Which term describes a substance that changes colour when it reacts with an acid or base and is usually a weak acid itself?

• A. Buffer
• B. Organic
• C. Inorganic
• D. Indicator

Answer: (D)

Indicators are substances that change color in response to the acidity or basicity of a solution because they exist in two forms that differ in color depending on whether they are protonated or deprotonated. They are usually weak acids (or bases) so the equilibrium between these forms shifts as the hydrogen ion concentration changes, producing a visible color change. This makes them ideal for signaling pH changes, such as at the endpoint of a titration. A buffer resists pH changes and doesn’t inherently show a color shift; labeling something as organic or inorganic describes composition, not a color-change behavior. Hence, the term that fits is indicator.