Follow these instructions to learn about acids and bases using red cabbage.

Safety

This activity requires the use of a knife, poisonous chemicals and hot water. Ask an adult to assist you. Always follow the safety advice on the products you are using.

You will need

  • fresh red cabbage
  • sharp knife
  • cutting board
  • hot tap water
  • 7 clear plastic disposable cups
  • 7 plastic spoons
  • large plastic bottle
  • range of the household substances which may include:
    • strongly acidic, e.g. powdered toilet cleaner
    • acidic, e.g. vinegar, lemon juice, white wine, lemonade or citric acid
    • weakly acidic, e.g. cream of tartar
    • neutral, e.g. pure water, shampoo or baby shampoo
    • slightly basic, e.g. bicarbonate of soda
    • basic, e.g. milk of magnesia, washing soda or floor cleaner
    • strongly basic, e.g. dishwashing liquid or powder.

What to do

  1. Make sure you get a red cabbage, not a green one, for this activity. © CSIRO, Robert Kerton

    Using a sharp knife and cutting board, finely slice three or four red cabbage leaves.
  2. Place the cabbage leaves in the plastic bottle, half fill the bottle with hot water and screw the lid on tightly.
  3. Shake the bottle for a few minutes until the water becomes a deep purple colour. Leave the solution to cool.
  4. Strain the solution and add sufficient water to the solution to make about 1 L.
  5. In each of the cups, place a small amount of one of the above household substances in the following order: strongly acidic; acidic; slightly acidic; neutral; slightly basic; basic and strongly basic.
  6. Now half fill each cup with the red cabbage water and stir the solution. If arranged in order, the jars should display a spectrum of colours from cherry red (strongly acidic), pink-red (acidic), lilac (slightly acidic), purple (neutral), blue (slightly basic), green (basic) and yellow (strongly basic).

What's happening

The things we eat and drink are all acidic, and the things we use for cleaning are basic. This is because basic substances taste unpleasant, but a cleaning agent usually needs to be basic to remove dirt and grease.

Red cabbage water changes colour depending on whether it is with an acid or a base.

Substances that are acidic or basic make the eyes sting, so baby shampoo is made neutral.

Acids

Acids are a very common group of chemical compounds, many of which occur naturally. Acids can be strong or weak.

Citric acid, which occurs naturally in lemons, is a weak acid. Hydrochloric acid (used for soldering) and sulfuric acid (battery acid) are very strong acids.

Bases

Bases (often called alkalis) are another group of chemical compounds that have different chemical properties from acids. When bases and acids are added together, they will neutralise each other's properties.

We describe whether things are acidic, basic or neutral by using a scale called the pH scale. The pH scale ranges from zero to 14. A substance with a pH of:

  • 0 is a very strong acid
  • 3 - 5 is a weak acid
  • 7 is neutral
  • 8 - 9 is a weak base
  • 13 - 14 is a very strong base.

Pure water has a pH of seven and is regarded as neutral.

Acids and bases can be detected by a group of chemical compounds called acid-base indicators. One of the first known naturally occurring indicators was a type of lichen called litmus. Lichens are plant-like growths that are often found on rocks and tree bark. Litmus turns red in the presence of an acid or blue with a base.

Most indicators used today to detect acids and bases are man-made. However, many plant pigments, such as the red cabbage you used, contain chemicals that act as acid-base indicators.

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