Question detail
In Pure substances, which answer best matches the evidence for pure substance?
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. A pure substance has a boiling point that varies.
- B. An impure substance has a boiling point that is constant.
- C. A pure substance has a constant boiling point.
- D. Boiling points are irrelevant to purity.
Answer
The correct answer is A pure substance has a constant boiling point.. 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 A pure substance has a constant boiling point.. This is correct because single element or compound / fixed melting point is the evidence expected for pure substance, and purity evidence is the result that supports the conclusion. Other options are weaker when they confuse gas tests, flame colours, ion-test precipitates, chromatography evidence, or pure-substance/formulation wording.
Common mistake
Identifying Purity from Melting Points
Students often think that a substance is pure if it has a melting point that matches the literature value, without considering the possibility of impurities affecting the melting point.
Students should remember that impurities can lower or raise the melting point, and they should analyze the melting point range to determine purity.
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