Answer the following question given this paragraph: 

A chemical reaction occurs when some substances change chemically to other substances. Chemical reactions are represented by chemical equations. Consider a simple chemical reaction, the burning of methane. In this reaction, methane (CH 4 ) combines with oxygen (O 2 ) in the air and produces carbon dioxide (CO 2 ) and water vapor (H 2 O). The reaction is represented by the following chemical equation:.


Q: What are chemical reactions represented by?


A:
chemical equations