Aluminium Hydroxide Formula: Preparation, Reactions, Uses

The aluminium hydroxide formula is articulated as \(\rm{Al(OH)}_3\). It can also be written as aluminium (III) hydroxide. Aluminium hydroxide appears as an amorphous white powder. It is naturally found as mineral gibbsite (also called hydrargillite).

The molar mass of aluminium hydroxide is approximately equal to \(78.00\,\rm{g/mol}\). Other polymorphs of aluminium hydroxide are ayerite, doyleite and nordstrandite. It is an amphoteric oxide. In this article, we are going to discuss more about aluminium hydroxide in detail. Continue reading to find out more!

Aluminium Hydroxide: Overview

The chemical formula of aluminium hydroxide is \(\rm{Al(OH)}_3\). Aluminium hydroxide is an ionic compound. That is, it contains aluminium \((\rm{Al}^{3+})\) and hydroxide \((\rm{OH}^-)\) ions.
Aluminium hydroxide can be represented as,

The molar mass of aluminium hydroxide is approximately equal to \(78.00\,\rm{g/mol}\).

Aluminium Hydroxide Chemical Formula Derivation

The chemical formula of a compound represents the number of atoms of each element present in one molecule of the compound. The chemical formula of aluminium hydroxide can be derived using the Criss-cross method.

In the criss-cross method, if we know the valency of each element of a compound, we can easily write its formula. The symbol of the positive part is written to the left-hand side and the symbol of the negative part to its right. The valency of each element is placed as a subscript on the side of the symbol of another element. Subscript \(1\) is not often written.

Using this, the chemical formula of aluminium hydroxide is found as,

Aluminium hydroxide contains aluminium \((\rm{Al}^{3+})\) and hydroxide \((\rm{OH}^-)\) radicals. The valency of aluminium radical is three, and that of hydroxyl radical is one. By criss-crossing these valency numbers, the chemical formula of aluminium hydroxide is \(\rm{Al(OH)}_3\). That is,

Preparation of Aluminium Hydroxide \([\rm{Al(OH)}_3]\)

Aluminium hydroxide is commercially produced by the Bayer Process. In this process, bauxite is dissolved in sodium hydroxide solution at a temperature of around \(270^\circ \rm{C}\). As a result of the process, aluminium hydroxide is precipitated from the solution containing sodium aluminate, and the waster solids and bauxite tailings are removed. This aluminium hydroxide obtained can be converted into alumina (aluminium oxide) by the process of calcination.

Properties of Aluminium Hydroxide

Let us discuss the physical and chemical properties of aluminium hydroxide \([\rm{Al(OH)}_3]\).

Physical Properties of Aluminium Hydroxide

The physical properties of aluminium hydroxide are as follows:

1. Aluminium hydroxide appears as a white amorphous powder.
2. The melting point of aluminium hydroxide is approximately equal to \(573\,\rm{K}\).
3. Aluminium hydroxide is almost insoluble in water and is soluble in alkalis and acids.
4. The density of solid aluminium hydroxide is approximately equal to \(2.42\,\rm{g/cm}^3\).

Chemical Properties of Aluminium Hydroxide

The physical properties of aluminium hydroxide are as follows:

Aluminium hydroxide is amphoteric in nature. It can act as both acid and a base. In acids, aluminium hydroxide acts as a Bronsted–Lowry base. As a result, it neutralises the acid-forming salt and water as the products.

For example, aluminium hydroxide reacts with hydrochloric acid to form aluminium chloride and water. The chemical equation for the reaction can be given as,

\(3\rm{HCl} + \rm{Al(OH)}_3 → \rm{AlCl}_3 + 3\rm{H}_2 \rm{O}\)

In bases, aluminium hydroxide acts as a Lewis acid by binding with hydroxyl \((\rm{OH}^-)\) ions. That is,

\({\rm{Al}}{\left( {{\rm{OH}}} \right)_{\rm{3}}}{\rm{ + O}}{{\rm{H}}^ – } \to {\rm{Al}}\left( {{\rm{OH}}} \right)_4^ -\)

Reactions of Aluminium Hydroxide

The various reactions of aluminium hydroxide are explained below:

Reaction of Aluminium Hydroxide with Sulphuric Acid

Aluminium hydroxide reacts with sulphuric acid to form aluminium sulphate and water as the products. The chemical equation for the reaction can be given as,

\(2\rm{Al(OH)}_3 + 3\rm{H}_2 \rm{SO}_4 → \rm{Al}_2 (\rm{SO}_4)_3 + 6\rm{H}_2 \rm{O}\)

