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New page: thumb|right|300px|Phase diagram of CO<sub>2</sub> A '''phase diagram''' is a graphical way of showing the physical state of a pure substance at varying [[pressur...
[[Image:CO2phase.jpg|thumb|right|300px|Phase diagram of CO<sub>2</sub>]]
A '''phase diagram''' is a graphical way of showing the [[physical state]] of a pure substance at varying [[pressure]]s and [[temperature]]s. As seen in the image to the right, a phase diagram is typically represented as a graph with temperature on the [[x-axis]] and pressure on the [[y-axis]].<ref name="atkins">Atkins, Peter and Julio de Paula. ''Physical Chemistry Volume 1, 8e.'' Oxford University Press: 2006.</ref>
A phase diagram will always have a number of lines on it, representing a phase [[equilibrium]], that is, the set of temperatures and pressures at which one phase is in equilibrium with another phase. These lines are called [[phase boundary|phase boundaries]], and they section the area of the plot into the three phases of [[matter]] ([[solid]], [[liquid]], and [[gas]]) as well as the [[supercritical fluid]] region.<ref name="atkins" />
Along with the phase boundaries, there are two important points on a phase diagram: the [[triple point]] and the [[critical point]]. The triple point is the unique temperature and pressure at which all three phases of matter coexist in equilibrium. The critical point defines the critical temperature (T<sub>c</sub>) and critical pressure (P<sub>c</sub>) above which the substance is a supercritical supercritical fluid.
==Other Types of Phase Diagrams==
While the pressure-temperature diagrams (PT diagrams) are useful for discerning the phase of a substance at certain operating conditions, other types of phase diagrams exist. For example, there are phase diagrams which describe the phase of a mixture of two compounds verses temperature. [[Metal]]lurgists also use a special type of phase diagram to describe solid-solid alloys, which have different "phases," or different ways the molecules [[crystal]]ize depending on the composition of the [[alloy]] at the temperature at which it is held.<ref name="Callister">Callister Jr., William D. ''Materials Science and Engineering: An Introduction, 6e". Von Hoffman Press: 2003.</ref>
A '''phase diagram''' is a graphical way of showing the [[physical state]] of a pure substance at varying [[pressure]]s and [[temperature]]s. As seen in the image to the right, a phase diagram is typically represented as a graph with temperature on the [[x-axis]] and pressure on the [[y-axis]].<ref name="atkins">Atkins, Peter and Julio de Paula. ''Physical Chemistry Volume 1, 8e.'' Oxford University Press: 2006.</ref>
A phase diagram will always have a number of lines on it, representing a phase [[equilibrium]], that is, the set of temperatures and pressures at which one phase is in equilibrium with another phase. These lines are called [[phase boundary|phase boundaries]], and they section the area of the plot into the three phases of [[matter]] ([[solid]], [[liquid]], and [[gas]]) as well as the [[supercritical fluid]] region.<ref name="atkins" />
Along with the phase boundaries, there are two important points on a phase diagram: the [[triple point]] and the [[critical point]]. The triple point is the unique temperature and pressure at which all three phases of matter coexist in equilibrium. The critical point defines the critical temperature (T<sub>c</sub>) and critical pressure (P<sub>c</sub>) above which the substance is a supercritical supercritical fluid.
==Other Types of Phase Diagrams==
While the pressure-temperature diagrams (PT diagrams) are useful for discerning the phase of a substance at certain operating conditions, other types of phase diagrams exist. For example, there are phase diagrams which describe the phase of a mixture of two compounds verses temperature. [[Metal]]lurgists also use a special type of phase diagram to describe solid-solid alloys, which have different "phases," or different ways the molecules [[crystal]]ize depending on the composition of the [[alloy]] at the temperature at which it is held.<ref name="Callister">Callister Jr., William D. ''Materials Science and Engineering: An Introduction, 6e". Von Hoffman Press: 2003.</ref>