Neutralisation

Bases and alkalis

A base is any substance that reacts with an acid to form a salt and water only. This means that metal oxides and metal hydroxides are bases.

Some bases are soluble in water. An alkali is a soluble base - it dissolves in water to form an alkaline solution. For example:

  • copper oxide is a base but it is not an alkali because it is insoluble in water
  • sodium hydroxide is a base and it dissolves in water, so it is also an alkali

Example

Explain why all alkalis are bases but not all bases are alkalis.

Alkalis are soluble bases, but only some bases are soluble in water, so these are not alkalis.

Neutralisation reactions

Neutralisation involves an acid reacting with a base or an alkali, forming a salt and water.

Reactions with metal oxides

In general:

acid + metal oxide → salt + water

For example:

sulfuric acid + copper(II) oxide → copper(II) sulfate + water

H2SO4(aq) + CuO(s) → CuSO4(aq) + H2O(l)

Reactions with metal hydroxides

In general:

acid + metal hydroxide → salt + water

For example:

nitric acid + sodium hydroxide → sodium nitrate + water

HNO3(aq) + NaOH(s) → NaNO3(aq) + H2O(l)

Aqueous neutralisation reactions

Remember:

  1. acids in solution form hydrogen ions, H+
  2. solutions of alkalis contain hydroxide ions, OH-

Acid-alkali neutralisation reactions involve the reaction between hydrogen ions from the acid, and hydroxide ions from the alkali:

H+(aq) + OH-(aq) → H2O(l)

Pure water is neutral (its pH is 7). A neutral solution can be produced if the correct amounts of acid and alkali react together.

curriculum-key-fact
Neutralisation reactions in solution can be generalised to hydrogen ions reacting with hydroxide ions to form water.