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.

Bases that are soluble in water are called alkalis and they dissolve in water to form alkaline solutions. 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

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

Alkalis are soluble bases, but only some bases are soluble in water. Bases that dissolve in water are alkalis.

Neutralisation reactions

A neutralisation reaction is a reaction between an acid and a base. Remember:

  • acids in solution are sources of hydrogen ions, H+
  • alkalis in solution are sources of hydroxide ions, OH-

In acid-alkali neutralisation reactions, hydrogen ions from the acid react with hydroxide ions from the alkali. Here are the ions involved in the reaction of hydrochloric acid with sodium hydroxide solution:

H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) → Na+(aq) + Cl-(aq) + H2O(l)

Now remove the spectator ions, Cl-(aq) and Na+(aq). This leaves:

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. The change in pH during a neutralisation reaction can be measured using a pH probe and meter, or estimated using universal indicator solution and a pH colour chart.

An acid-alkali neutralisation is the reaction between hydrogen ions and hydroxide ions, forming water.