1. 1
   A food scientist heats a sample of what is labelled as pure cooking oil and measures its melting point. The measured melting point is lower than, and spread over a wider range than, the known melting point of the pure oil.

   (a)

   What does this suggest about the sample?

   [1 mark]

   • AThe sample is definitely pure, since oils always melt over a range of temperatures
   • BThe sample is likely impure, since impurities generally lower and broaden the melting point range
   • CThe sample must be a different substance entirely
   • DThe measurement is meaningless because melting point cannot indicate purity

   (b)

   How would the melting point behaviour of a genuinely pure sample of the oil differ from what the food scientist observed?

   [1 mark]

   • AA pure sample would melt at a single, sharp, fixed temperature
   • BA pure sample would melt over an even wider range of temperatures
   • CA pure sample would not melt at all, regardless of temperature
   • DA pure sample would boil before it melted

   (c)

   Define a pure substance, and explain why measures of purity, such as melting point testing, are important for foodstuffs.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A pharmaceutical laboratory tests a batch of a drug that should be a pure compound. The known, published melting point of the pure drug is 152 degrees Celsius, but the batch sample melts gradually between 138 and 148 degrees Celsius.

   (a)

   What does this data suggest about the batch?

   [1 mark]

   • AThe batch is pure, since it still has a melting point close to 152 degrees Celsius
   • BThe batch must be a completely different drug
   • CThe batch is likely impure, since it melts at a lower temperature and over a range rather than sharply at 152 degrees Celsius
   • DMelting point data cannot be used to assess drugs

   (b)

   Why is it particularly important for a pharmaceutical laboratory to check the purity of its drugs using melting point data?

   [1 mark]

   • ABecause melting point testing increases the mass of the drug
   • BBecause melting point testing is required to change the colour of the drug
   • CBecause pure drugs always taste different from impure ones
   • DBecause impurities in medicines could affect their safety or effectiveness for patients

   (c)

   Explain how melting point data can be used to assess the purity of the drug sample, referring to both the sharpness and the temperature of the melting point observed.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A quality control chemist at a bottling plant is asked to check whether a batch of drinking water contains any dissolved impurities, using boiling point data.

   (a)

   Explain how boiling point measurements could be used for this purpose, and what result would indicate the water is pure.

   [3 marks]

   (b)

   The chemist finds that the water sample boils over a range from 100 to 103 degrees Celsius. Explain what this result suggests, and why measuring purity is particularly important for drinking water intended for public consumption.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A forensic laboratory receives a small white crystalline sample, suspected to be either a pure compound or a mixture containing an unknown contaminant.

   (a)

   Explain in full how the forensic scientists could use melting point (and/or boiling point) data to assess the purity of the sample, including what results would indicate purity versus impurity.

   [6 marks]

   (b)

   Discuss the limitations of using only melting point or boiling point data to assess purity, and evaluate why forensic scientists might need to use additional methods alongside this technique.

   [6 marks]

   Total for question 4: 12 marks

End of questions