Which description correctly distinguishes a eutectic reaction from a peritectic reaction in phase diagrams?

• A. Eutectic: solid transforms to liquid at a single temperature; peritectic: liquid plus a solid transform to a different solid at a fixed temperature.
• B. Eutectic: liquid transforms to two solids at a single temperature; peritectic: liquid plus a solid transform to a different solid at a fixed temperature.
• C. Eutectic: two liquids form a solid at a single temperature; peritectic: a single solid transforms into a liquid and a solid.
• D. Eutectic: liquid transforms to a single solid at a fixed temperature; peritectic: liquid and solid transform into two solids at varying temperatures.

Answer: (B)

In phase diagrams, a eutectic reaction is when a liquid at the eutectic composition solidifies into two different solid phases at one fixed temperature. This means the liquid turns directly into two solids (often labeled α and β) at the eutectic temperature. A peritectic reaction, on the other hand, involves a liquid reacting with an existing solid to form a different solid at a fixed temperature: L + α → β. So the key difference is that the eutectic reaction yields two solids from a liquid at one temperature, while the peritectic reaction combines liquid and a solid to produce a different solid at a fixed temperature. This aligns with the described correct choice.