Matter is anything that takes up space and has mass (or weight which is the influence of gravity on mass.) It is distinguished from energy, which causes objects to move or change, but which has no volume or mass of its own. Matter and energy interact, and under certain circumstances behave similarly, but for the most part remain separate phenomena. They are, however, inter-convertible according to Einstein's equation E=mc2, where E is the amount of energy that is equivalent to an amount of mass m, and c is a constant: the speed of light in a vacuum.
In 1804, the English scientist John Dalton formulated the atomic theory, which set out some fundamental characteristics of matter, and which is still used today. According to this theory, matter is composed of extremely small particles called atoms, which can be neither created nor destroyed. Atoms can, however, attach themselves (bond) to each other in various arrangements to form molecules. A material composed entirely of atoms of one type is an element, and different elements are made of different atoms. A material composed entirely of molecules of one type is a compound, and different compounds are made of different molecules. Pure elements and pure compounds are often referred to collectively as pure substances, as opposed to a mixture in which atoms or molecules of more than one type are jumbled together in no particular arrangement.
Elements and compounds can undergo chemical processes (reactions), which rearrange, break, or form bonds between atoms. Substances can also change by physical processes, which may alter the observable characteristics of the substance, but do not rearrange the internal structures of any molecules. The chemical compound water, for example, can be split into the element hydrogen and oxygen by electricity. That is a chemical reaction, because bonds in the water molecules break, and new bonds form. Water can also freeze into ice, or boil into vapor. Those are both physical processes because the water molecules do not change their internal bonding.
At Earth's surface, matter exists in one of three physical states (or phases)—solid, liquid, or gas—categorized by the extent of attraction between the molecules or atoms of the substance. (Other intermediate states are possible under more extreme conditions.) Solids have a very orderly, rigid arrangement of their atoms or molecules, with strong forces holding the atoms or molecules together. Gas molecules or atoms, on the other hand, have almost no intermolecular forces holding them together. Liquids have intermediate properties; their molecules or atoms have some attractive force for each other, but are not fixed in place like those of a solid.
See also Element, chemical.