Which term describes a substance that changes colour in response to acidic or basic conditions and is commonly a weak acid?

• A. Buffer
• B. Equivalence Point
• C. Organic
• D. Indicator

Answer: (D)

An indicator is a substance that changes color in response to acidic or basic conditions because it exists in two forms that differ in color. It’s usually a weak acid (HIn) that equilibrates with its conjugate base (In−): HIn ⇌ H+ + In−. In acidic solutions, the HIn form dominates and has one color; in basic solutions, the In− form dominates and has another color. This pH-dependent color change is what lets indicators signal the endpoint of a titration or the overall acidity/basicity of a solution. Buffers resist pH change, so they don’t rely on a color change; equivalence point is a stage in titration, not a color change; organic simply describes composition, not the color-change property.