Alums: We often observe that a transparent, crystal-like stone is added to impure water or muddy water to get pure water. Have you ever thought about this stone-like appearing substance? The answer is ‘Alum’. Chemically alum is a special class of chemical compound having double sulphate salts. It acts as a coagulant. Thus, when an alum is added to the impure water, all dirt gets coagulated with alum particles to form large particles that can be removed by using a sieve or by decantation as heavier particles will settle down the container.

The general chemical formula of alum is \(\mathrm{AM}\left(\mathrm{SO}_{4}\right)_{2} .12 \mathrm{H}_{2} \mathrm{O}\), where ‘\(\mathrm{A}\)’ represents a monovalent cation such as potassium or ammonium and ‘\(\mathrm{M}\)’ represents a trivalent metal ion such as aluminium or chromium (III). Each molecule of an alum is surrounded by water molecules, i.e., it is hydrated, and upon heating, it loses water molecules and forms an amorphous powder. Alums are used in several industrial and household works such as purification of water; since olden days, alums are used to stop bleeding and prevent infection in case of minor cuts, they are also present in toothpaste, etc.

Definition of Alums

In chemistry, alums are crystalline double sulphates of a monovalent metal (or group or cation) and a trivalent metal having general chemical formula as \(\mathrm{AM}\left(\mathrm{SO}_{4}\right)_{2} \cdot 12 \mathrm{H}_{2} \mathrm{O}\), where ‘\(\text {A}\)’ represents a monovalent cation such as potassium or ammonium and ‘\(\text {M}\)’ represents a trivalent metal ion such as aluminium or chromium (III). It is a colourless astringent compound which is a hydrated double sulphate of metals like aluminium and potassium, used in solution in dyeing and tanning. Mostly, “alum” refers to potassium alum, which is the hydrated potassium aluminium sulfate with the chemical formula \(\mathrm{KAl}\left(\mathrm{SO}_{4}\right)_{2} \cdot 12 \mathrm{H}_{2} \mathrm{O}\).

Types of Alums

1. Potassium Alum: Potassium alum is popularly known as potash alum. It is hydrated aluminium potassium sulphate having chemical formula \(\mathrm{KAl}\left(\mathrm{SO}_{4}\right)_{2} \cdot 12 \mathrm{H}_{2} \mathrm{O}\). This is one of the most common types of alum that we find in the grocery store used for pickling and baking powder.

2. Soda Alum: Soda alum or Sodium alum has the chemical formula \(\mathrm{NaAl}\left(\mathrm{SO}_{4}\right)_{2} \cdot 12 \mathrm{H}_{2} \mathrm{O}\). It is mostly used in baking powder and as an acidulant in food.

3. Ammonium Alum: Ammonium alum is made up of aluminium hydroxide, sulphuric acid and ammonium sulphate and forms a solid solution with potassium alum. Its chemical formula is \(\left(\mathrm{NH}_{4}\right) \mathrm{Al}\left(\mathrm{SO}_{4}\right)_{2} .12 \mathrm{H}_{2} \mathrm{O}\). Ammonium alum is used in tanning, dyeing textiles, producing textiles flame retardant, in the production of porcelain cement and vegetable glues, in water purification and in some deodorants.

4. Chrome Alum: It is also known as Chromium (III) potassium sulphate. It is a hydrated form of the potassium-double sulphate salt of chromium. The chemical formula of chrome alum is \(\mathrm{KCr}\left(\mathrm{SO}_{4}\right)_{2} \cdot 12 \mathrm{H}_{2} \text {O}\). This deep violet compound is used in leather tanning and can be added to other alums to develop lavender or purple crystals.

5. Selenate Alum: These alums contain selenium in place of sulphur in the sulphate anion, making selenate \(\mathrm{SO}_{4}^{2-}\) are known as selenium or selenate alums. The selenium-containing alums are strong oxidising agents. Hence they are used in antiseptics.

6. Aluminum Sulfate: This compound is sometimes called alum or papermaker’s alum. Its chemical formula is represented as \(\text {XAl}\left(\text {SO}_{4}\right)_{2} .12 \text {H}_{2} \text {O}\) where \(\text {X}\) represents a monovalent cation such as potassium or ammonium. It is a good coagulating agent and is used in the purification of water in the water treatment plant and in paper manufacturing industries.

Chemical Properties of Alum

Aluminium-based alums have various common chemical properties. Some of them are discussed below:

1. Mostly Sodium alums are water-soluble. At the same time, Rubidium and Caesium alums are sparingly soluble in water.
2. Alums have a sweet taste.
3. Alum react acid to litmus; they turn blue litmus to red. They are crystallised into regular octahedra.
4. Crystals of alums are divided into three classes: \(\alpha, \beta\) and \(\gamma\).
5. In alums, each metal ion is surrounded by six molecules of water. When heated, they liquefy, and on further heating, the water of crystallisation is lost, leading to the formation of amorphous powder, also known as alum powder.

