Eutectic Temperature
Definition and meaning of Eutectic Temperature in chemistry.
Eutectic temperature is the single, invariant temperature at which a mixture of two or more substances melts or freezes sharply as one phase, marking the lowest possible melting point across all compositions of that system.
In more detail
This minimum occurs only at one specific composition, the eutectic composition, where liquid and both solid phases coexist in equilibrium simultaneously. Any other composition of the same two components solidifies over a range of temperatures (between its liquidus and the eutectic temperature) rather than at a single point, because one component crystallizes out first while the melt composition drifts toward the eutectic. Below the eutectic temperature, the eutectic composition freezes into a finely intermixed solid of both pure phases rather than a solid solution. Eutectic temperatures are central to reading binary phase diagrams and to designing low-melting alloys and de-icing brines.
Key facts
| Field | Physical Chemistry |
|---|---|
| Classic alloy example | Sn-Pb solder: 61.9 wt% Sn, eutectic temperature 183°C |
| Pure component melting points | Sn 232°C; Pb 327°C |
| Salt-water eutectic | 23.3 wt% NaCl, eutectic temperature −21.1°C |
Tin-lead solder is eutectic at 61.9% tin by mass and melts sharply at 183°C, well below the melting points of pure tin (232°C) and pure lead (327°C), which is why this ratio is favored for electronics soldering.
Frequently asked questions
How is eutectic temperature different from ordinary freezing-point depression?
Freezing-point depression describes how any dissolved solute lowers a solvent's freezing point; the eutectic temperature is the specific minimum this depression can reach, occurring only at the exact composition where solid solute and solid solvent crystallize out together.
Does every composition of a mixture freeze at the eutectic temperature?
No. Only the eutectic composition freezes sharply at that single temperature; other compositions begin freezing at a higher liquidus temperature and finish at the eutectic temperature as the last liquid solidifies.