1. 1
   A student adds a few drops of silver nitrate solution, along with dilute nitric acid, to a solution of an unknown potassium halide salt. A white precipitate forms immediately.

   (a)

   Which halide ion is present in the salt?

   [1 mark]

   • AChloride
   • BBromide
   • CIodide
   • DFluoride

   (b)

   Why is dilute nitric acid added to the solution before, or together with, the silver nitrate solution in this test?

   [1 mark]

   • ATo dissolve the halide salt fully
   • BTo remove other ions (such as carbonate ions) that could also form a precipitate with silver nitrate and give a false positive
   • CTo speed up the precipitate reaction
   • DTo change the colour of the precipitate

   (c)

   State the colours of the precipitates formed with silver nitrate solution for bromide and iodide ions, and explain how these colours allow all three common halide ions to be distinguished from one another.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A water company tests a sample of seawater for its halide content by adding silver nitrate solution and dilute nitric acid, and observes a cream-coloured precipitate forming.

   (a)

   Which halide ion does this indicate is present?

   [1 mark]

   • AChloride
   • BNone; cream indicates no halide is present
   • CIodide
   • DBromide

   (b)

   The company also tests a second seawater sample, from a different location, and observes a yellow precipitate instead. Which halide ion is indicated by this result?

   [1 mark]

   • AChloride
   • BBromide
   • CIodide
   • DSulfate

   (c)

   The colours cream and white can sometimes look similar under poor lighting. Suggest one way the company could more reliably distinguish a cream precipitate (bromide) from a white precipitate (chloride), and explain why this method helps.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A food safety laboratory needs to check whether a batch of table salt has been contaminated with a different halide salt (such as potassium iodide) rather than being pure sodium chloride.

   (a)

   Describe the procedure the laboratory should use to test a solution of the salt for its halide content.

   [3 marks]

   (b)

   The laboratory observes a precipitate that looks mostly white but with a very slight cream tinge, raising the possibility that the salt is mostly sodium chloride but contains a small amount of a bromide contaminant. Explain how this could arise, and describe how the laboratory could confirm whether a genuine, if minor, bromide contamination is present.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemical supplier receives three unlabelled drums of solid salt, each known to contain a pure sodium halide (sodium chloride, sodium bromide, or sodium iodide), and must correctly label each drum before shipping them to customers, since supplying the wrong halide could cause serious problems downstream.

   (a)

   Describe, in detail, a complete testing procedure the supplier should use to reliably identify the contents of all three drums using the silver nitrate test.

   [6 marks]

   (b)

   For one of the three drums, two different technicians independently carry out the silver nitrate test and disagree: one records the precipitate as white (suggesting chloride), while the other records it as pale cream (suggesting bromide). Evaluate the possible reasons for this disagreement, and discuss how the supplier should resolve it before committing to a final label for the drum.

   [6 marks]

   Total for question 4: 12 marks

End of questions