Reaction of Aluminium Hydroxide with Nitric Acid

Aluminium hydroxide reacts with nitric acid to form aluminiumnitrate and water. The chemical equation for the reaction can be given as,

\(3 \rm{HNO}_3 + \rm{Al(OH)}_3 → \rm{Al(NO}_3)_3 + 3\rm{H}_2 \rm{O}\)

Reaction of Aluminium Hydroxide with Carbonic Acid

Aluminium carbonate and water are produced when aluminium hydroxide reacts with carbonic acid. The chemical equation for the reaction can be given as,

\(\rm{Al(OH)}_3 + \rm{H}_2 \rm{CO}_3 → \rm{H}_2 \rm{O} + \rm{Al}_2 (\rm{CO}_3)_3\)

The above reactions are examples of neutralization reactions.

Decomposition of Aluminium Hydroxide

At higher temperatures, aluminium hydroxide decomposes to give aluminium oxide and water. That is,

\({\text{2Al}}{\left( {{\text{OH}}} \right)_{{\text{3}}}} \xrightarrow{{{\text{Heat}}}}{\text{A}}{{\text{l}}_{\text{2}}}{{\text{O}}_{\text{3}}}{\text{ + 3}}{{\text{H}}_{\text{2}}}{\text{O}}\)

The above reaction takes place at a temperature of about \(575\,^\circ {\rm{C}}.\)

Uses of Aluminium Hydroxide

Aluminium hydroxide has got several uses. They include:

1. In polymer applications, aluminium hydroxide is used as a fire retardant filler because of its inexpensive, colourless appearance and good fire retardant properties.
2. In polyesters, acrylics, PVC, epoxies, and rubber, it is also used as a smoke suppressant.
3. In humans and animals, aluminium hydroxide is used as an antacid under the name ‘algeldrate’. It helps in the neutralization of stomach acid, and hence it gives relief from the stomach acidity.
4. Aluminium hydroxide is also used to control elevated phosphate, or phosphorus, levels in the blood (hyperphosphatemia).
5. In vaccines, precipitated aluminium hydroxide is used as an adjuvant. Vaccine formulations containing aluminium hydroxide enhances the immune system.
6. It is used as an additive for glass and paper.
7. It is used as mordant in dyeing.
8. It is used for the manufacture of pottery, fire clay and printing inks.

Summary

Aluminium hydroxide is an inorganic compound with the chemical formula \(\rm{Al(OH)}_3\). It is naturally found as the mineral gibbsite, which is also known as hydrargillite. It is commercially produced using the Bayer process. Aluminium hydroxide is amphoteric in nature. That is, in acids, it behaves as a Bronsted–Lowry base, and in bases, it behaves as a Lewis acid. Aluminium hydroxide has got many uses also. It is used as an additive for glass and paper. Aluminium hydroxide is also utilised as a mordant in dyeing.

Learn About Aluminium Oxide Formula Here

FAQs on Aluminium Hydroxide

Q.1. How do you write the formula of aluminium hydroxide?
Ans: Aluminium hydroxide is a chemical compound that contains aluminium \((\rm{Al}^{3+})\) and hydroxide \((\rm{OH}^-)\) radicals. The valency of aluminium radical is three, and that of hydroxyl radical is one. By criss-crossing these valency numbers, the chemical formula of aluminium hydroxide can be written as \(\rm{Al(OH)}_3.\)

Q.2. How is \(\rm{Al(OH)}_3\) produced?
Ans: Aluminium hydroxide \([\rm{Al(OH)}_3]\) is commercially produced by the Bayer Process. In this process, bauxite is dissolved in sodium hydroxide solution at a temperature of around \(270\,^\circ {\rm{C}}.\) As a result of the process, aluminium hydroxide is precipitated from the solution containing sodium aluminate.

Q.3. What is aluminium hydroxide used for?
Ans: Aluminium hydroxide has got many uses. They include:
1. In polymer applications, aluminium hydroxide is used as a fire-retardant filler.
2. In polymers, it is used as a smoke suppressant.
3. In humans and animals, aluminium hydroxide is used as an antacid under the name ‘algeldrate’.
4. It is used for the manufacture of pottery, fire clay and printing inks.

Q.4. Is aluminium hydroxide bad?
Ans: Aluminium hydroxide is approved by FDA as safe. Therefore, in permitted amounts, it is not bad to use. However, in higher amounts, it can cause toxic effects such as osteomalacia, kidney issues, brain damage, etc.

Q.5. Is aluminium hydroxide healthy?
Ans: Aluminium hydroxide is approved by FDA as safe and effective. Therefore, in permitted amounts, it is healthy to use.