States of Matter
States of Matter, in classical physics, three forms in which matter occurs—solid, liquid, and gas. Plasma , the collection of charged gaseous particles containing nearly equal numbers of negative and positive ions, is sometimes called the fourth state of matter (see Ion ; Ionization ). Solid matter is characterized by resistance to any change in shape, caused by a strong attraction between the molecules of which it is composed. In liquid form, matter does not resist forces that act to change its shape, because the molecules are free to move with respect to each other (see Molecule ). Liquids, however, have sufficient molecular attraction to resist forces tending to change their volume. Gaseous matter, in which molecules are widely dispersed and move freely, offers no resistance to change of shape and little resistance to change of volume. As a result, a gas that is not confined tends to diffuse infinitely, increasing in volume and diminishing in density. Most substances are solid at l