Question: Why do alkalis turn blue/purple and acids red/orange?

Sandra Greive answered on 10 Mar 2017:

I am guessing you are asking why litmus turns blue in alkali solutions and red in acid solutions. This is a pH indicator that changes colour depending the pH (acid or base) of the solution. In an acid solution the dye has an extra hydrogen atom, which can help balance the charge from the electrons in the dye to stabilise one chemical structure which is coloured. In base solutions this hydrogen atom is lost and the overall molecule becomes more negatively charged and so has a different chemical structure which is a different colour. You do this with ‘tea’ made from red cabbage. Add it to vinegar and see what colour it turns. What colour is it in a solution of baking soda in water?