A pH indicator is a substance that changes color in response to the acidity or alkalinity of a solution. It helps determine the pH level, which ranges from 0 to 14, with values below 7 indicating acidity, 7 being neutral, and above 7 indicating alkalinity. Common pH indicators include litmus paper, phenolphthalein, and universal indicator. These indicators are often used in laboratories, educational settings, and various industries to test the pH of liquids. By observing the color change, users can quickly assess the chemical properties of a solution, aiding in experiments, quality control, and environmental monitoring.