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What is Alum?

What is Alum?

Alum is a group of chemical compounds that form when alkali metals react with the trivalent metals. It is basically a type of salt that has found numerous industrial, medicinal, and cosmetic uses.
Chandramita Bora
In chemistry, alum refers to a class of chemical compounds, which are formed due to the reaction between alkali metals (like potassium, sodium or ammonia) and the trivalent metals (like aluminum, iron, and chromium). So, it is basically a type of salt that forms when an alkali metal reacts with a trivalent metal. The most commonly used alum is known as potash alum, which is actually potassium aluminum sulfate. This compound has several industrial, cosmetic, as well as culinary uses.


Alum, as has been mentioned already, is a type of salt. The term 'alum' is basically used for potassium aluminum sulfate. The other important types of alum are, soda alum or sodium aluminum alum, ammonium alum, and chrome alum.

Alum can be found in the form of large or small white crystals, and also as a powder. It can be dissolved in water, and the resulting solution is acidic and slightly sweet to taste. It is known to possess astringent and antibacterial properties. It liquefies when heated, and if the heating is continued, it begins to froth and swell, and ultimately leaves behind an amorphous powder.

Important Uses

As mentioned already, this chemical compound has found several uses in the industrial and medicinal sectors. It is also used for cosmetic and culinary purposes. The following are some of the noteworthy uses of this compound:
  • It is used for water treatment or purification, as it can cause the small particles and colloids present in water to stick together, and form larger and heavier particles. Such heavier particles can be easily removed when they settle down at the bottom of the water tank. This process is termed as flocculation.
  • Potassium aluminum sulfate is traditionally used as an underarm deodorant due to its antibacterial properties. In the past, it was used for this purpose in many parts of the world, including Europe, Thailand, Philippines, and Mexico.
  • It has flame retardant properties, which is why it is used in treating wood, paper materials, and clothes to make them more resistant to fire. It can also be found in foamite, which is used in fire extinguishers.
  • Due to its astringent properties, crystals or blocks of potassium alum can be used to prevent bleeding from small cuts. So, it can be used as an aftershave to prevent bleeding from small shaving cuts.
  • This compound is used in hair removal products, especially in the middle Eastern countries.
  • It is used for medicinal purposes as well. For example, it is used as an adjuvant in many vaccines, basically to enhance the response to immunogens.
  • It is also effective in treating canker sores and acne. It is an ingredient in some toothpastes and tooth powder as well.
  • Ammonium alum can be found in porcelain cement, and it is also used for tanning and dyeing.
Culinary Uses

Potassium aluminum sulfate is primarily used for food processing and pickling. It is basically used in pickling for maintaining the crispiness of the pickled fruits and vegetables. Another type of alum, known as sodium aluminum sulfate, is used for making baking powder. Baking powder is used as a raising agent in baking food, as it helps increase the volume and lighten the texture of the baked food.

Though alum (potassium and soda alum) can be used as a food additive, and has been approved for this purpose by the Food and Drug Administration (FDA) of the United States, this compound can be toxic when ingested in large quantities. So, care must be taken while using it for culinary purposes.