An indicator is placed in water - HSC - SSCE Chemistry - Question 25 - 2013 - Paper 1

An indicator functions by changing color when there is a shift in equilibrium, which occurs in response to pH changes. The solution containing Ind^- (green) and HInd (red) demonstrates this equilibrium behavior:

$ext{HInd}_{(aq)} + ext{H}_2 ext{O}_{(l)} \rightleftharpoons \text{Ind}^-{(aq)} + ext{H}_3 ext{O}^+_{(aq)}$

When an acid is added to the solution, the concentration of H₃O⁺ increases, shifting the equilibrium to the left and favoring the formation of HInd, which is red. Conversely, when a base is added, the concentration of H₃O⁺ decreases, shifting equilibrium to the right, resulting in the green color of Ind^-. This characteristic color change enables the solution to act as an effective indicator.