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Difficulty: MediumPure and Impure Substances and Criteria of Purity

A student tested a sample of liquid ethanol in the laboratory. The literature boiling point of pure ethanol is 78.4∘C78.4^\circ\text{C} at 1 atm1\text{ atm}. When heated, the liquid sample began boiling at 76.2∘C76.2^\circ\text{C} and the temperature continuously rose until distillation finished at 81.5∘C81.5^\circ\text{C}. Which of the following conclusions best accounts for this experimental observation?

  1. The sample is impure because a pure liquid boils at a sharp, constant temperature, whereas impurities broaden the boiling range.Answer
  2. B
    The sample is pure because the average of the boiling range is approximately equal to the literature boiling point of pure ethanol.
  3. C
    The sample is pure because impurities only cause boiling to occur at a single lower fixed temperature.
  4. D
    The sample is impure because pure ethanol must boil at a temperature significantly higher than 85.0∘C85.0^\circ\text{C}.

Answer

The sample is impure because a pure liquid boils at a sharp, constant temperature, whereas impurities broaden the boiling range.
A key criterion of chemical purity for any liquid is a sharp, fixed boiling point at constant pressure. When a liquid contains impurities, distillation occurs over a temperature interval (range) rather than at a single constant temperature.

Step-by-Step Solution

1
Recall the fundamental physical criteria for chemical purity.
Pure liquids boil at a sharp, constant temperature at standard pressure, whereas pure solids melt at a sharp temperature.
Physical constants such as boiling point and melting point are characteristic of pure substances.
2
Analyze the observed temperature profile during boiling.
The sample boils over a range from 76.2∘C76.2^\circ\text{C} to 81.5∘C81.5^\circ\text{C} rather than at a constant 78.4∘C78.4^\circ\text{C}.
Impurities or mixed components alter phase change behavior, causing boiling to take place across a temperature interval.
3
Conclude the purity status of the ethanol sample.
The broad temperature range confirms that the ethanol sample is impure.
A constant boiling temperature is a required criterion for confirming liquid purity.

Key Concept

Criteria of Purity: Sharp Boiling and Melting Points
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Topics

Pure and Impure Substances and Criteria of Purity