Question

Why does indicators change their colour in acidic or basic medium

Answer 1

A variety of indicators change color at various pH levels. A properly selected acid-base indicator can be used to visually "indicate" the approximate pH of a sample. An indicator is usually some weak organic acid or base dye that changes colors at definite pH values. The weak acid form (HIn) will have one color and the weak acid negative ion (In-) will have a different color. The weak acid equilibrium is:

HIn --> H+ + In- 

Answer 2

Answer:

Tell students that when an acid is added to an indicator solution, the acid donates protons to the water molecules. This increases the concentration of H3O+ ions in the solution. The H3O+ ions donate protons to the indicator molecules causing the indicator to change color toward red.