What is the term for a compound that changes color in response to acids or bases?

What is the term for a compound that changes color in response to acids or bases?

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

Answer: (C)

An indicator is a compound that changes color when the hydrogen ion concentration in a solution changes, signaling whether the solution is acidic or basic. These dyes respond to pH by shifting structure as H+ or OH- levels vary, so they show a visible color at different pH ranges. For example, litmus may turn red in acidic solutions and blue in basic ones, while phenolphthalein is colorless in acid and pink in base. This color change lets us read how acidic or basic a solution is without precise measurements. The other terms don’t describe this behavior: a base is a substance that can accept protons or donate hydroxide but isn’t defined by changing color; a salt is an afterreaction product of an acid and base; a hydrogen ion is just a proton, not a dye that reveals pH.

An indicator is a compound that changes color when the hydrogen ion concentration in a solution changes, signaling whether the solution is acidic or basic. These dyes respond to pH by shifting structure as H+ or OH- levels vary, so they show a visible color at different pH ranges. For example, litmus may turn red in acidic solutions and blue in basic ones, while phenolphthalein is colorless in acid and pink in base. This color change lets us read how acidic or basic a solution is without precise measurements. The other terms don’t describe this behavior: a base is a substance that can accept protons or donate hydroxide but isn’t defined by changing color; a salt is an afterreaction product of an acid and base; a hydrogen ion is just a proton, not a dye that reveals pH.