Acids and Alkali

In the previous sections, we grouped substances as Acids or Alkali depending on how they changed the colour of an Indicator. But what is it about those substances that make them an Acid or Alkali?

The tables below show some common acids and alkali, as well as their chemical formulae:-

The Element Hydrogen is present in all Acids. It must be this Element that is responsible for the acidity of Acids.

The Elements Hydrogen and Oxygen are present in all Alkalis. They always form an OH group which is called a Hydroxide. It must be this Hydroxide group that is responsible for the alkalinity of Alkali.


Strong Acids and Alkali can be very useful, but it is often useful to make them weaker or get rid of them entirely by making a neutral solution. By adding Acid and Alkali together in the correct amounts, a neutral solution will be formed.

The diagram below shows the process of Neutralisation:-

Note - The above diagram show colour within the solutions, this is shown here only to show pH, in the experiment the solutions themselves will be colourless.

A neutralisation reaction occurs when an Acid reacts with a neutraliser. During a neutralisation reaction, the pH of the solution will move towards pH 7 (neutral).

When an Acid and Alkali react together, they form water and a salt. The type of salt depends on the Acid and Alkali.

The general formula for a neutralisation reaction is:-

Acid + Alkali → Salt + Water

Formation of Salts

In a neutralisation reaction, a salt is formed, with the type of salt depending on the Acid and Alkali used.

The name of a salt is formed of parts of the Acid and Alkali names:-

  1. Alkali - Full name used (e.g. Sodium etc) for first part of salt name.

  2. Acid - Part of name used (see table below) for second part of salt name.

The table below shows the type of salts produced by different Acids:-

For Example -

Hydrochloric Acid + Sodium Hydroxide → Sodium Chloride + Water

Sulphuric Acid + Sodium Hydroxide → Sodium Sulphate + Water

Nitric Acid + Potassium Hydroxide → Potassium Nitrate + Water

Phosphoric Acid + Calcium Hydroxide → Calcium Phosphate + Water

Acids and Bases

So far we have only looked at reactions between Acids and Alkali causing neutralisation. It turns out, however, that there are other substances that can neutralise an Acid. This larger group of substances are called Bases.

The three main substances that make up the Bases are:-

  1. Hydroxides - Substances with an OH group.

  2. Metal Carbonates - Substances with a CO3 group.

  3. Metal Oxides - Substances that are a compound of a Metal and Oxygen.

None of these groups are called 'Alkali'. This is because the real definition of an Alkali is:-

"An Alkali is a Base that can dissolve in water"

The Hydroxides are the only Bases that can dissolve in water, which is why we normally just call that group Alkali.

When Acids and Bases react together, a neutralisation reaction occurs. The products of that reaction depend on the type of Base used.

The table below shows the products of neutralisation of Acids with the different Bases:-

For Example -

Hydrochloric Acid + Potassium Hydroxide → Potassium Chloride + Water

Hydrofluoric Acid + Lithium Carbonate → Lithium Fluoride + Water + Carbon Dioxide

Nitric Acid + Copper Oxide → Copper Nitrate + Water