Any domain or collection of homogeneous domains with the same properties is called a phase.
Examples Consider a glass of iced tea.
Consider a mixture of graphite and diamond. Consider a vessel containing solid carbon dioxide and gaseous carbon dioxide. There are two phases, solid and gas. The phases are in equilibrium if the gas pressure is equal to the vapor pressure of solid CO 2 at the given temperature. In order to specify the composition of the system, one must specify both the number of different chemical species and the number of independent constraints on the amounts of these species.