Follow these instructions to learn about acids and bases using red cabbage.
Caution: 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.
What you need
To do this activity you will need:
What to do
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Using a sharp knife and cutting board, finely slice three or four red cabbage leaves.
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Place the cabbage leaves in the plastic bottle, half fill the bottle with hot water and screw the lid on tightly.
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Shake the bottle for a few minutes until the water becomes a deep purple colour. Leave the solution to cool.
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Strain the solution and add sufficient water to the solution to make about one litre.
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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.
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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.
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:
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.
For more fun activities, join CSIRO's Double Helix Science Club.
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