Chemistry the science that deals with the composition and changes in composition of substances. Water is an example of this composition; it is composed of two gases, hydrogen and oxygen. Water also changes form from liquid to solid to gas but does not necessarily change composition. Matter anything that has weight (mass) and occupies space. Kinds of matter include elements, compounds, and mixtures. Solids substances that maintain definite size and shape. Liquids have a definite volume, but not shape, that will fill containers to certain Ievels and form free level surfaces. Gases do not have definite volume or shape; they completely fill any container in which they are placed. Element the simplest form of chemical matter. Each element has chemical and physical characteristics different from all other kinds of matter. Compound a substance of two or more chemical elements chemically combined. Example: water (H20) is a compound formed by hydrogen and oxygen. Mixture a physical, not chemical, intermingling of two or more substances. Example: sand and salt stirred together. Atom smallest particle of an element that can unite chemically with other elements. All the atoms of an element are the same in chemical behavior, although they may differ slightly in weight. Most atoms can combine chemically with other atoms to form molecules. Molecule the smallest particle of matter or a compound that has the same composition and characteristics as the rest of the substance. A molecule may consist of a single atom, two or more atoms of the same kind, or two or more atoms of different kinds. Radical two or more atoms that unite in a solution and behave chemically as if they were a single atom. Solvent the component of a solution that does the dissolving.