states of matter, forms of matter differing in several properties because of differences in the motions and forces of the molecules (or atoms, ions, or elementary particles) of which they are composed. The states of matter are also known as phases of matter or states of aggregation. There are three commonly recognized states of matter: solid, liquid, and gas. The molecules of a solid are limited to vibration about a fixed position. This restriction gives a solid both a definite volume and a definite shape. As energy in the form of heat is added to a solid, its molecules begin to vibrate more rapidly until they break out of their fixed positions and the solid becomes a liquid. The change from solid to liquid is called melting and occurs at a definite temperature, the melting point. The molecules of a liquid are free to move throughout the liquid but are held from escaping from the liquid by intermolecular forces (see adhesion and cohesion). This gives a liquid a definite volume but no definite shape. As more heat is added to the liquid, some molecules gain enough energy to break away completely from the liquid and escape into the surrounding space (see evaporation). Finally a temperature is reached at which molecules throughout the liquid are becoming energetic enough to escape and bubbles of vapor form and rise to the surface. The change of the liquid to a vapor, or gas, in this manner is called boiling and occurs at the boiling point. The molecules of a gas are free to move in every possible way; a gas has neither a definite shape nor a definite volume but expands to fill any container in which it is placed. In addition to these three states of matter, scientists also distinguish three additional states—plasma and the Bose-Einstein and the fermionic condensates. A plasma is formed by adding still more heat to the molecules of a gas. Eventually a point is reached where the molecules are moving so rapidly that the molecules become torn apart into their component atoms and individual electrons are pulled away from the atoms. This very hot mixture of negatively charged electrons and positively charged ions has properties distinct from those of the other states of matter. Bose-Einstein condensate and fermionic condensates are formed by chilling the molecules of a gas. As temperatures approach absolute zero ( - 273.15°C), the motion of the individual atoms slows to the point where they combine to form a single "super atom" with properties distinct from those of other states of matter.
W. H. Bragg, Concerning the Nature of Things (2004).