A substance is a form of [[Matter|matter]] with a uniform a definite composition. Substances are characterized by specific properties such as [[Melting Point|melting point]], [[Boiling Point|boiling point]], [[Specific Volume and Density|density]], [[Pressure|pressure]], [[Temperature|temperature]], or [[Reactivity|reactivity]]. Substances can exist in various [[Phase|phases]] and compositions of phases. Substances may either be [[Pure Substance|pure substances]] or [[Mixtures|mixtures]].