pH indicators are substances that change color in response to the acidity or alkalinity of a solution. They are commonly used in laboratories and classrooms to measure the pH level, which ranges from 0 (very acidic) to 14 (very basic). A pH of 7 is considered neutral. Common pH indicators include litmus paper, phenolphthalein, and universal indicator. These indicators work by undergoing a chemical change when they come into contact with hydrogen ions in a solution. For example, litmus paper turns red in acidic solutions and blue in basic ones. This color change helps scientists and students quickly assess the pH of various substances.