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  The Materials

  For this experiment you will need:

  • About half a small head of red cabbage
  • Baking soda
  • Very hot water — [be careful and ask for help if you need it]
  • A medium-sized mixing bowl
  • A grater
  • A strainer
  • A jug
  • Six clear plastic cups
  • Some “test liquids” such as lemon juice, vinegar, cola

The Cabbage Juice Indicator

Some acid can be very harmful and dangerous, but many ordinary things in you kitchen have acids that are relatively safe, such as orange juice or vinegar.

Acids and alkalis have the ability to change the colour of certain vegetable materials. So this experiment uses an ordinary red cabbage to find out if a liquid is an acid or an alkali.

Red cabbage juice includes a pigment called flavin. Acids and alkalis turn this pigment into different colours. The juice will turn various shades of red when mixed with acids, and various shades of blue (or greeny yellow) when you mix it with alkalis.

So you can use the cabbage juice as a simple but effective pH indicator. PH is a measure of how “acidic” a solution is — the lower the pH, the more acidic the solution.

First you need to make some cabbage juice. Grate the red cabbage into the bowl. Or put it in a blender and about a cup of very hot water, and blend for a couple of minutes until it is finely chopped. Cover the cabbage with hot water and leave it for half an hour.

Strain the juice into the jug and discard the cabbage. From the jug, pour an equal amount of cabbage juice into each plastic cup.

Add one teaspoon of baking soda to five of the plastic cups. The baking soda is a base, so it will turn your cabbage juice blue.

Now take one of your “test liquids” and see how much of it you need to turn the cabbage juice back to its original colour. Add the test liquid one teaspoon at a time to your first cup. If the juice looks like it is turning back, the liquid you added is probably an acid.

Remember that the sixth cup — the one without the baking soda — is called the “control”. It will show the colour that you want to get all of your mixtures to match.

Now try another one of your test liquids in your second cup. Remember to keep a note of the amount of teaspoons you need to add to get back to your original colour. If the juice stays blue, though, the test liquid is probably not an acid. Other test liquids you might try could include:

• A clear fizzy drink (e.g. 7-Up or Sprite)
• Milk
• Salty water
• Sugar
• Milk of magnesia
• Sparkling mineral water