Acid or alkali? How to make a red cabbage indicator

Acids and alkalis are substances that can be found in the science laboratory and at home all around us. We can make a very easy red cabbage indicator to find out whether a substance is an acid or alkali.

Acids can be found in the food we eat, in our body and some acids can be very harmful.

Alkali’s are another group of chemicals that react with acids. Substances such as soap are alkalis and bicarbonate of soda which we use in baking but also harmful things like bleach.

How to make a red cabbage indicator

We use a substance called an indicator to test it. An indicator will change colour to determine an acid or alkali. You can make an indicator using red cabbage.

 

What you will need

  • A grater or blender
  • Red cabbage
  • Hot water
  • A saucepan
  • A sieve
  • Cups
  • Different substances to test (I use bicarbonate of soda and vinegar and water but you could try any other substance in the home as long as supervised and act with caution) Wearing latex gloves and goggles are good safety practices when using chemicals.

Method

1. Grate or blend the cabbage.

red cabbage

2. Tip into a saucepan and pour over boiling water from the kettle.

3. Heat until simmering and leave for 10 minutes.

4. Sieve the water and cabbage into a jug – you will notice that the fluid is very purple in colour!

5. Leave to cool a bit – your indicator is ready

6. Place out cups of the amount of substances you plan to test.

7. Add equal quantities of indicator to each cup

8. Dissolve some Bicarbonate of soda in water – measure out a set amount e.g 20ml Pour into the first cup. What happens?

How to make a red cabbage indicator

9. Repeat this with the same quantity of each substance you test and record the colour change.

Safety note

Wear safety goggles if using strong acids/bases.

An adult should help with the chopping and heating of the cabbage.

Results

In this case of red cabbage indicator the colour will change from purple to red if it is an acid and from purple to green if it is an alkali. The different shades of colour will depend on the strength of the acid or alkali. If there is no colour change the substance is said to be neutral.

Why does this work?

Acid and bases are opposites, acids have a low pH and bases have a high pH.

Red cabbage contains a pigment called anthocyanin which is what changes colour.

Extension tasks

  • Make your own pH indicator strips by soaking filter paper in the red cabbage indicator and leave to dry. Once dry cut the filter paper into strips and dip into test substances. Try testing milk, fizzy drinks or soap. Can you predict their pH before testing?
  • Try using beetroot juice instead of red cabbage, which works the best?
  •  You could also try blowing into the indicator. What happens?

red cabbage indicator

The indicator should turn red, as the carbon dioxide we breathe out reacts with the water to form carbonic acid.

Did you know you can make your own pH indicator test strips too?

How to make a red cabbage indicator

 

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