What Is An Alkali In Chemistry?

by Kate Onissiphorou

Alkali chemicals contain alkali metals. Alkali compounds are water-soluble compounds that are hydroxides of alkali metals, like sodium hydroxide or NaOH.

Alkalis are highly corrosive to organic compounds because they can easily break the bonds between carbon chains and their functional groups. They can denature and dissolve proteins and start the saponification process, or breakdown, of fats.

Alkalis can also refer to the hydroxides of alkaline earth metals, such as calcium hydroxide or Ca(OH)2. Alkaline compounds were identified hundreds of years ago and originally referred to the ashes of plants that contained high amounts of sodium or potassium.

Examples of an Alkali

Alkalis have a special definition in chemistry.

They are a subset of bases that contain the alkali metals hydrogen (H), lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs) and francium (Fr).

Hydrogen is technically considered part of the alkali metals. Water is a hydroxide of hydrogen but pure water is generally neutral, meaning it is neither acidic nor basic. However, when the ions aren’t balanced, water can become either basic or acidic. See the ionic equation below.

2H2O ⇌ H3O+ + OH

Here are some examples of common alkali hydroxides:

  • Sodium hydroxide with the chemical formula NaOH, sodium hydroxide is otherwise known as caustic soda. Sodium hydroxide is important in manufacturing various products, including paper and soap. In paper manufacturing, sodium hydroxide is used to dissolve cellulose, turning it into pulp. The pulp is easier to spread as thin layers of paper. The sodium hydroxide is thoroughly washed away and the pulp is then bleached and washed again.  When combined with oil or fat, caustic soda can also be used to make soap and detergent.

The interior of a paper manufacturing facility

  • Potassium hydroxide potassium hydroxide has the chemical formula KOH and is commonly known as potash lye. Potassium hydroxide has several uses, including agricultural, industrial, and household applications. For example, it’s commonly used by farmers to balance the pH of acidic soil. In food processing, potassium hydroxide acts as a stabiliser and thickening agent. It also serves as an electrolyte in alkaline batteries and is often used in electroplating and lithography.
  • Calcium hydroxide it has the formula Ca(OH)2 and is known by many other names including hydrated lime, builders’ lime, and slaked lime. Calcium hydroxide is commonly used in food preparation during the pickling process, hence why it’s often called pickling lime. The most important and oldest use of calcium hydroxide is in the preparation of lime mortar for plastering walls. It’s also very useful as a flocculant when treating sewage water, as it causes the colloidal particles in water to precipitate.
  • Ammonium hydroxide – ammonium hydroxide is represented by the chemical formula NH4OH. Ammonium hydroxide is an aqueous solution of ammonia, which is dissolved gas in water. It’s mainly used as a cleaning agent in industries, although it’s also used in manufacturing plastic, rayon, fertilisers, and rubber.

A rubber manufacturing machine that uses ammonium hydroxide

What Type of Base is an Alkali?

Alkalis are inorganic water-soluble bases. They are generally either alkali earth metal or alkali metal bases. Ammonium hydroxide may appear to be a non-alkali, but it’s classified as a weak alkali. This is because it’s soluble in water and contains hydrogen, which belongs to the alkali metal group. 

Alkalis are categorised as Brønsted-Lowry, which means they are proton acceptors.

What is the Difference Between an Acid and an Alkali?

Not all bases are alkalis, but all alkalis are classified as water-soluble bases. They fall under the category of Brønsted-Lowry bases, which make them proton acceptors during neutralisation reactions with acids.

As bases, they have pH levels that are above 7. Meanwhile, acids are the chemical opposites of alkalis and have pH levels below 7. When an acid reacts with an alkali, they form salt and water.

This is an example of a reaction between an alkali and an acid. In this case, the acid is sulphuric acid and the alkali is calcium hydroxide. Their reaction produces calcium sulphate salt and water.

H2SO4 + Ca(OH)2 → CaSO4 + 2H2O

In this reaction, the sulphuric acid donates hydrogen ions and combines with the hydroxide ions to form water.

What is the Difference Between a Base and an Alkali?

While alkalis are a subcategory of bases, not all bases are considered alkalis.

The latter are characterised by the metals they contain, namely, alkali metals and alkaline earth metals. They are generally classified as Brønsted-Lowry bases, although not all of them exhibit the characteristics of Brønsted-Lowry bases.


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