Cooling an exactly eutectic solution: As a binary solution of eutectic composition is cooled, what happens before the eutectic temperature is reached?
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AThe solvent begins to freeze out even before reaching the eutectic temperature
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BIt undergoes no phase change until the eutectic temperature is reached
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CIt will not solidify even on reaching and passing the eutectic temperature
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DNone of these
Answer
Correct Answer: It undergoes no phase change until the eutectic temperature is reached
Explanation
Introduction / Context:Eutectic systems are crucial in alloy design, soldering, and crystallisation. A eutectic composition freezes at a single, sharp temperature where two solid phases form simultaneously from the liquid.
Given Data / Assumptions:
- Binary system at exactly eutectic composition.
- Equilibrium cooling under negligible undercooling.
- No impurities shifting the phase diagram.
Concept / Approach:At eutectic composition, the liquid remains entirely liquid until the eutectic temperature Te is reached. Then the eutectic reaction L → α + β occurs isothermally at Te, producing a characteristic fine microstructure.
Step-by-Step Solution:
Identify eutectic point on the phase diagram.Follow vertical line at eutectic composition during cooling.No intersection with liquidus lines before Te, so no solid appears earlier.At Te, liquid transforms to two solids simultaneously.Verification / Alternative check:Compare to hypoeutectic or hypereutectic compositions, where primary crystals appear before Te; that contrast confirms the unique “no change until Te” behavior at the eutectic composition.
Why Other Options Are Wrong:
- Solvent freezing before Te: Occurs only off-eutectic (primary solid forms prior to Te).
- No solidification at or past Te: Contradicts the definition of the eutectic reaction.
- None: Not applicable since (b) is correct.
Common Pitfalls:Confusing eutectic with azeotropic behavior or misreading phase diagrams; pay attention to composition versus temperature paths.
Final Answer:It undergoes no phase change until the eutectic temperature is reached