Chemical Reactions Change Substances Into Different Substances By. Chemical changes occur through the process of chemical reactions, and the resulting substances have different properties because their atoms and molecules are arranged differently. Catalysts can also increase rate of reaction.
Reactants and products the oxygen molecules (o 2) that you breathe in are part of a series of chemical reactions. Each chemical reaction is characterized by a stoichiometric relationship between the substances and by a reaction rate. In a chemical process, the atoms that make up the original substances are regrouped into different molecules, and these new substances have different properties from those of the reactants.