ChemThink: Chemical Reactions In a chemical reaction, the starting material is called the The materials left in the end are called the l l m DUB An is placed between them indicating a chemical change has taken place. Chemical reactions always involve I , or both. In chemical reactions, there can never be or when the reaction is oven Chemical reactions only rearrange the in that are already there. A reaction is when it shows us not only what is present before and after the reaction but also shows of each atom present before and after the reaction. This is called the Law of of Correct Water formation 9 KC|O3 9 KCI + 02 Al + 02 -) A|203