Why does a eutectic alloy solidify at a single temperature into two solid phases?

• A. Because the liquid transforms into two solid phases at a fixed temperature
• B. Because the composition becomes completely homogeneous and forms a single phase
• C. Because diffusion ceases entirely at eutectic composition
• D. Because the melting point increases with cooling

Answer: (A)

At the eutectic composition, the liquid can transform into two solid phases at a single, fixed temperature because this is an invariant reaction: liquid → two solids. At that exact temperature, the liquid is in equilibrium with both solid phases, so the Gibbs free energies balance and the transformation proceeds without a change in temperature. As cooling passes through that point, the liquid crystallizes into a mixture of the two solid phases (often in a lamellar or fine interwoven structure) rather than forming a single homogeneous phase. Other compositions would solidify over a range of temperatures, and the eutectic point represents the lowest melting temperature for the mixture, not a rise in melting point.