Which of the following is an acid-base indicator?

Concept

An acid-base indicator is a substance whose visible colour depends on the acidity or alkalinity of the medium.

Natural indicators contain pigments that change structure, and therefore colour, when the hydrogen-ion environment changes.

Application

Turmeric contains the pigment curcumin. It remains yellow in acidic or neutral media but becomes reddish-brown in an alkaline medium, so it can distinguish acid-base conditions.

Contrast

• Vinegar is mainly a dilute solution of acetic acid; it is a sample whose acidity can be tested.

• Baking soda is sodium hydrogen carbonate; its aqueous solution is mildly alkaline and can be tested with an indicator.

• More than one of the above would require at least two listed substances to serve as indicators.

• None of the above would require every listed substance to lack indicator behaviour.

Cross-check

Adding an alkaline substance such as soap solution to turmeric produces the characteristic reddish-brown change. Hence, the acid-base indicator among the listed substances is turmeric.