Showing True Colors with Acid-Base Indicators

Indicators are substances (organic dyes) that change color in the presence of an acid or base. You may be familiar with an acid-base indicator plant — the hydrangea. If it’s grown in acidic soil, it turns pink; if it’s grown in alkaline soil, it turns blue.

In chemistry, indicators are used to indicate the presence of an acid or base. Chemists have many indicators that change at slightly different pHs, but the two indicators used most often are litmus paper and phenolphthalein. I discuss both in this section.