Atoms and Molecules
Bonds and Octet Rules
VISUAL AIDSFunctional Groups
QUIZZESPractice Quiz I
Practice Quiz II
Practice Quiz III
Matter can undergo a variety of changes through processes called reactions. Reactions can make matter more complex, such as the reaction that takes oxygen atoms and hydrogen atoms and turns them into water molecules. Reactions can also make matter simpler, such as the reaction in a car engine that turns gasoline into water, carbon dioxide, and carbon monoxide.
A reaction has a beginning and an end. At the beginning of the reaction the matter present is called the reactants, and once the reaction is completed, the matter is called the products.
Scientists have discovered millions of different reactions in nature and have created millions of reactions in laboratories. Almost all of these reactions can be classified into three basic reaction types.
Antoine Laurent Lavoisier (1743-1794) established chemistry as a quantitative science. In 1773, Lavoisier performed an experiment that had already been discovered by Karl Wilhelm Scheele (1742-1786) and Joseph Priestley (1733–1804) that involved the decomposition of mercury oxide into metallic mercury and oxygen gas. However, Lavoisier's experiment was unique in that he was the first to measure the masses of all the substances before and after the reaction.
Lavoisier discovered that when the chemical reaction was carried out, the total mass of all the substances before, during, and after the reaction did not change. He summarized his findings into the Law of Conservation of Mass, which states that matter is neither created nor destroyed during a chemical reaction. The total mass of the matter before a reaction (reactants) is always equal to the total mass of the matter after a reaction (products).
Law of conservation of mass