Turn red cabbage into a liquid that changes colour in acids and bases, then test what's in your kitchen: lemon juice, vinegar, soap and baking soda.
Age 8 and up. About an hour. Starter level.
Chop the cabbage into small pieces and put them in the jug. Ask an adult to pour in hot water until the cabbage is just covered. Leave it for ten minutes, stirring now and then, until the water is dark purple.
Pour the liquid through the sieve into a clean jug and let it cool. This purple liquid is your indicator. The cabbage has done its job.
Put the same amount of indicator in each cup, about a quarter full. Label the cups. Keep one with just indicator, so you have a colour to compare against.
Before each test, write down what colour you think it will turn. Then add a spoonful of the thing you're testing, one per cup, and stir. For baking soda, dissolve a spoonful in a little water first.
Acids turn it pink or red: lemon juice and vinegar are strongly acidic. Water stays purple, because it is neutral. Bases turn it blue or green: baking soda is a weak base, soap usually a bit stronger. Put your cups in order from red to green and you have made a pH scale.
Red cabbage contains pigments called anthocyanins, whose shape changes with how acidic their surroundings are, and a different shape reflects a different colour. The same pigments colour blueberries and some flowers, which is why some hydrangeas change colour with the soil they grow in.
Go further:
Science courses at Kautuk