm.
Transfer 10 g aluminium sulphate crystals to the other 250 ml beaker after washing it with water. To prevent aluminium sulphate hydrolysis, add about 20 mL of water and 1 mL of dilute sulphuric acid. Five minutes on high heat If the milkiness persists, filter the solution.
Place the china dish on a wire gauze over a burner and combine the two solutions. Using a glass rod, stir the solution. Concentrate the solution until it reaches the crystallisation point. Place the dish on top of a beaker filled with cold water.
Potash alum crystals separate quickly. Remove the mother liquor and rinse the crystals with a small amount of ice-cold water.
Place the crystals between filter paper pads or spread them over a porous plate to dry.
Observations
Result
Potash alum crystals are octahedral in shape.
Precautions
To get good crystals, slowly cool the solution.
Do not disturb the solution when cooling.
Lab Manual Questions
1. How do you test the purity of alum?
Ans. The first test is to find the melting temperature of the compound and compare this value with the accepted (published) value for alum (92.5°C). The second test determines the water of hydration present in the alum crystals. The third test is a chemical test to determine the per cent sulfate in your sample of alum.
2. What happens when you add KOH to alum?
Ans. A wispy, gelatinous precipitate will form upon adding a small amount of KOH to the aqueous alum solution. Further addition of KOH will cause the precipitate to re-dissolve.
3. What happens when alum is mixed with water as a coagulant?
Ans. When alum is added to water, it reacts with the alkalinity present in water. It leads to the formation of a sticky gelatinous precipitate of Aluminium hydroxide, which attracts fine suspended impurities in water over its surface and gets easily settled in the following sedimentation process.
4. Why is alum the most preferred coagulant?
Ans. Alum is preferred over other coagulants because it reduces the taste and odour in addition to turbidity and produces lighter flocs that can be broken easily. The ferric sulphate and ferric chloride so formed are called Chlorinated Copperas.
Viva Questions
1. Which process can purify the sample of potash alum?
Ans. Crystallisation can be used to purify the sample of potash alum.
2. Why do we use distilled water in potash alum?
Ans. The alum used for purifying water is known as potash alum. Potash alum is generally used to remove the hardness of the water. Alum precipitates the insoluble colloidal materials in the water and kills or lowers the total bacterial content of the water.
3. How do you separate alum from water?
Ans. In crystallisation, the alum is heated. During heating, the pure alum separates from its solution of impurities in the form of a crystal.
4. Why is potash alum called double salt?
Ans. Potash alum is double potassium sulphate of aluminium, i.e. a double salt, as it contains more than one cation.
5. Where is alum found?
Ans. Alum is a mineral found in nature in both pure and impure forms. It is a mineral salt. It is obtained from the soil ore found in Nepal, Bihar, Punjab, and the Kathiawar area.
6. What is the role of alum in water purification?
Ans. Aluminium sulfate or alum is used as a flocculant to remove colour and turbidity from water supplies. The alum promotes the coagulation of fine particles. Once coagulated, they sediment down.
7. Is alum basic or acidic?
Ans. The aqueous solution of potash alum is acidic because it is a double salt of strong acid.
8. What happens when alum is heated?
Ans. On heating potash alum, it losses water of crystallisation. When heated they liquefy and if the heating is continued, the water of crystallization is driven off.
9. Is alum a mineral?
Ans. Yes. Alum is a hydrous potassium aluminium sulphate mineral.
Practical Based Questions
1. Potash alum formula is
(a) $${{K}_{2}{S}{O}_{4}{.}{Al}_{2}{(}{S}{O}_{4}{)}_{3}{.}{24}{H}_{2}{O}}$$
(b) $${{K}{H}{S}{O}_{4}{.}{Al}_{2}{(}{S}{O}_{4}{)}_{3}{.}{24}{H}_{2}{O}}$$
(c) $${{K}_{2}{S}{O}_{4}{.}{Al}_{2}{(}{S}{O}_{4}{)}{3}{.}{21}{H}_{2}{O}}$$
(d) None of the above
Answer: (a)
2. What is the common name of the potash alum?
(a) Alum
(b) Fitkari
(c) Both (a) and (b)
(d) None of the above
Answer: (c)
3. What is the colour of the potash alum?
(a) White
(b) Colourless
(c) Off-white
(d) None of the above
Answer: (b)
4. What is the IUPAC name of the potash alum?
(a) Aluminium potassium sulphate dodecahydrate
(b) Alumio potassium sulphate dodecahydrate
(c) Potassium aluminium sulphite dodecahydrate
(d) Potassium alumino sulphite dodecahydrate
Answer: (a)
5. Potash alum is a
(a) Simple salt
(b) Complex salt
(c) Acid salt
(d) Double salt
Answer: (d)
6. Potash alum is used in the purification of water because
(a)It kills the microorganisms
(b)It precipitates the colloidal matter
(c)It removes the hardness of water
(d)It catalyses the removal of impurities
Answer:(b)
