Matter is classified into two major groups, pure substances and mixtures. There are two types of pure substances: An element is composed of identical atoms. Diamond, which is composed of only carbon atoms, is an example. A compound is composed of identical molecules. The molecules of a compound are made up of more than one type of atom that are chemically bonded together. Carbon dioxide is an example. Each identical particle is composed of one carbon atom bonded to two oxygen atoms.
Mixtures are made up of more than one type of particle. Unlike pure substances, mixtures have variable compositions. A homogeneous mixture has its particles evenly distributed and looks the same throughout. Salt water is an example of a homogeneous mixture. The concentration of the salt dissolved in water can vary. Homogeneous mixtures are also called solutions.
A heterogeneous mixture is not evenly distributed. Examples are orange juice with pulp and mixtures of sand and gravel.