Dhristi JEE 2022-24

What is Alum?

Alum is an inorganic compound composed of Water molecules, Aluminum, other Metal than Aluminium, and Sulphates. Alum is a double salt in the hydrated form. Alum exists in different forms: Potash Alum, Soda Alum, ammonium Alum, and chrome Alum. The general chemical formula for Alum is XAl(SO4)2·12H2O.


Definition

We can define Alum as an inorganic compound that is made up of Water molecules, Sulphates and Aluminium. It is a hydrated form of a double salt.


This compound exists in different forms like Potash Alum, Ammonium Alum, Chrome Alum, Soda Alum. Alum has a general chemical formula, it is XAl(SO4)2·12H2O.


Types

There are different types of Alum including:

Alum

Common Name

Chemical Formula

Chemical Name

Appearance

Molar Mass

1. Potash Alum

Potassium Alum; Fitkari

KAl(SO4)2·12H2O

Potassium Aluminium Sulphate

It exists in a white crystal form and smells like Metallic Water.

258.192g/mol

2. Soda Alum

Sodium Alum

NaAl(SO4)2·12H2O

Sodium Aluminium Sulphate

It exists in a white crystal form and smells like Metallic Water.

458.28g/mol

3. Ammonium Alum

Ammonium Sulphate Alum

NH3Al(SO4)2·12H2O

Ammonium Aluminium Sulphate

It exists in a white crystal form and smells like Metallic Water.

132.14g/mol

4. Chrome Alum

Chromium Alum

KCr(SO4)2·12H2O

Chromium Aluminium Sulphate

It exists in a purple crystal form and smells like Metallic Water.

283.22g/mol

5. Selenate Alum

Selenium Alum

Al2O12Se3

Selenium Aluminium Sulphate

It exists in a white crystal form and smells like Metallic Water.

482.9g/mol


  1. Potash Alum - It is also known as Potassium Alum. The common name of Potash Alum is fitkari. The chemical formula for Potash Alum is KAl(SO4)2·12H2O. Its chemical name is Potassium Aluminium Sulphate. The common Alum is Potash Alum or Potassium Alum.


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The molar mass of the Potash Alum is 258.192 g/mol. It exists in a white crystal form. It smells like Metallic Water. It is also commonly known as white Alum.


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  1. Soda Alum - It is also known as sodium Alum. Its common name is SAS. The chemical formula for sodium Alum is NaAl(SO4)2·12H2O. Its chemical name is sodium Aluminium Sulphate. The molar mass of the sodium Alum is 458.28 g/mol. It exists in a white crystal form. It smells like Metallic Water.


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  1. Ammonium Alum - It is also known as ammonium Sulphate Alum. Its common name is AAS. The chemical formula for Potash Alum is NH3Al(SO4)2·12H2O. Its chemical name is ammonium Aluminium Sulphate. The molar mass of the ammonium Alum is 132.14 g/mol. It exists in a white crystal form. It smells like Metallic Water.


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  1. Chrome Alum -  It is also known as chromium Alum. Its common abbreviation for chromium Alum is CAS. The chemical formula for Potash Alum is KCr(SO4)2·12H2O. Its chemical name is chromium Aluminium Sulphate. The molar mass of the chromium Alum is 283.22 g/mol. It exists in a purple crystal form. It smells like Metallic Water.


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  1. Selenate Alum - In this type of the Alum selenium takes the place of the sulphur. The anion present in this form of Alum is selenate in place of Sulphate. These Alums possess the property of strong oxidizing agents. The molecular formula for selenate Alum is Al2O12Se3. The molecular mass of the selenate Alum is 482.9 g/mol.


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Alum is available both offline and online. Alum offline is generally available at grocery shops and medical stores. Alum Online is available on different e-commerce websites. Alums are generally sold by their common names like white fitkari, red fitkari, and white Alum stones or red Alum stones. Alums possess the property of anti-inflammatory agents. It is used in the gargling process to reduce teeth gum inflammation and pain.


Properties of Alum

  • Alums are highly soluble in Water.

  • These compounds are sweet in taste.

  • They generally crystallize in the regular octahedral form.

  • The Alum crystals get liquified when heated.

  • Alums generally exist in the form of a white and transparent crystalline form.

  • Its boiling point is around 200 degrees celsius.

  • Its melting point is 92.5degrees celsius.

  • Its density is 1.725 g/cm3.

  • They are highly soluble in Water.

  • They are sweet in taste.

  • Their density is 1.725g/cm3

  • Their melting point is 92.5° Celsius.

  • Their boiling point is around 200° Celsius.

  • They generally crystallize in a regular octahedral form.

  • The Alum crystals liquify when they are heated.


Alum Water Treatment 

Alum Water treatment is generally carried to treat the polluted Water. These compounds act as a coagulant. It is used in the Coagulation-Flocculation process of polluted Water. It is a chemical Water treatment technique typically applied prior to sedimentation and filtration to enhance the ability of a treatment process to remove particles.


Coagulation - it destabilizes the charges of the particles. Coagulants with charges opposite to those of the suspended solids are added to the Water to neutralize the negative charges on dispersed non-settable solids such as clay and organic substances.


\[ Al_{2}(SO_{4})_{3}.18H_{2}O + 6HCO^{3-} \rightarrow 2Al(OH)_{3} + 6CO_{2} +18H_{2} + 3SO_{4}^{-2} \]


Once the charge is neutralized, the small-suspended particles are capable of sticking together. The slightly larger particles formed through this process are called microflocs and are still too small to be visible to the naked eye.


