Red Cabbage Indicator Some dyes act as indicators - they change colour depending on whether they have acid or alkali mixed with them. You Will Need: half a red cabbage a saucepan water a sieve a bottle and a dropper Directions: Chop half a red cabbage, place it in a saucepan and add just enough water to barely cover the cabbage. Bring the water to the boil, and simmer for about 15 minutes. Pour the water through a sieve. This purple water is your indicator dye. Testing the Indicator: Place a small amount of indicator dye into three separate containers. Add a few drops of vinegar or lemon juice to one sample. Add a small amount of bicarbonate of soda (or baking powder) or cloudy ammonia to another sample. Compare each of the mixtures with the third sample of indicator dye. What happened? The bicarbonate of soda or ammonia (alkalis) should have turned the cabbabge water a pale greeny-blue. The vinegar or lemon juice (acids) should have turned the cabbage water a reddish colour. The red cabbage dye is behaving as an indicator, a chemical substance which changes colour, depending on the acidity, or alkalinity of its environment.

Bibliography Parker, S. Simple Chemistry (London: Kingfisher, 1990).