Acid-base indicators are substances that change color in response to changes in pH, helping to determine whether a solution is acidic or basic. Common indicators include litmus, phenolphthalein, and methyl orange. These indicators work by undergoing a chemical change when they encounter different concentrations of hydrogen ions (H⁺) in a solution. In practical applications, acid-base indicators are often used in titrations, where they signal the endpoint of a reaction between an acid and a base. By observing the color change, scientists and students can easily identify the pH level of a solution, making these indicators valuable tools in chemistry and biology.