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GATE 2021 MT – Question 40

Metallurgical Thermodynamics · Phase equilibria and phase diagrams · 2 marks · Multiple choice

Which condition gives the binary A–B phase diagram shown? $\Delta H_{mix}^{solid}$ and $\Delta H_{mix}^{liquid}$ denote solid and liquid mixing enthalpies.

Diagram for GATE 2021 MT question 40
  1. $\Delta H_{mix}^{solid}=0$, $\Delta H_{mix}^{liquid}=0$
  2. $\Delta H_{mix}^{solid}\ll0$, $\Delta H_{mix}^{liquid}=0$
  3. $\Delta H_{mix}^{solid}>0$, $\Delta H_{mix}^{liquid}=0$
  4. $\Delta H_{mix}^{solid}=0$, $\Delta H_{mix}^{liquid}\ll0$

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Show answer and explanation

Correct answer: (B) $\Delta H_{mix}^{solid}\ll0$, $\Delta H_{mix}^{liquid}=0$

Explanation

A strong negative solid mixing enthalpy stabilizes the intermediate solid relative to the liquid. More heat is then required to melt it, producing the shown congruent maximum. The listed combination is **strongly negative solid mixing enthalpy and zero liquid mixing enthalpy, B**.