Elements, Compounds and Mixtures
5 marks · EasyA student is given a sample that may be pure aspirin, or may be aspirin mixed with impurities. The sample is white powder.
Explain how you could use melting point to decide if the sample is pure or a mixture.
The student finds the melting point is between 127°C to 134°C. What does this suggest about the sample? Give a reason.
Suggest a separation technique the student could use to obtain pure aspirin from this sample, and briefly describe how it works.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) A pure substance melts at a fixed temperature. If the sample is pure aspirin, it will melt sharply at its melting point (138°C). If it is a mixture or contains impurities, it will melt over a range of temperatures below 138°C. · must show pure substance: sharp melting point, mixture: melts over a range
(b) The sample is impure or a mixture because it melts over a range below the melting point of pure aspirin. · must show impure/as mixture: lower and spread melting point, reasons with comparison
(c) Crystallisation. Dissolve the sample in a suitable solvent and then slowly evaporate the solvent. Pure aspirin will form crystals, leaving impurities behind. · must show crystallisation method described, solvent, evaporation, crystals form
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · States pure substances melt at a fixed temperature.
M2 (a) · 1 mark · Explains mixtures/impure substances melt over a range below the pure melting point.
B1 (b) · 1 mark · Correctly concludes and explains that melting over a range below pure melting point shows impurity.
M3 (c) · 1 mark · Names crystallisation as a suitable technique.
M4 (c) · 1 mark · Describes dissolving, slow evaporation and separation of crystals.
Worked solution
a) A pure substance melts at one temperature (e.g. pure aspirin melts at 138°C). If the sample is pure, it should melt exactly at this temperature. If it is a mixture, it will probably melt over a range below 138°C.
b) The fact that the sample melts from 127°C to 134°C suggests it is not pure and contains impurities, since a pure sample would melt exactly at 138°C.
c) The student should use crystallisation: dissolve the sample in a solvent, evaporate the solvent until crystals begin to form, then allow complete crystallisation and collect the pure crystals.