Match each physical analytical observation of a chemical sample with its corresponding int… — JAMB UTME

Match each physical analytical observation of a chemical sample with its corresponding interpretation regarding purity.

A solid sample exhibits a sharp melting point at exactly $156.5^\circ\text{C}$ .Confirms the sample is a pure crystalline compound.
A solid sample melts over a broad temperature range of $142.0^\circ\text{C}$ to $150.5^\circ\text{C}$ .Indicates the presence of impurities that depress and broaden the melting point.
A liquid sample boils at a steady temperature higher than the literature value of the pure solvent.Indicates the presence of a non-volatile dissolved impurity in the liquid.
Paper chromatography of a dye sample produces a single spot under varied solvent systems.Demonstrates chemical homogeneity and single-component composition.

Answer

1. Solid with a sharp melting point at

156.5^\circ\text{C}

matches 'Confirms the sample is a pure crystalline compound'.
2. Solid melting over

142.0^\circ\text{C}

150.5^\circ\text{C}

matches 'Indicates the presence of impurities that depress and broaden the melting point'.
3. Liquid boiling at a steady temperature higher than literature value matches 'Indicates the presence of a non-volatile dissolved impurity in the liquid'.
4. Single spot on paper chromatography matches 'Demonstrates chemical homogeneity and single-component composition'.

Pure compounds exhibit constant physical properties such as sharp melting points, constant boiling points, and single chromatographic spots. Impurities alter lattice stability and vapor pressure, leading to melting point depression, boiling point elevation, and broad temperature transition ranges.

Step-by-Step Solution

Analyze solid sample melting characteristics

A sharp melting point indicates high purity, while a broad melting range below the expected temperature indicates contamination.

Impurities disrupt the orderly crystal lattice, requiring less thermal energy to melt and causing melting across a range of temperatures.

Analyze liquid boiling point behavior

Boiling point elevation at constant pressure signifies a non-volatile solute dissolved in the liquid.

Solute particles reduce the escaping tendency of liquid molecules into the vapor phase, requiring a higher temperature to match atmospheric pressure.

Evaluate chromatographic criteria

A pure substance contains only one chemical species and produces a single chromatogram spot.

If multiple components were present, differing partition coefficients would separate them into multiple spots.

Key Concept

Criteria of Purity for Substances

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

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