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a liquid that is in equilibrium at constant pressure with crystal phases of chemical compounds or solid solutions; the number of crystal phases equals the number of components in the system and reduces to unity upon a change in temperature. The number of crystal phases serves to distinguish a peritectic from a eutectic, which is a liquid in equilibrium with crystal phases whose number does not change upon a decrease in temperature (a eutectic is completely converted into a solution or melt upon an increase in temperature). For example, in a binary system that consists of water and sodium sulfate (Na2SO4 at 32.4° C, the solution (which contains 33.2 percent Na2SO4 by weight) is in equilibrium with two crystal phases: anhydrous Na2SO4 and the decahydrate Na2SO4• 10H2O. Only Na2SO4• 10H2O crystals are in equilibrium with the solution at temperatures below 32.4°C, whereas only Na2SO4 crystals are in equilibrium with the solution above this temperature.
The term “peritectic,” or “peritectic point,” is often used to designate the point of intersection between the temperature lines on a phase diagram at the onset of crystallization of two solid phases in equilibrium with a peritectic liquid.