Here's why:
* It names something: A reaction is a response to something.
* It can be the subject or object of a sentence: "The reaction was immediate." "He caused a strong reaction."
However, "reaction" can sometimes be used as an adjective in specialized contexts, especially in chemistry. For example, you might say "the reaction mixture" or "the reaction rate."
Let me know if you have any other words you'd like to analyze!