1. 1
   A student is given a sample of rock salt, which is a mixture of sand (insoluble in water) and salt (soluble in water). The student wants to obtain a pure, dry sample of salt crystals. The student first stirs the rock salt with water, then processes the mixture through two further stages to separate and purify the salt.

   (a)

   Which separation technique should the student use first to remove the insoluble sand from the mixture?

   [1 mark]

   • APaper chromatography
   • BCrystallisation
   • CSimple distillation
   • DFiltration

   (b)

   After removing the sand, which technique should the student use to obtain dry salt crystals from the salt solution?

   [1 mark]

   • AFiltration
   • BSedimentation
   • CPaper chromatography
   • DCrystallisation

   (c)

   The student obtains dry salt crystals and measures their melting point, finding they melt sharply at 801°C. Explain how this melting point result shows that the salt sample is pure.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A forensic scientist is investigating a black ink recovered from a ransom note. She wants to find out which dyes make up the ink and confirm whether a suspect's pen matches the ink used. She first uses simple distillation to separate the solvent from the ink, then uses paper chromatography to separate and identify the different coloured dyes within the ink.

   (a)

   During the simple distillation stage, what does the scientist obtain as the distillate?

   [1 mark]

   • AInsoluble pigment residue
   • BThe solid dye particles
   • CA mixture of all the dyes still combined
   • DThe solvent used in the ink

   (b)

   In paper chromatography, which phase is the paper itself?

   [1 mark]

   • AThe eluent
   • BThe mobile phase
   • CThe solvent front
   • DThe stationary phase

   (c)

   In the chromatogram, a dye spot travels 4.2 cm from the baseline while the solvent front travels 6.0 cm. Calculate the Rf value of the dye, giving your answer to 2 decimal places.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A coastal town with no natural fresh water supply obtains its drinking water in two stages. First, seawater is treated at a desalination plant using distillation to remove dissolved salts. The resulting fresh water is then piped to a separate water treatment works, where it undergoes further treatment before being supplied to homes, following the same steps used to treat water from reservoirs and rivers: sedimentation, filtration and chlorination.

   (a)

   Explain how distillation is used to produce potable water from seawater.

   [3 marks]

   (b)

   Describe the three stages used at the water treatment works to make the fresh water from the desalination plant safe to supply to homes, explaining the purpose of each stage.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A food scientist suspects that a batch of 'pure honey' sold at a market has been adulterated with cheap sugar syrup. She has samples of the suspect honey, a sample of pure authentic honey, and a sample of the sugar syrup used by other companies to adulterate honey. She plans to use paper chromatography, alongside melting point analysis of solid sugar extracted from each sample, to investigate whether the suspect honey has been adulterated.

   (a)

   Explain how the food scientist could use paper chromatography to determine whether the suspect honey sample has been adulterated with sugar syrup, including how she would interpret her results.

   [6 marks]

   (b)

   Evaluate the use of melting point analysis, compared with paper chromatography, as a method of testing whether the honey has been adulterated.

   [6 marks]

   Total for question 4: 12 marks

End of questions