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Chemistry · Glossary

What is Eutectic?

Also known as: eutectic point · eutectic mixture

Definition 8.10 University Chemistry — Year 2 · Chapter 8 — Binary Solid–Liquid Diagrams

The eutectic point of a binary system is the point where the liquid is in equilibrium with two solids at once; it is the lowest temperature at which a liquid exists in that part of the diagram. A liquid of the eutectic composition, and the fine intimate mixture of the two solids into which it freezes, are called a eutectic mixture.

Benzene–naphthalene, computed with the Schröder–van Laar equation for each solid. The two branches meet at the eutectic E, -3.5 C and 0.13 naphthalene, below which all is solid.
Benzene–naphthalene, computed with the Schröder–van Laar equation for each solid. The two branches meet at the eutectic E, −3.5∘C-3.5{}^{\circ}\mathrm{C} and 0.13 naphthalene, below which all is solid.

Examples

Example 8.12 (Cooling a benzene–naphthalene mixture)

A liquid of naphthalene mole fraction 0.5 cools until the naphthalene branch of the liquidus, near 46∘C46{}^{\circ}\mathrm{C}, where pure solid naphthalene starts to crystallise. The liquid, losing naphthalene, slides down the branch; at −3.5∘C-3.5{}^{\circ}\mathrm{C} it reaches the eutectic, and the rest freezes at constant temperature into the eutectic mixture. By the lever rule, just above the eutectic, the solid naphthalene is (0.5−0.133)/(1−0.133)=42 %(0.5 - 0.133)/(1 - 0.133) = 42~\% of the moles, the eutectic liquid 58 %.

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