1. 1
   A student adds a few drops of barium chloride solution, together with dilute hydrochloric acid, to a solution of an unknown salt. A white precipitate forms immediately.

   (a)

   Which ion is most likely present in the salt?

   [1 mark]

   • AChloride
   • BSulfate
   • CCarbonate
   • DNitrate

   (b)

   Why is dilute hydrochloric acid added along with the barium chloride solution in this test?

   [1 mark]

   • ATo dissolve the sulfate precipitate so it can be measured accurately
   • BTo speed up the reaction between barium and chloride ions
   • CTo change the colour of the precipitate to confirm the result
   • DTo remove ions such as carbonate that could also form a white precipitate with barium ions, avoiding a false positive

   (c)

   Explain why the precipitate formed in this test is insoluble, and state the name of the precipitate that is formed.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A water treatment engineer wants to check whether a groundwater sample contains high levels of sulfate ions, which could indicate contamination from a nearby industrial site. The engineer adds barium chloride solution and dilute hydrochloric acid to the sample.

   (a)

   Which type of result would indicate the presence of sulfate ions?

   [1 mark]

   • AA blue solution forms
   • BThe sample bubbles vigorously
   • CA white precipitate forms
   • DThe sample changes from clear to yellow

   (b)

   The engineer observes a dense white precipitate forming almost immediately after adding the reagents to the groundwater sample. What does the speed and density of this precipitate most likely suggest?

   [1 mark]

   • AA relatively high concentration of sulfate ions is present
   • BA very low concentration of sulfate ions is present
   • CNo sulfate ions are present
   • DThe sample contains only chloride ions

   (c)

   Explain why the engineer should be cautious about estimating the exact concentration of sulfate ions based only on how quickly or how dense the precipitate appears, rather than treating this test as a purely qualitative (yes/no) indicator.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A brewery's quality control team wants to check the mineral water used in beer production for sulfate content, since high sulfate levels can affect the taste of the final product.

   (a)

   Describe the procedure the team should follow to test the water for sulfate ions.

   [3 marks]

   (b)

   The team tests two different water samples: Sample A produces a dense white precipitate within seconds, while Sample B produces only a very faint, slow-forming white haze after several minutes. Explain what these two results suggest about the samples, and describe how the team could obtain a more precise comparison of sulfate levels between them.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An environmental agency is investigating whether an industrial plant is illegally discharging sulfate-rich waste into a nearby river, and needs to test water samples taken from multiple points along the river (upstream, at the discharge point, and downstream) to build a case.

   (a)

   Describe, in detail, a complete testing procedure the agency should follow to reliably compare sulfate levels at each point, and explain how the results should be interpreted.

   [6 marks]

   (b)

   The agency's results show a clear increase in sulfate precipitate mass between the upstream and discharge-point samples, but the downstream sample, taken further away, shows only a slightly higher precipitate mass than upstream. Evaluate what this pattern might suggest, and discuss what further investigation the agency should carry out before concluding whether the plant is responsible for ongoing sulfate contamination.

   [6 marks]

   Total for question 4: 12 marks

End of questions