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Chemistry · Glossary

What is Liquidus and solidus?

Also known as: liquidus · solidus

Definition 8.5 University Chemistry — Year 2 · Chapter 8 — Binary Solid–Liquid Diagrams

On a solid–liquid diagram the liquidus is the curve above which everything is liquid; it gives the composition of the liquid in equilibrium with a solid. The solidus is the curve below which everything is solid; where the solid is a solution, it gives the composition of the solid in equilibrium with the liquid.

Copper–nickel, computed for ideal solid and liquid solutions from the melting points and enthalpies of fusion of the two metals. At 1300 C an alloy of composition 0.58 is split into a liquid of 0.541 and a solid of 0.609 (dashed tie line).
Copper–nickel, computed for ideal solid and liquid solutions from the melting points and enthalpies of fusion of the two metals. At 1300∘C1300{}^{\circ}\mathrm{C} an alloy of composition 0.58 is split into a liquid of 0.541 and a solid of 0.609 (dashed tie line).

Examples

Example 8.8 (An alloy at 1300∘C1300{}^{\circ}\mathrm{C})

For the alloy of nickel mole fraction 0.58 at 1300∘C1300{}^{\circ}\mathrm{C}, the fraction of the atoms in the solid is (0.58−0.541)/(0.609−0.541)=0.57(0.58 - 0.541)/(0.609 - 0.541) = 0.57. The first solid to appear, near 1315∘C1315{}^{\circ}\mathrm{C}, is richer in nickel than the alloy, the last liquid poorer.

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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