Final answer:
In a titration reaction, indicators change color depending on the acidity or basicity of the solution. Phenolphthalein is a commonly used indicator that turns from colorless to pink in a strong acid-strong base titration.
Step-by-step explanation:
In a titration reaction, the colors of indicators change because indicators are substances that change color depending on the acidity or basicity of the solution. When an indicator is added to the solution being titrated, it undergoes a pH titration just like any other acid or base. As the concentration of the indicator's acidic form decreases and the concentration of its basic form increases, the color of the solution slowly changes.
For example, phenolphthalein is a commonly used indicator for strong acid-strong base titrations. When a few drops of phenolphthalein are added to the acidic solution, it turns from colorless to pink as the solution becomes basic.