7. Which one of the following is not a use of potash alum?
(a)As a styptic in arresting bleeding
(b)As a pesticide
(c)As a mordant in dyeing
(d)As coagulant for colloidal clay in water
Answer: (b)
8. The molar mass of the potash alum is
(a)474.3884 g/mol
(b)305.3884 g/mol
(c)400.3884 g/mol
(d)200.3884 g/mol
Answer: (a)
9. What quantity of potash-alum is required to prepare 500ml solution having strength 1.5F?
(a)711 gm
(b)355.5 gm
(c)35.55 gm
(d)71.1 gm
Answer: (b)
10. Potash alum is used as
(a)Water softener
(b)Mordant
(c)Stop bleeding
(d)Drying agent
Answer: (a)
Conclusion
Potash alum, also known as potassium sulphate dodecahydrate, is a chemical compound that is widely used. It is double salt that is commonly used in medicine and water treatment. In medicines, potassium alum is primarily used as an astringent (or styptic) and antiseptic. Styptic pencils are rods made of potassium alum that is used topically to reduce bleeding in minor cuts and abrasions, nosebleeds, and haemorrhoids and relieve pain from stings and bites. After shaving, potassium alum blocks are rubbed over wet skin.
Potash alum is made by dissolving an equimolar mixture of hydrated aluminium sulphate and potassium sulphate in a small amount of water containing a little sulphuric acid and then crystallising the resulting solution, which produces octahedral crystals of potash alum.
FAQs on To Prepare a Pure Sample of Potash Alum
1. What is the chemical formula for potash alum, and why is it classified as a double salt in the CBSE Class 12 syllabus?
The chemical formula for potash alum is K₂SO₄·Al₂(SO₄)₃·24H₂O. It is called a double salt because it is a stable crystalline compound formed from two different simple salts—potassium sulphate (K₂SO₄) and aluminium sulphate (Al₂(SO₄)₃)—that crystallise together from an equimolar solution in a fixed stoichiometric ratio.
2. What is the most important reason for adding dilute sulphuric acid while preparing a pure sample of potash alum?
In the preparation of potash alum, dilute sulphuric acid is added to prevent the hydrolysis of aluminium sulphate. Without the acid, aluminium sulphate would react with water to form a gelatinous precipitate of aluminium hydroxide, Al(OH)₃, which would make the solution turbid and result in an impure sample.
3. How does the rate of cooling affect the quality of potash alum crystals formed? What is the expected shape of the crystals?
The rate of cooling is critical for obtaining high-quality crystals. Slow and undisturbed cooling of the hot, concentrated solution allows for the formation of large, well-defined, and pure crystals. Rapid cooling leads to the formation of small, poorly-formed, and impure crystals. The expected geometric shape of pure potash alum crystals is octahedral.
4. What are two crucial precautions a student must take to maximise the yield for this practical exam?
To maximise the yield and score well in the practical exam, a student must focus on the following precautions:
- Avoid Over-concentration: Heat the solution only until the point of crystallisation is reached. Do not heat it to complete dryness, as this can decompose the salts and reduce the yield.
- Wash with Ice-Cold Water: The final crystals must be washed with a very small amount of ice-cold water. Using warm water or too much water will dissolve a significant portion of the product, leading to a lower yield.
5. Why are the crystals of potash alum colourless? What does this imply about its constituent ions?
Potash alum crystals are colourless because their constituent ions (K⁺, Al³⁺, and SO₄²⁻) do not absorb light in the visible region of the spectrum. The metal cations, potassium (K⁺) and aluminium (Al³⁺), are not transition elements and lack partially filled d-orbitals. Therefore, d-d electron transitions, which are responsible for colour in many inorganic compounds, cannot occur.
6. What would a student observe if they used scrap aluminium and potassium hydroxide (KOH) as starting materials instead of pre-made sulphates?
If starting with scrap aluminium and KOH, a student would first observe the vigorous evolution of hydrogen gas as the aluminium dissolves in the strong alkali to form potassium aluminate. After this, sulphuric acid would be carefully added to first neutralise the excess KOH and then to provide the sulphate ions needed to form both aluminium sulphate and potassium sulphate in the solution, from which the alum would eventually crystallise.
7. What is the standard confirmatory test for the presence of sulphate ions in the prepared sample of potash alum, as expected in a viva voce?
The confirmatory test for sulphate ions (SO₄²⁻) is the Barium Chloride (BaCl₂) test. To perform this, dissolve a small amount of the prepared alum in water, add a few drops of dilute hydrochloric acid, and then add barium chloride solution. The formation of a thick, white precipitate of barium sulphate (BaSO₄), which is insoluble in HCl, confirms the presence of sulphate ions.
