Definition

1. Noun:
   • A chemical substance that changes color when the acidity or alkalinity of a solution changes: An acid-base indicator is a compound used in chemistry to visually signal the pH of a solution, typically by undergoing a distinct color change as the solution transitions from acidic to basic, or vice versa.

Usage

• An acid-base indicator is used to determine the approximate pH of a solution or to signal the endpoint of an acid-base titration.
• It is added in small amounts to the solution being tested.

Examples

• Noun:
  • Litmus paper is a common acid-base indicator that turns red in acidic solutions and blue in basic ones.
  • Phenolphthalein is an acid-base indicator often used in titrations; it is colorless in acidic solutions and pink in basic solutions.
  • The student selected a suitable acid-base indicator for the experiment based on the expected pH at the equivalence point.

Advanced Usage

• "to act as an acid-base indicator": to function as a substance that shows a color change with pH.
  • The pigment in red cabbage juice can act as a natural acid-base indicator.

Variants and Related Words

• Indicator (n): a broader term for any substance that shows a visible change, often in color, to signal a condition (e.g., chemical concentration, endpoint of a reaction).
  • A redox indicator changes color based on the oxidation state of a solution.

Synonyms

• pH indicator: A more specific synonym for an acid-base indicator.
• Acid-alkali indicator: An alternative term used primarily in British English.

Related Phrases

• Universal indicator: A mixture of several different acid-base indicators that produces a spectrum of colors to indicate a specific pH value across a wide range.
  • A universal indicator, unlike a single acid-base indicator, can show the approximate pH number on a scale from 1 to 14.

Noun

1. an indicator that changes color on going from acidic to basic solutions