Why Does Red Cabbage Juice Change Colour with Acids and Bases?

Purple liquid extracted from red cabbage turns reddish with lemon juice and may shift towards blue-green with a solution of bicarbonate of soda. The temperature has not changed, and acids and bases are not adding colours of their own. The responsive material is the cabbage’s anthocyanin pigments.

Anthocyanins are water-soluble pigments. As acidity changes, their molecules gain or lose hydrogen ions and move between several structures. Each structure absorbs different wavelengths of visible light. The light that remains reflected or transmitted reaches our eyes as red, purple or blue-green. This colour change is a reversible chemical equilibrium: restoring the conditions can often restore the colour, although strong alkali, heat or long storage may break the pigments down.

Red cabbage juice can therefore act as a natural acid–base indicator, but it is not a precise pH meter. The apparent colour also depends on extract concentration, lighting, temperature and the sample’s own colour, so it shows only an approximate range. A safe household comparison uses the same batch of cabbage extract with plain water, lemon juice and a solution of bicarbonate of soda. Do not taste the mixtures, and do not test or combine household cleaning products.

American Chemical Society:
https://www.acs.org/education/activities/red-cabbage-indicator.html