We use Alum to treat the polluted Water mainly. These compounds can act as a coagulant and can be used in the Coagulation-Flocculation process to treat contaminated Water.


In other words, it is a chemical Water treatment technique done before the sedimentation and filtration process to strengthen the proficiency of the treatment and its ability to remove polluted and dirty particles.


We used two terms in the explanation of this treatment, they are:


Coagulation: This is the destabilization performed on the charges of the particles. Coagulants with charges that are opposite to that of the suspended solids are added to the Water. We stabilize and neutralize the negative charges in the non-settable solids like clay and other organic substances.


Flocculation: This is the process where a chemical coagulant is added to the Water to stimulate bonding between different types of particles. This leads to the creation of larger aggregates that can separate easily.


This is the chemical equation for them:


\[ Al_{2}(SO_{4})_{3}.18H_{2}O + 6HCO^{3-} \rightarrow 2Al(OH)_{3} + 6CO_{2} +18H_{2} + 3SO_{4}^{-2} \]


Once this charge is neutralized, it allows the minuscule and suspended particles to stick together, whereas the slightly larger particles (microflocs) are still too small for the naked eye to see.


Uses of Alum

  • It is used in the pickling and baking process.

  • It is used in the tanning process of leather.

  • It is used in the Coagulation and Flocculation process of Water treatment.

  • It is used as an acidulating agent in cooking.

  • It is used as a drying agent in a textile company.

  • It is used as an antiseptic agent.

  • We use it in the baking and picking process.

  • We use it for the Flocculation and Coagulation process during the Water treatment.

  • We use it as a drying agent in textiles companies.

  • We use it as an antiseptic agent.

  • We use it as an acidulating agent in the cooking process.

  • We had it to tan leather.

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FAQs (Frequently Asked Questions)

1. What is an alum?

Alum is an inorganic compound composed of water molecules, aluminium, other metal than aluminium, and sulphates. Alum is a double salt present in the hydrated form. The general chemical formula for alum is XAl(SO4)2·12H2O.

2. Write the properties of the alum.

The properties of the alum are given below:

  1. Alums are highly soluble in water.

  2. These compounds are sweet in taste.

  3. They generally crystallize in the regular octahedral form.

  4. The alum crystals get liquified when heated.

  5. Alums generally exist in the form of a white and transparent crystalline form.

3. What is Phitkari?

Phitkari is a common name that is used for the chemical compound which we use for medicinal purposes and other treatment processes. It has a chemical formula of KAl(SO4)2·12H2O.


It is a mixed salt of a compound with Potassium Sulphate and Aluminium Sulphate which is being hydrated by Water molecules.


This compound is widely known as Potash Alum or Potassium or chemically known as Potassium Sulphate Dodecahydrate. We used to extract it from alunite minerals and nowadays we can produce it industrially.

4. Can we separate Alum from a solution?

We can separate an Alum from a solution using the process of Electrodialysis.


Electrodialysis is the process of separating charged membranes and electrical membranes to separate ionic particles and other uncharged components from an aqueous solution.


Electrodialysis (ED) has the ability to do this separation, in other words it removes salt ions and charged organic matter using an ion exchange membrane to purify brackish Water.


The major advantage of conducting this process before filtration is that it doesn't use pressure and can do the task with less energy consumption.

5. What are the uses of Potash Alum?

Potash Alum is used for various purposes in real life:

  • Fire Retardant: Potash Alum adds flame resistance to clothes, wood, and other materials.

  • Gourmet Food: It has a specific acidic composition perfect for use in baking powder; bakeries in England use this Alum in their bread-making process.

  • Dissolving Iron and Steel: It is used to dissolve Iron and Steel to create castings of various machinery.

  • Lake Pigmentation: It acts as a base for the pigmentation of most of the lakes.

6. What is the difference between Coagulation and Flocculation?

The primary definition of both Coagulation and Flocculation is not so evident in the industry that we can differentiate them properly. But we can compare their functions to differentiate between them:


Coagulation is the process of creating particles that aggregate by themselves without any aid. For example, influencing the particles and changing their pH.


Flocculation is the process of creating particles that aggregate with the help of polymers that binds them together.


Even if we differentiate between these terms, we usually use them hand in hand as they are both used together to purify brackish Water or waste Water.


Coagulation is also known in regular life, a well-known example is Sour Milk, when milk gets sour its pH decreases, which causes destabilization of fat particles which then coagulate.

7. How can I learn more about Alums on Vedantu?

On this page, you learned about Alum, its definition, types, properties, and application in real life. To see essential questions related to this concept or any other concept in chemistry that might be vital for appearing in entrance exams or scoring good marks in board exams, you can check out Vedantu.


We provide students with micro-courses, live lectures, online help, and multiple offline studying options like mock tests, sample papers, and their in-depth solutions, along with a proper syllabus structure. Students can also download this material in the form of PDFs.

8. Is Alum soluble in Water?

Yes, Alum is soluble in Water. Alum is an ionic compound consist of Water of hydration. These large number of Water molecules contribute to the solubility of Alum in Water. It gets completely dissociated when dissolving in Water.

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