What is Alum? Formula, Types, Preparation, and Practical Uses
Alum is a key compound in inorganic chemistry and an everyday life substance. Understanding alum helps students connect chemistry concepts to water treatment, medicine, and other common uses. This page explains alum’s formula, preparation, properties, types, and importance for exams and real-world problem-solving.
What is Alum in Chemistry?
A Alum refers to a group of hydrated double salts containing aluminum sulfate, water of crystallization, and the sulfate of another metal such as potassium or sodium. The most common alum is potassium alum (KAl(SO₄)₂·12H₂O), also called potash alum or fitkari in Hindi. This concept appears in chapters related to double salts, water purification, and hydrated salts, making it a foundational part of your chemistry syllabus.
Molecular Formula and Composition
The molecular formula of alum is KAl(SO₄)₂·12H₂O (for potash alum). It consists of potassium ions, aluminium ions, sulfate ions, and 12 water molecules of crystallization. Alum belongs to a family of hydrated double sulfates, and its structure forms large, transparent crystals. Other types of alum follow the general formula XAl(SO₄)₂·12H₂O, where X = K, Na, NH₄, etc.
Preparation and Synthesis Methods
Alum can be prepared both in laboratories and on an industrial scale:
1. Dissolve equimolar amounts of potassium sulfate (K₂SO₄) and aluminum sulfate (Al₂(SO₄)₃) in water and mix thoroughly.2. Let the solution stand and cool; large colorless crystals of alum will form upon slow evaporation.
3. Industrially, alum is obtained by treating bauxite (aluminum ore) with sulfuric acid to get aluminum sulfate, then adding potassium sulfate and evaporating the solution to crystallize out alum.
Physical Properties of Alum
Alum displays these physical properties:
- Appearance: Colorless or white, glassy, crystalline solid
- Solubility: Highly soluble in water
- Taste: Sweet and astringent
- Odor: Odorless
- Density: 1.725 g/cm³ (potash alum)
- Melting Point: Starts to liquefy at 92.5°C (decomposes on further heating)
- Crystalline Form: Regular octahedral crystals
Chemical Properties and Reactions
Alum shows several chemical behaviors classified as:
- Acidic aqueous solution (slightly acidic pH)
- Dissociates in water into ions: K+, Al3+, and SO₄2-
- Reacts with alkalies to give gelatinous aluminum hydroxide [Al(OH)₃]
- Acts as a coagulant for colloidal particles in water treatment
- On heating, loses water of crystallization and decomposes
Frequent Related Errors
- Confusing alum with neutral molecules or different acids.
- Not specifying the type of alum (potash, soda, chrome, etc.) in formula-based questions.
- Ignoring hydrated structure while writing formula or calculating molar mass.
- Mislabeling fitkari as simple aluminum sulfate instead of double salt alum.
Uses of Alum in Real Life
Alum is widely used in real life and industries:
- Water purification as a coagulant: helps remove suspended impurities from drinking water
- In medicine: as an astringent and antiseptic for small cuts, shaving, and oral health (gargles)
- In baking powder as an acidulant (food-grade alum only)
- Used in textile dyeing and tanning leather
- In the production of lake pigments and fireproofing of materials
- As a preservative in pickling (regulated amounts)
Relevance in Competitive Exams
Students preparing for NEET, JEE, and Olympiads should be familiar with alum basics, its chemical formula, uses, and preparation. Concept-testing questions often include identification of alum as a double salt, writing equations for its preparation, and its role in water treatment. Knowing the difference between simple salts and double salts or hydrated salts is highly exam-relevant.
Relation with Other Chemistry Concepts
Alum is closely related to topics such as hydrated salts and water purification methods, helping students build conceptual bridges between physical chemistry, inorganic reactions, and chemical technology chapters. Its preparation and crystallization link to practical lab work in schools.
Step-by-Step Reaction Example
- Start with the reaction setup.
Mix potassium sulfate and aluminum sulfate in water.
- Write the balanced equation.
K₂SO₄ (aq) + Al₂(SO₄)₃ (aq) + 24H₂O (l) → 2KAl(SO₄)₂·12H₂O (crystals)
- Explain each intermediate or by-product.
