Solid

Solid, the physical state of matter in which samples maintain their shape and size. Some highly viscous liquids, such as cold molasses, flow so slowly that they seem to retain their size and shape and thus appear to be solids. X-ray examination, however, reveals an important difference in microscopic structure. Solids exhibit a regular arrangement of atomic, ionic, or molecular particles—solid objects have a crystalline structure. In contrast, the molecules of liquids are arranged irregularly—liquids have no crystalline structure. See Matter; Matter, States of.