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Difficulty: EasyPure and Impure Substances and Criteria of Purity

The presence of a soluble non-volatile impurity in a pure solid sample depresses its melting point and broadens its melting temperature range.

Answer: Answer

Answer

The statement is TRUE. Soluble non-volatile impurities depress (lower) the melting point of a pure solid and cause it to melt over a broader temperature range.
A pure solid exhibits a sharp and distinct melting point. The presence of a soluble non-volatile impurity destabilizes the crystal lattice, causing the substance to melt at a lower temperature and over a broader range.

Step-by-Step Solution

1
Identify the melting characteristics of a pure solid.
A pure crystalline solid melts at a sharp, characteristic temperature.
Uniform intermolecular forces throughout the pure crystal lattice break simultaneously at a specific thermal energy threshold.
2
Evaluate the structural effect of adding a soluble impurity.
Impurity molecules disrupt the regular arrangement of the crystal lattice, weakening lattice stability.
Lattice disruption lowers the overall thermal energy required for the phase change from solid to liquid.
3
Determine the impact on melting point value and temperature range.
The impure solid melts below its literature melting point and across a wider range of temperatures.
Melting point depression and a broad melting range serve as definitive physical indicators of impurity.

Key Concept

Melting Point Depression and Melting Range as Criteria of Purity
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Topics

Pure and Impure Substances and Criteria of Purity