A pH indicator is a substance that changes color in ways that lets you identify a substance. pH indicators are used to measure the acidity or basicity of a solution. The pH of a solution is a measure of the concentration of hydrogen ions (H+) in the solution. A pH indicator changes color when the concentration of hydrogen ions changes.

There are many different types of pH indicators. Some common pH indicators include:

* Litmus: Litmus is a natural pH indicator that is extracted from lichens. Litmus turns red in acidic solutions and blue in basic solutions.

* Phenolphthalein: Phenolphthalein is a synthetic pH indicator that is colorless in acidic solutions and pink in basic solutions.

* Methyl orange: Methyl orange is a synthetic pH indicator that is red in acidic solutions and yellow in basic solutions.

pH indicators are used in a variety of applications, including:

* In the laboratory: pH indicators are used to measure the pH of solutions in the laboratory.

* In industry: pH indicators are used to control the pH of industrial processes.

* In medicine: pH indicators are used to measure the pH of body fluids.

pH indicators are an important tool for measuring the acidity or basicity of a solution. By changing color, pH indicators allow us to identify substances and control the pH of solutions.