Phase Rule (Physics) A rule stating that the number of degrees of freedom in a material system at equilibrium is equal to the number of Components minus the number of Phases plus the constant 2. For example, the system of water vapor, liquid water, and solid ice has zero degrees of freedom because the three phases of vapor, liquid, and solid coexist in one component, water. A solution of salt in water, for example, is a chemical system in which the components are salt and water. The chemical components of a system can exist as gas, liquid, or solid phases. The phase rule is applicable only to systems, called heterogeneous systems, in which two or more physically distinct phases are in equilibrium. A system cannot contain more than one gas phase but can contain any number of liquid and solid phases. A water solution of salt contains three phases: Salt comprises the solid phase, water comprises the liquid phase, and water vapor formed by evaporation of the water comprises the gas phase. Water is an example of a heterogeneous chemical system of one component. The liquid and gas phases, water and water vapor, coexist over a wide range of temperatures and pressures. At one temperature and pressure, called the Triple Point, all three phases of water (liquid), water vapor (gas), and ice (solid) coexist at equilibrium. The phase rule is expressed by the equation F = C – P + 2, where “F” is the number of variables (usually temperature, pressure, and concentration) that can be changed without causing the disappearance of a phase or the appearance of a new one. “C” represents the number of chemical components of the system and “P” the number of phases present.
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Author of the Water Words Dictionary source of text: Gary A. Horton
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