Successive dilutions are each 10-fold more dilute than the previous solution. What is the significance of this 10-fold dilution?

Students should notice that their solutions each differ by one pH number. Encourage them to explain that they have had to dilute each solution by 10 in order to achieve this change of one pH number. For example, a solution with a pH of 3 will have ten times as many H⁺ ions in the solution as the next solution of pH 4.

How can the activity be organised so that it doesn't take a long time?

If it's a class experiment, then there is the opportunity to work in teams. Each pair of students preparing the acidic solutions can be teamed with another pair preparing the alkaline solutions. The pairs preparing the alkaline solutions need to remember that they are starting at the highest pH number and working down their way down, numbering from 13 to 8. This could be made explicit in their instructions at the start for clarity and saving time.

Some of the colours are not quite the same as those of the pH colour chart. Why is this?

If any of the colours don't quite match the colour chart, it is most likely that these are the ones in the middle of the range (around pH 7). As these are the most dilute solutions, other factors such as the quality and pH of the water being used can become more significant. The self-ionisation of water, which becomes more significant for more dilute solutions towards pH 7, is considered in the next topic.