Colour magic

Cut a red cabbage leaf into small pieces and soak in a cup of boiling water. After half an hour pour the violet-coloured cabbage water into a glass. You can now use it for crazy colour magic. Place three glasses on the table, all apparently containing pure water.

In fact only the first glass contains water, in the second is white vinegar and in the third water mixed with bicarbonate of soda. When you pour a little cabbage water into each glass, the first liquid remains violet, the second turns red and the third green.

The violet cabbage dye has the property of turning red in acid liquids and green in alkaline. In neutral water it does not change colour. In chemistry one can find out whether a liquid is acid or alkaline by using similar detecting liquids (indicators).