Simple differences between mixtures and compounds
3 marks · NormalParacetamol is a pure compound. Explain, using ideas about melting and boiling points, how you could prove that a sample of paracetamol is not a mixture. (3 marks)
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A pure compound has a fixed, sharp melting (or boiling) point, while a mixture will melt (or boil) over a range of temperatures. If the paracetamol melts sharply at a single temperature (its published melting point) and does not show a range, this shows it is pure and not a mixture.
Must show: States fixed/sharp melting or boiling point for pure compound, Explains range of temperature for mixtures, Applies to paracetamol sample
Also accept: Explains using only melting point or only boiling point
Common mistakes: Repeating that a mixture just has more than one substance without reference to temperature ranges
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Marking points
M1 · 1 mark · States pure compound melts/boils at a fixed (sharp) temperature
A1 · 1 mark · States mixture melts/boils over a range of temperatures
B1 · 1 mark · Applies this to paracetamol sample (states how observation confirms sample is pure and not mixture)
Worked solution
If you heat a pure sample of paracetamol, it will melt sharply at one temperature (its melting point). If it were a mixture, it would start to melt at one temperature but not finish until a higher temperature, so it would melt over a range of temperatures. Observing a sharp melting point shows the sample is pure and not a mixture.