Question detail
Which answer avoids confusing pure substance with another qualitative analysis result?
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
MCQ
Type
practice
Style
Topic
Purity, formulations and chromatography
Question
- A. It melts at a range of temperatures
- B. It boils at a range of temperatures
- C. It melts and boils at specific temperatures
- D. It decomposes instead of melting
Answer
The correct answer is It melts and boils at specific temperatures. It matches pure substance because the evidence is single element or compound / fixed melting point and the expected result is purity evidence.
Explanation
The correct option is It melts and boils at specific temperatures. Use this as an exam check: if the observation is not purity evidence, the conclusion about pure substance is not properly supported. Other options are weaker when they confuse gas tests, flame colours, ion-test precipitates, chromatography evidence, or pure-substance/formulation wording.
Common mistake
Misinterpreting a narrow melting range
Students often think a substance with a slightly broadened melting range is still pure, assuming the range is just experimental error.
Explain that a pure substance should have a sharp, narrow melting point range (typically 1–2 °C). A broadened range indicates the presence of impurities, which lower and broaden the melting point. Therefore, a substance with a noticeably wider range should be considered impure, not pure.
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