Which term describes a substance that changes color in response to pH?

Which term describes a substance that changes color in response to pH?

• A. Base
• B. Salt
• C. Hydrogen Ion
• D. Indicator

Answer: (D)

A substance that changes color in response to pH is a pH indicator. These materials exist in forms that react to the concentration of hydrogen ions in a solution, shifting their structure and the wavelengths of light they absorb, which we see as a color change. This makes indicators useful for telling whether a solution is acidic or basic and roughly how acidic or basic it is. The other terms describe roles in acid–base chemistry but don’t inherently signal pH with a color change: a base raises pH, a salt is a product of an acid–base reaction, and a hydrogen ion is the proton that determines acidity. An indicator specifically provides that color cue for pH, so it’s the best fit.

A substance that changes color in response to pH is a pH indicator. These materials exist in forms that react to the concentration of hydrogen ions in a solution, shifting their structure and the wavelengths of light they absorb, which we see as a color change. This makes indicators useful for telling whether a solution is acidic or basic and roughly how acidic or basic it is. The other terms describe roles in acid–base chemistry but don’t inherently signal pH with a color change: a base raises pH, a salt is a product of an acid–base reaction, and a hydrogen ion is the proton that determines acidity. An indicator specifically provides that color cue for pH, so it’s the best fit.