Students may be familiar with the pH scale from their introductory study of acids and alkalis, giving a number that indicates the degree of acidity or alkilinity. They may typically associate these numbers with the colour range of Universal Indicator.

Students can observe that diluting an acid or an alkali with water changes its pH. Diluting an acid makes its pH go up towards 7. Diluting an alkali shifts the pH down towards 7.

[INSERT pH_CI_05_ani]

[Explaining dilution and link with pH scale is an important concept. This visually shows relative volumes, making a clearer explanation and one that teachers could do as demo/commentary]

How would your students explain pH changes upon dilution?

The number of moles (n) of hydrogen ions present in the acid (or number of moles of hydroxide ions in the alkali) hasn't changed (n is the same) but the volume (v) increases with the addition of water. This results in the concentration decreasing and the pH will show the corresponding change.

Eventually the solution can be diluted to reach a neutral solution. Note that this is not the fastest or most efficient way to achieve neutrality.

A simple practical activity, exploring the link between pH and concentration more fully, offers further support in the Developing Understanding section.