1. 1
   A student adds a few drops of sodium hydroxide solution to a solution of an unknown metal salt, and a white precipitate forms. On adding excess sodium hydroxide solution, the precipitate completely dissolves.

   (a)

   Which metal ion is present in the salt?

   [1 mark]

   • ACalcium
   • BMagnesium
   • CAluminium
   • DCopper(II)

   (b)

   The student repeats the test on a different unknown salt solution. This time, a white precipitate forms and remains insoluble even in excess sodium hydroxide solution. Which two metal ions could this precipitate be consistent with?

   [1 mark]

   • ACopper(II) and iron(II)
   • BCalcium and magnesium
   • CIron(II) and iron(III)
   • DAluminium and copper(II)

   (c)

   Explain why the addition of excess sodium hydroxide solution is a necessary extra step, rather than simply observing the initial white precipitate, when trying to identify which of aluminium, calcium or magnesium ions is present.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A technician adds sodium hydroxide solution to a blue solution of an unknown copper compound, and a distinctly coloured precipitate forms.

   (a)

   What colour precipitate would confirm the presence of copper(II) ions?

   [1 mark]

   • AGreen
   • BBrown
   • CWhite
   • DBlue

   (b)

   The technician then tests a pale green solution suspected of containing iron(II) ions, using the same sodium hydroxide test. Which precipitate colour would confirm iron(II) is present rather than iron(III)?

   [1 mark]

   • AGreen precipitate
   • BBrown precipitate
   • CBlue precipitate
   • DWhite precipitate

   (c)

   Explain how the technician could distinguish between iron(II) and iron(III) ions using the sodium hydroxide precipitate test, stating both expected colours.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A water company suspects that a sample of borehole water contains dissolved metal ions, possibly including calcium, magnesium or iron, and wants to use the sodium hydroxide precipitate test to investigate.

   (a)

   Describe the full procedure the company's laboratory should follow to test the water sample and interpret the results.

   [3 marks]

   (b)

   The laboratory finds a white precipitate that does not fully dissolve in excess sodium hydroxide, but they need to determine whether it is calcium or magnesium specifically, since the sodium hydroxide test alone cannot distinguish between these two. Explain this limitation and suggest what the laboratory should do next.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An industrial waste management company receives a batch of six unlabelled solutions, each known to contain only one of the following metal ions: aluminium, calcium, magnesium, copper(II), iron(II) or iron(III), and needs a systematic method for identifying each solution using the sodium hydroxide precipitate test, so the solutions can be disposed of or recycled correctly.

   (a)

   Describe, in detail, a complete procedure the company should follow to test all six solutions and interpret the results reliably.

   [6 marks]

   (b)

   One of the solutions produces a precipitate that appears to be somewhere between blue and green, making the company's technician unsure whether it is copper(II) or iron(II). Evaluate why this ambiguity might occur, and discuss the further steps the company should take to reach a scientifically reliable identification of this particular solution.

   [6 marks]

   Total for question 4: 12 marks

End of questions