1. 1
   A water testing laboratory suspects that a sample of well water contains dissolved carbonate ions and also wants to check for the presence of chloride ions in the same sample.

   (a)

   The technician adds dilute hydrochloric acid to a sample and passes any gas produced through limewater. Which observation would confirm the presence of carbonate ions?

   [1 mark]

   • AThe limewater turns milky/cloudy
   • BA yellow precipitate forms in the limewater
   • CThe limewater turns blue
   • DA pungent gas with a sharp smell is released directly, with no change to the limewater

   (b)

   The technician then tests the same water sample for chloride ions, first acidifying it with dilute nitric acid before adding aqueous silver nitrate. Which observation confirms the presence of chloride ions?

   [1 mark]

   • AA yellow precipitate forms
   • BA cream precipitate forms
   • CA white precipitate forms
   • DNo precipitate forms and the solution turns green

   (c)

   Explain why the sample must first be acidified with dilute nitric acid before adding aqueous silver nitrate to test for chloride ions.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student is given an unknown gas released from a chemistry demonstration and carries out a series of standard gas tests to identify it, then repeats the process with a second, different unknown gas.

   (a)

   The gas is tested with a glowing splint, which relights. Which gas has the student most likely identified?

   [1 mark]

   • AHydrogen
   • BOxygen
   • CCarbon dioxide
   • DAmmonia

   (b)

   The student tests the second unknown gas with a piece of damp red litmus paper held at the mouth of the tube, and the paper turns blue. Which gas has the student identified?

   [1 mark]

   • AHydrogen
   • BChlorine
   • CSulfur dioxide
   • DAmmonia

   (c)

   Describe the test and expected positive result for identifying hydrogen gas.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A metallurgist analysing an unknown blue-green crystalline compound from an old mine suspects it may contain copper(II) ions. The metallurgist also flame-tests a portion of the sample to gather further supporting evidence.

   (a)

   Describe the test using aqueous sodium hydroxide that could be used to help confirm the presence of Cu2+ ions, including the expected observation.

   [3 marks]

   (b)

   The metallurgist also carries out a flame test on a small sample of the crystals and observes a lilac flame colour. Explain what this observation suggests about the composition of the compound, and describe why using more than one test (the sodium hydroxide test and the flame test) gives a more reliable identification of the compound overall.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A quality control chemist at a fertiliser company receives an unlabelled sample believed to be either ammonium sulfate or ammonium nitrate.

   (a)

   Describe a full sequence of tests the chemist could carry out to confirm the presence of the ammonium cation and to distinguish between the sulfate and nitrate anions, explaining the reagents used and the expected observations at each stage.

   [6 marks]

   (b)

   The chemist later receives a second unlabelled sample suspected to be an aqueous solution containing either iron(II) or iron(III) ions, and also containing a sulfite impurity that must be checked for. Describe how aqueous sodium hydroxide could be used to distinguish between iron(II) and iron(III) ions, and describe a separate test that could confirm the presence of sulfite ions, explaining why comparing the two cations' hydroxide precipitates is a reliable method of distinguishing them.

   [6 marks]

   Total for question 4: 12 marks

End of questions