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Difficulty: MediumPure and Impure Substances and Criteria of Purity

The presence of a soluble non-volatile impurity in a pure liquid sample depresses its boiling point and causes it to boil over a narrow temperature range.

Answer: Answer

Answer

False. Soluble non-volatile impurities elevate the boiling point of liquids and broaden the temperature range over which boiling occurs.
Soluble non-volatile impurities elevate the boiling point of liquids rather than depressing it, and cause the liquid to boil over a broad range of temperatures instead of a narrow, sharp temperature point.

Step-by-Step Solution

1
Analyze how non-volatile impurities affect the vapor pressure of a liquid.
Solute molecules occupy surface area, reducing the rate of solvent evaporation and lowering the vapor pressure.
Lower vapor pressure means the liquid cannot boil at its normal boiling point.
2
Determine the effect on boiling point and boiling range.
The liquid must be heated to a higher temperature to reach atmospheric pressure, causing boiling point elevation and a non-sharp boiling range.
A sharp, fixed boiling point is a criterion of purity; impure liquids boil over a range of elevated temperatures.

Key Concept

Effect of Soluble Impurities on Boiling Point and Purity Criteria
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Topics

Pure and Impure Substances and Criteria of Purity