Answer the following question given this paragraph: 

A chemical reaction is a process that changes some chemical substances into other chemical substances. The substances that start a chemical reaction are called reactants . The substances that form as a result of a chemical reaction are called products . During the reaction, the reactants are used up to create the products. For example, when methane burns in oxygen, it releases carbon dioxide and water. In this reaction, the reactants are methane (CH 4 ) and oxygen (O 2 ), and the products are carbon dioxide (CO 2 ) and water (H 2 O).


Q: What is a process that changes some chemical substances into other chemical substances?


A:
a chemical reaction