- phase diagrams are useful for seeing equilibrium regions and understanding the different behavior in different environments, e.g. the lower boiling point at high attitudes
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At any point in the areas separated by the curves, the pressure and temperature allow only one phase (solid, liquid, or gas) to exist, and changes in temperature and pressure, up to the points on the curves, will not alter this phase. At any point on the curves, the temperature and pressure allow two phases to exist in equilibrium: solid and liquid, solid and gas, or liquid and gas.
For example, the line drawn for the variation with temperature of gas pressure for the liquid is the boundary between liquid and gas; only gas can exist on the low-pressure, high-temperature side of the line, while the substance must be liquid on the high-pressure, low-temperature side; liquid and gas exist together at temperatures and pressures corresponding to points on the line; at the place where this line vanishes, called the critical point, the liquid and its gas become indistinguishable. Along the line between liquid and solid, the melting temperatures for different pressures can be found. The junction of the three curves, called the triple point, represents the unique conditions under which all three phases exist in equilibrium together.
A substance triple point can have an enormous impact on the evolution of that substance. For example, life developed on the Earth because the Earth's mean temperature and air pressure are near the triple point of water. This allows water to exist in all three forms on the Earth's surface and serve as an environment for biochemical reactions and a universal solvent for the construction of complex macromolecules. The surface of Titan, Saturn's largest moon, has environment conditions near the triple point of methane (methane lakes, snow, rains, ice) and so methane life may have formed there.
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