Peritectic

peritectic [¦per·ə¦tek·tik]

(physical chemistry)

An isothermal reversible reaction in which a liquid phase reacts with a solid phase during cooling to produce a second solid phase.

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Peritectic 

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.

REFERENCES

See references under BINARY SYSTEMS.