sentences: iron in contact with water and oxygen will rust & rust forming is a kind of chemical change ->  iron in contact with water and oxygen is a chemical change  iron in contact with water and oxygen is a chemical change & chemical change is a kind of chemical reaction ->  iron in contact with water and oxygen is a chemical reaction  iron in contact with water and oxygen is a chemical reaction & chemical reactions cause new substances to form -> iron in contact with water and oxygen will form new substance