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Difficulty: MediumPure and Impure Substances and Criteria of Purity

Match each physical observation or analytical test result with its corresponding purity condition or chemical interpretation.

  • A solid sample melts sharply at a single, fixed temperature of 122.5∘C122.5^\circ\text{C}.Characteristic physical criterion confirming a pure solid compound.
  • A liquid sample distills across a broad temperature range of 74∘C74^\circ\text{C} to 81∘C81^\circ\text{C}.Indication of an impure liquid containing a mixture of volatile substances.
  • Chromatographic analysis of a dye yields a single distinct spot on the paper chromatogram.Confirmation of a single chemical component present in the sample.
  • A liquid sample exhibits a boiling point measured above its standard literature value.Effect caused by the presence of a non-volatile dissolved impurity.

Answer

A solid melting sharply at 122.5∘C122.5^\circ\text{C} corresponds to the criterion for a pure solid compound. Distilling across 74∘C−81∘C74^\circ\text{C}-81^\circ\text{C} indicates an impure liquid mixture. Producing a single chromatogram spot confirms a single chemical component. A boiling point elevated above literature value indicates the presence of a non-volatile dissolved impurity.
Each physical criterion uniquely identifies pure vs. impure substances: sharp melting point defines pure solids; broad boiling ranges define liquid mixtures; single chromatographic spots confirm a single substance; and elevated boiling points indicate non-volatile solutes in liquids.

Step-by-Step Solution

1
Analyze the solid melting point behavior
A sharp melting point indicates high purity for a solid sample.
Pure crystalline solids have uniform intermolecular forces that break simultaneously at a specific temperature.
2
Evaluate the liquid boiling range
A broad boiling range indicates a mixture or an impure liquid.
Different components or impurities alter the vapor pressure progressively as temperature changes.
3
Interpret the chromatographic result
A single spot on chromatography confirms homogeneity/purity.
Multiple components would separate into distinct spots based on differing solubilities and affinities.
4
Determine the cause of boiling point elevation
A boiling point higher than the literature value points to a non-volatile dissolved impurity.
Solute particles reduce the solvent's vapor pressure, requiring higher kinetic energy (temperature) to match atmospheric pressure.

Key Concept

Criteria of purity (fixed sharp melting point, fixed boiling point, single chromatographic spot, density, refractive index).
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Topics

Pure and Impure Substances and Criteria of Purity