[INSERT pH_EU_01_cc]

[cc introduces the difficulty of pH versus concentration and that the pH of a substance can and does vary with concentration. Also notion that textbook images are not necessarily 'absolute' or the final word]

Following the students' discussion in the above character conversation, what sort of response would you expect from their teacher?

Something like: 'Ah, your observations are absolutely right! It depends on the concentrations of these solutions. The hydrochloric acid in your beaker is very dilute so it is indeed less acidic than the cola.'

Students often find it tricky to appreciate the link between pH and concentration, and they also require a secure understanding of concentration itself.

What else might your students find tricky about the pH scale?

The higher concentrations of hydrogen ions in solution give a lower pH, which can be confusing as they may perceive this scale to be 'backwards' or inverse.

The mathematics of the pH scale, such as base 10 logs, can also be problematic and students need to understand what they mean and practise manipulating powers of 10. This is considered in more detail on the following page, including links to further support and practice.

The conversation above highlights another potential difficulty arising when the pH scale is often introduced as a colour picture with a range of acids and bases placed at a pH number. A typical representation may show hydrochloric acid placed at pH 1 – but this is not always true, depending upon how dilute the solution is, eg 0.001 mol dm^–3 is pH 3.