• Pestle and mortar
• Clear containers
• Household acids and alkalis such as washing powder or soap. 
• Edible acidic and alkaline food such as lemon juice, bicarbonate of soda, white vinegar, lemonade, fizzy water or tap water
• Plant material - red cabbage, beetroot, blackberries, blackcurrant, pink and purple rose petals
• Droppers

A discussion of the meaning of the words ‘acid’, ‘alkali’ and neutral would be a good way to introduce the experiment. Find out what pupils already know about acids and ask for suggestions of ones they are already familiar with at home.

Ask for a prediction (and then demo) what happens if a known acid is tested with universal indicator solution and how we could test for the degree of acidity.

Depending on the age of the learners, either demo or have pupils working in small groups to prepare the indicator solution.

• Cut raw red cabbage into small strips and soak in hot water
• Drain and decant the liquid into a container
• The test solutions need to be set out in transparent, labelled plastic cups
• Using a dropper put a small sample of the solution to be tested into a transparent cup and add a dropper full of indicator solution
• Groups use one of these indicator sheets to assess the pH of the solution and record their results

Feedback results into a class table and discuss what they have found out. Establish that there are two classes of solution (acid - already discussed - and alkali) and that natural dyes can be used to indicate which is which.

Differentiation

• Have solutions of red cabbage indicator prepared in advance or use litmus paper to test for acids and alkalis.
• For early finishers: Using a pestle and mortar with warm water and a range of alternative plant material, e.g. dark coloured rose petals, raw beetroot, blackcurrant juice, blackberries, ask pupils to produce indicator solutions and test if they can indicate the presence of acid and alkali
• Does the colour of the plant affect its ability to work as an indicator?

Plenary

Demonstrate the use of universal indicator paper or solution to obtain a pH number for a few solutions already tested. Compare with the natural indicators. Can children see the difference in the colour changes and say which is acid and which is alkali.

Can learners explain how a neutral solution can be obtained? Demonstrate this. Relate the pH value of an acid or alkali to its hazardous and corrosive nature.

Assessment questions

• Which common substances are acids and which are alkalis?
• What is an indicator and what does it do?
• What is the pH scale?
• When could doing a pH test be particularly important?
• Is fizzy water good for us?

The scale for measuring the acidity or basicity of a solution is called pH. The pH scale ranges from 0 to 14. Water is neutral and has a pH of about 7. Solutions with a pH of less than 7 are acidic and if the pH is higher than 7, the solution is basic/alkaline.

When red cabbage is boiled or soaked it releases pigment molecules called anthocyanins. These pigment molecules are also found in some flower petals and fruits. One function of anthocyanin to plants is as an antioxidant.

Anthocyanins change colour depending on the pH of the environment they are in. Very acidic solutions should turn them red and very alkaline ones green/yellow. In neutral solutions the anthocyanin should remain dark blue/purple.