[INSERT ABA_CI_08_ima]

How indeed can we tell the difference? The substance can be tested with an indicator – either as a solution or as indicator paper. A pH indicator shows one colour when added to an acidic solution and a different colour when added to an alkaline solution. For example, the two colours of bromothymol blue and another indicator, phenolphthalein, are shown below.

[INSERT ABA_CI_10_ima and ABA_CI_11_ima side by side]

How acidic is a particular acid? And how alkaline is a particular alkali? Universal Indicator solution is a 'full range' indicator, able to display a range of different colours, as shown below, in response to differing acids and bases. Universal Indicator is a mixture of indicators that is discussed in more detail in the topic Indicators. Different colours correspond to different 'pH values' of the pH scale as a measure of acidity or alkalinity.

[INSERT ABA_CI_04_ima]

The pH scale is a continuous range from below 0 to above 14. In practice, values in the range 0 to 14 are the most commonly met in schools.

Solutions that are:

• acids have a pH  7;
• alkalis have a pH > 7;
• neutral have a pH = 7.

Predict the pH numbers of the following items from the kitchen cupboard by dragging each item to your choice of pH number:

[INSERT ABA_CI_05_int]

How do your predictions compare with the actual pH values?