On cooling and evaporation, alum crystals separate due to reduced solubility.
- State reaction conditions.
Slow evaporation, no catalyst needed. Room temperature for crystal growth.
Lab or Experimental Tips
Remember alum by the rule of “double salt + 12 H₂O” for exam and lab recall. For growing large alum crystals, use saturated hot solution and allow slow cooling without disturbance. Vedantu educators often highlight slow evaporation and undisturbed environment for perfect crystal growth during live sessions.
Try This Yourself
- Write the IUPAC name of potash alum.
- Classify alum as simple salt, double salt, or mixed salt.
- List two uses of alum in homes or schools.
- Draw and describe the crystal shape of alum.
Final Wrap-Up
We explored alum—its chemical structure, formula, types, preparation, properties, and broad uses, connecting chemistry theory with laboratory and daily-life relevance. For more in-depth explanations, solved problems, and exam-prep tips on alum and chemistry topics, join a live class or explore free notes on Vedantu for confident learning.
Find more related concepts here: Crystallization.
FAQs on Alum – Formula, Types, Uses & Properties in Chemistry
1. What is alum?
Alum is a group of chemical compounds, generally hydrated double salts, most commonly known as potassium aluminum sulfate (KAl(SO₄)₂·12H₂O). It's widely used in water purification, as an astringent, and in various industrial processes. The term 'alum' often refers to potash alum, but other types exist, like ammonium alum and chrome alum.
2. What is the chemical formula of alum (fitkari)?
The most common type of alum, potassium alum, has the chemical formula KAl(SO₄)₂·12H₂O. This shows it contains potassium (K), aluminum (Al), sulfate (SO₄), and twelve water molecules (12H₂O).
3. What are the different types of alum?
Several types of alum exist, differing in the cation replacing potassium:
- Potash alum: KAl(SO₄)₂·12H₂O
- Ammonium alum: NH₄Al(SO₄)₂·12H₂O
- Sodium alum: NaAl(SO₄)₂·12H₂O
- Chrome alum: KCr(SO₄)₂·12H₂O
4. What are the properties of alum?
Alum typically exists as colorless crystals. Key properties include:
- High solubility in water
- Astringent taste
- Ability to form octahedral crystals
- Acts as a coagulant in water treatment
5. How is alum used in water purification?
Alum acts as a coagulant, neutralizing the negative charges of suspended impurities in water. This causes the impurities to clump together, forming larger particles (flocs) that settle easily, leaving clearer water. This is part of a larger water treatment process.
6. What are the uses of alum?
Alum has diverse applications:
- Water purification
- Tanning leather
- Dyeing textiles
- Baking powder (as a leavening agent)
- Medicine (as an astringent for minor cuts and antiseptic)
7. Is alum safe for skin?
Alum's use on skin is traditional, often as an aftershave astringent or for minor cuts. However, prolonged or excessive use can irritate sensitive skin. It's best to use it sparingly and avoid contact with broken skin or eyes.
8. What is the difference between alum and aluminum sulfate?
Alum is a double salt, a combination of two different salts. Aluminum sulfate (Al₂(SO₄)₃) is a single salt, a component of alum. Alum contains aluminum sulfate, but aluminum sulfate itself is not an alum.
9. How is alum prepared?
Alum can be prepared through various methods. One common method involves reacting aluminum hydroxide with sulfuric acid, followed by the addition of a soluble potassium salt to crystallize potash alum. Industrial methods often utilize bauxite ore as a starting material.
10. What are some common lab experiments involving alum?
Common experiments include growing alum crystals from a saturated solution to observe crystal formation and exploring alum's use as a coagulant in simulated water purification experiments. These experiments demonstrate key chemical concepts and practical applications.
11. What is fitkari in Hindi?
Fitkari is the Hindi word for alum, specifically referring to potassium alum.
12. Can alum be used in food?
Food-grade alum has limited culinary uses, such as in some pickling recipes. However, it's crucial to use only food-grade alum in appropriate amounts, as excessive consumption can pose health risks.
