Final answer:
An acid-base indicator is a compound that reversibly changes color in response to pH changes, such as phenolphthalein which turns red or pink in basic conditions above pH 8.3. These substances can exist in different forms with varying colors, depending on the pH level.
Step-by-step explanation:
A compound that can reversibly change color depending on conditions such as pH is known as an acid-base indicator. These are typically intensely colored organic molecules that exhibit a noticeable color change within a specific pH range. Notably, phenolphthalein is an indicator that turns from colorless to red or pink in a pH range above 8.3, indicating a transition from acidic to basic conditions. Acid-base indicators are generally weak organic acids or bases that can exist as either a protonated or deprotonated species, with each form having a different color, which is the basis of the color change.
Indicators like phenolphthalein are valuable tools in chemistry for determining the pH of solutions. The color-change interval is the range of pH over which this color change is observed. Acid-base indicators work by establishing an equilibrium between their conjugate acid and conjugate base forms, each with distinct colors, depending on the surrounding pH.