A chemical indicator is a substance that changes color or physical properties in response to changes in the chemical environment, such as pH or concentration of a specific ion. These indicators are commonly used in laboratories and educational settings to visually demonstrate chemical reactions or to determine the acidity or basicity of a solution. One well-known example of a chemical indicator is litmus, which turns red in acidic solutions and blue in basic solutions. Other indicators, like phenolphthalein, change color at different pH levels, making them useful for titrations and other analytical procedures.