react

If you react to a person or event, you do something because of the person or event.

Example: When someone shouts at you, you usually react by shouting back.

If you react to a medicine or other substance, something happens in your body because of it.

Example: She reacts badly to asprin. Her skin becomes red all over her body.

When two or more chemicals react, they combine to form new chemical compounds.

Example: Acids and alkalis normally react to form salt and water.