Reactions+and+Stoich

//The only constant is change.// So spoke the Greek philosopher Heraclitus 2500 years ago. Much of chemistry is spent trying to describe these changes. Most often, the changes we study occur in a //chemical reaction//, a process in which atoms are rearranged to form new substances, but are not created or destroyed in the process. We begin this study of change by first representing it, both in pictures and with symbols (as in a //chemical equation//). We next tackle the easiest aspect of change: mass. Although the total mass of all substances remains constant, the process of //stoichiometry// allows us to calculate the amounts of reactants used and products formed in a given chemical reaction.