ABSTRACT
A pure substance is a sample of matter that cannot be separated into simpler components without chemical change. Physical changes can alter the state of matter but not the chemical identity of a pure substance. The Law of Conservation of Mass is the law that states that in a chemical reaction the total mass of the products is equal to the total mass of the reactants. The Law of Definite Proportions is the law that states that every chemical compound contains fixed and constant proportions (by weight) of its constituent elements. Many of the important topics in chemistry, such as chemical bonding, the shape of molecules, and so on, are based on where the electrons in an atom are located. Valence bond theory and molecular orbital theory are two basic theories that use quantum mechanics methods to explain chemical bonding.
