A binary solid mixture present at its exact eutectic composition melts sharply at a single… — JAMB UTME

A binary solid mixture present at its exact eutectic composition melts sharply at a single fixed temperature, demonstrating that a narrow melting point range alone does not serve as absolute proof of chemical purity.

Answer: Answer

Answer

True

The statement is true because a binary mixture at its exact eutectic composition melts completely at a single characteristic temperature (

T_{\text{eutectic}}

) without displaying the broad melting range typical of non-eutectic mixtures. Therefore, while a sharp melting point is a strong indicator of purity, it is not an absolute guarantee without further verification such as mixed melting point determination.

Step-by-Step Solution

Analyze standard physical criteria used to evaluate solid substance purity.

Pure solid compounds typically exhibit sharp melting points over a narrow range (

\le 0.5^\circ\text{C}

1.0^\circ\text{C}

), whereas ordinary impure mixtures melt over a broad temperature range below the pure substance melting point.

Establishing the general baseline behavior of pure and impure samples.

Examine the solid-liquid phase equilibria of binary mixtures at the eutectic point.

At the specific eutectic ratio, both solid components freeze and melt simultaneously at a constant minimum temperature (

T_{\text{eutectic}}

), behaving thermodynamically like a single pure component.

Identifying theoretical exceptions to the rule that mixtures always melt over broad ranges.

Evaluate the adequacy of a sharp melting range as an absolute criterion of purity.

Because eutectic mixtures also possess sharp melting points, a narrow melting range alone is an insufficient guarantee of chemical purity unless supported by mixed melting point determination or chromatography.

Confirming the statement is true.

Key Concept

Eutectic Mixtures and Criteria of Purity

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