There are two kinds of substances-elements and compounds. Elements are substances that cannot be broken down into simpler substances by ordinary chemical means. Elements cannot be made by the combination of simpler substances. There are slightly more than 100 elements, and every material object in the universe consists of one or more of these elements. Familiar substances that are elements include carbon, aluminum, iron, copper, gold, oxygen, and hydrogen.
Compounds are substances consisting of two or more elements chemically combined in definite proportions by mass to give a material having a definite set of properties different from that of any of its constituent elements. For example, the compound water consists of 88.8% oxygen and 11.2% hydrogen by mass. The physical and chemical properties of water are distinctly different from those of both hydrogen and oxygen. For example, water is a liquid at room temperature and pressure, while the elements of which it is composed are gases under these same conditions. Chemically, water does not burn; hydrogen may burn explosively in oxygen (or air). Any sample of pure water, regardless of its source, has the same composition and the same properties.
There are two kinds of mixtures-homogeneous mixtures and heterogeneous mixures. Homogeneous mixtures are also called solutions, and heterogeneous mixtures are sometimes simply called mixtures. In heterogeneous mixtures, it is possible to see differences in the sample merely by looking, although a microscope might be required. In contrast, homogeneous mixtures look the same throughout the sample, even under the best optical microscope.