Acids or alkalis can dissociate - or ionise - in solution to different extents. A strong acid is an acid that is fully ionised or completely dissociated to form ions in aqueous solution.

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What can you say about the number of acid molecules present in a solution of the acid shown above?

There are none (or very few) as the acid is fully ionised. The equilibrium lies fully to the right.

What are other examples of strong, fully ionised acids?

Hydrochloric acid, sulfuric acid, phosphoric acid.

Weak acids or alkalis only ionise to a limited extent in water. At equilibrium most of the acid or alkali remains as molecules and only a small amount as ions.

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What can you say about the number of acid molecules present in a solution of ethanoic acid relative to the number of ions, as represented above?

Most molecules of a weak acid remain as molecules in solution. Only a very tiny proportion of the acid molecules ionise. The equilibrium lies to the left. Students can find it helpful put another way - in dilute ethanoic acid about one in every hundred acid molecules is dissociated.

What might shift the equilibrium to the right, increasing ionisation to H₃O⁺ ions?

Dilution with water results in more ionisation. Consider the equilibrium, where adding water will favour a shift in the position of equilibrium to the right, increasing the amount of H₃O⁺ ions. The practical demonstration showing the dilution of concentrated ethanoic acid in Developing understanding section of the Importance of water topic shows this dramatic effect.

What are other examples of weak, partially ionised, acids?

Methanoic acid, propanoic acid, any carboxylic acids, hydrofluoric acid (HF)

What are examples of strong, fully ionised, alkalis?

Sodium hydroxide, potassium hydroxide

What are examples of weak, partially ionised, alkalis?

ammonia or other amines

The most important point to emphasise with students is that the terms 'strong' and 'weak' are nothing to do with the concentration  where the terms 'dilute' and 'concentrated' should be used. Of course, there can be a dilute solution of a strong acid. This is drawn out further in the section, Developing understanding.