Alum Uses

Alum has various household and industrial uses. Potassium alum or potash alum is used most, although ammonium alum, ferric alum, and soda alum may be used for many similar purposes. Some of the uses of alums are given below:

1. Alums are widely used in water treatment plants and in the purification of drinking water as a chemical flocculant.
2. In kitchens, they are used as a pickling agent to help keep pickles crisp.
3. Alums are commonly used in styptic pencils to stop bleeding from minor cuts and prevent any infection.
4. They are also used as a flame retardant.
5. Alums are used in baking powder as an acidic component.
6. Alums have various uses in cosmetics, such as skin whiteners and hair removal waxes.
7. As alums also whiten teeth, they are used as an ingredient in some brands of toothpaste.

Summary

In short, we can say that chemically alum is a special class of chemical compound having double sulphate salts and acts as a coagulant. Thus, when an alum is added to the impure water, all dirt gets coagulated with these alum particles to form large particles that can be removed easily by decantation as heavier particles will settle down the container. The general chemical formula of alum is \(\mathrm{AM}\left(\mathrm{SO}_{4}\right)_{2} \cdot 12 \mathrm{H}_{2} \mathrm{O}\), where ‘\(\mathrm{A}\)’ represents a monovalent cation such as potassium or ammonium and ‘\(\mathrm{M}\)’ represents a trivalent metal ion such as aluminium or chromium (III).

Each molecule of an alum is hydrated as it is surrounded by water molecules. It loses water molecules and forms amorphous powder on heating to high temperatures. Different types of alums are potassium alum, sodium alum, ammonium alum, chrome alum, selenate alum, aluminium sulphate, etc. Alums are used in several industrial, and household works such as in water treatment plants for purification of water; alums are used to stop bleeding and prevent infection in case of minor cuts; they are also present in toothpaste, other cosmetic products, as a flame retardant, etc.

FAQs on Alums

Q.1. What are alums used for?
Ans: Alums are used in wide industrial as well as domestic uses such as: (i) In water treatment plants and in the purification of drinking water as a chemical flocculant. (ii) Alums are commonly used in styptic pencils to stop bleeding from minor cuts and prevent any infection. (iii) In kitchens, they are used as a pickling agent to help keep pickles crisp. (iv) Alums are used in baking powder as an acidic component. (v) They are also used as a flame retardant. (vi) As alums also whiten teeth, they are used as an ingredient in some brands of toothpaste. (vii) Alums have a wide variety of uses in cosmetics, such as skin whiteners and in hair removal waxes.

Q.2. What is the general formula of alums?
Ans: Alums are crystalline double sulphates of a monovalent metal (or group or cation) and a trivalent metal having general chemical formula as \(\mathrm{AM}\left(\mathrm{SO}_{4}\right)_{2} \cdot 12 \mathrm{H}_{2} \mathrm{O}\), where ‘\(\text {A}\)’ represents a monovalent cation such as potassium or ammonium and ‘\(\text {M}\)’ represents a trivalent metal ion such as aluminium or chromium (III).

Q.3. What is an alum chemical name?
Ans: The chemical name of alum is crystalline double sulphates of a monovalent metal (or group or cation) and a trivalent metal having general chemical formula as \(\mathrm{XM}\left(\mathrm{SO}_{4}\right)_{2} \cdot 12 \mathrm{H}_{2} \mathrm{O}\), where ‘\(\mathrm{X}\)’ represents a monovalent cation such as potassium or ammonium and ‘\(\text {M}\)’ represents a trivalent metal ion such as aluminium or chromium (III).

Q.4. Why do we use alum in chemistry?
Ans: In chemistry, alum is used as a coagulating and flocculating agent. It is a hydrate, as we know that alum molecule has water molecules trapped within the solid. Upon heating, they will release all the water molecules to form an amorphous powder.

Q.5. What are alums giving an example?
Ans: In chemistry, alums are crystalline double sulphates of a monovalent metal (or group or cation) and a trivalent metal having general chemical formula as \(\text {AM}\left(\mathrm{SO}_{4}\right)_{2} \cdot 12 \mathrm{H}_{2} \mathrm{O}\), where ‘\(\mathrm{A}\)’ represents a monovalent cation such as potassium or ammonium and ‘\(\text {M}\)’ represents a trivalent metal ion such as aluminium or chromium (III). It is a colourless astringent compound that is a hydrated double sulphate of metals like aluminium and potassium. Mostly, “alum” refers to potassium alum, which is the hydrated potassium aluminium sulfate with the chemical formula \(\mathrm{KAl}\left(\mathrm{SO}_{4}\right)_{2} \cdot 12 \mathrm{H}_{2} \mathrm{O}\). Other examples of alums are potassium alum, sodium alum, ammonium alum, chrome alum, selenate alum, aluminium sulphate, etc.

Q.6. What are the side effects of alum?
Ans: Alum has several side effects on the human body like some allergic reactions, like rashes, itching, redness, swollen, blistered, or peeling of the skin accompanied with fever, tightness in the chest or throat, trouble in breathing, swallowing, or even talking, hoarseness or swelling of the mouth, face, lips, or throat. Due to these reasons, consumption of drinking water purified with alum is not recommended.

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