<P> The term phase is sometimes used as a synonym for state of matter, but a system can contain several immiscible phases of the same state of matter . </P> <P> In a solid, the particles (ions, atoms, or molecules) are closely packed together . The forces between particles are strong so that the particles cannot move freely but can only vibrate . As a result, a solid has a stable, definite shape, and a definite volume . Solids can only change their shape by force, as when broken or cut . </P> <P> In crystalline solids, the particles (atoms, molecules, or ions) are packed in a regularly ordered, repeating pattern . There are various different crystal structures, and the same substance can have more than one structure (or solid phase). For example, iron has a body - centred cubic structure at temperatures below 912 ° C, and a face - centred cubic structure between 912 and 1394 ° C. Ice has fifteen known crystal structures, or fifteen solid phases, which exist at various temperatures and pressures . </P> <P> Glasses and other non-crystalline, amorphous solids without long - range order are not thermal equilibrium ground states; therefore they are described below as nonclassical states of matter . </P>

Substances that can be in two physical states