Cabbage Chemistry

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How can you use a cabbage to tell if something is acidic?

Try this and see:

What you need:

• Red cabbage
• Knife
• Chopping board
• Large bowl
• Water
• Wooden spoon
• Sieve
• Measuring jug
• Clear plastic cups
• Kitchen items to test. You can use whatever you like, but good items to start with are lemon juice, ketchup, soap, vinegar, fizzy drinks and bicarbonate of soda.

How to:

1. Take half a cabbage.
2. Chop the cabbage into small pieces.
3. Put the cabbage pieces in the bowl and add about 250ml of water.
4. Mash the cabbage up using a wooden spoon.
5. When you see a bright purple liquid in the bottom of the bowl, sieve the liquid into a measuring jug.
6. Pour about 1cm of liquid into several different cups - each cup will be used to test a different kitchen item.
7. Leave one of the cups as your 'control'. This will remind you what the original colour was and you can compare the colour changes to this.
8. Now you can test your kitchen items! Start with something that you think is acidic - how about vinegar or lemon juice?
9. Then try something basic - also known as alkaline - try bicarbonate of soda.

What else can you test in your kitchen?

What colours did you see?

When the cabbage juice goes red, it means the kitchen item is acidic. When the cabbage juice goes blue, it means the substance is alkaline.  If the colour doesn't change it means the substance is neutral.

Which item was the most acidic? Which item was the most alkaline? Were any of the items neutral?

Why does the cabbage change colour?

Red cabbage contains a pigment called flavin, which gives the cabbage its colour.

Acids and bases affect the number of hydrogen ions in a solution. There are more hydrogen ions in an acidic solution and fewer hydrogen ions in a basic - alkaline - solution. Bases produce hydroxide ions, which neutralise the hydrogen ions. The ratio of hydrogen ions to hydroxide ions determines how acidic a solution is.

The colour of flavin - the pigment in cabbage - depends on the ratio of hydrogen ions to hydroxide ions. At high concentrations of hydrogen ions, flavin is a red-pink colour. At low concentrations of hydrogen ions, the flavin pigment breaks up and turns a blueish colour.