1. 1
   A jeweller suspects that a gold ingot sold to her as '24 carat' (pure) gold might actually be a gold alloy mixed with cheaper metals. She heats a small sample of the ingot slowly and carefully records the temperature at which it starts to melt and the temperature at which it has fully melted.

   (a)

   If the ingot is genuinely pure gold, what would the jeweller expect to observe as she heats the sample?

   [1 mark]

   • AIt melts sharply, starting and finishing melting at one single, specific temperature
   • BIt melts gradually over a wide range of temperatures
   • CIt does not melt at any temperature
   • DIt changes colour but never melts

   (b)

   The jeweller finds that her sample actually starts melting below the known melting point of pure gold and continues melting gradually over a range of several degrees. What does this suggest?

   [1 mark]

   • AThe sample is pure gold
   • BThe sample is an impure mixture (an alloy), not pure gold
   • CThe thermometer used must be faulty
   • DGold has more than one melting point

   (c)

   Explain why melting point data can be used to distinguish a pure substance from an impure mixture.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A bakery's quality control team tests a new batch of baking powder (a raising agent) to make sure it releases the correct gas when it reacts with the moisture and acidic ingredients in cake batter. They mix a sample of the baking powder with a little water in a test tube and collect the gas produced, bubbling it through a test tube of limewater.

   (a)

   The limewater turns milky (cloudy) when the gas is bubbled through it. What does this confirm about the gas?

   [1 mark]

   • AIt is hydrogen
   • BIt is chlorine
   • CIt is carbon dioxide
   • DIt is oxygen

   (b)

   The quality control team wants to check whether a second, unlabelled gas cylinder in the storeroom contains oxygen. Which test should they use?

   [1 mark]

   • ABubble the gas through limewater and check if it turns milky
   • BHold a lit splint at the mouth of a test tube of the gas and listen for a pop
   • CPlace damp litmus paper in the gas and check if it bleaches
   • DInsert a glowing splint into a sample of the gas and check if it relights

   (c)

   Explain why confirming that the gas produced is carbon dioxide, and not some other gas, is important quality control for this baking powder.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An antiques restorer is analysing green-blue corrosion products found on an old bronze ornament, and a separate white crust found on a different silver-coloured decorative item, to identify which metal ions are present before deciding how to clean and preserve each piece.

   (a)

   Describe how the restorer could use a flame test to help identify a metal ion suspected to be present in a small sample of the corrosion, including what a positive result for copper ions would look like.

   [3 marks]

   (b)

   The restorer also suspects the corrosion sample might contain iron(II) or iron(III) ions rather than copper ions. Describe a chemical test, other than a flame test, that could distinguish between iron(II) ions, iron(III) ions and copper(II) ions, including the expected result for each.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An environmental agency is checking a sample of leachate (liquid draining) from a landfill site for dissolved metal ions. A junior chemist first carries out flame tests and sodium hydroxide precipitate tests, then the sample is sent away to be re-analysed by flame emission spectroscopy, an instrumental method, to double-check the results and measure exact concentrations.

   (a)

   The junior chemist's flame test on the leachate produces a lilac flame, and adding sodium hydroxide solution produces no precipitate at all with any of the metal ions tested. Identify which metal ion is indicated by the lilac flame, explain why this particular metal ion would not be expected to produce a precipitate with sodium hydroxide, and suggest why relying only on these traditional tests might not be enough for accurate environmental monitoring.

   [6 marks]

   (b)

   Explain how flame emission spectroscopy would allow the agency to overcome the limitations you identified in part (a), and evaluate why an environmental agency monitoring many samples from multiple sites might choose to use instrumental methods as standard, despite them typically being more expensive than simple chemical tests.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   An anti-doping laboratory tests an athlete's urine sample using paper chromatography to check for the presence of a banned substance. A small spot of the sample is placed near the bottom of a sheet of chromatography paper, alongside a reference spot of the known banned substance, and the paper is stood upright in a shallow layer of solvent, with the spots kept above the solvent level.

   (a)

   After the solvent has risen most of the way up the paper, how would the scientists use the chromatogram to check whether the banned substance is present in the athlete's sample?

   [1 mark]

   • ABy comparing the number of spots and their positions (Rf values) in the sample lane with the position of the reference spot
   • BBy measuring the total mass of the paper before and after
   • CBy checking whether the paper changes colour completely
   • DBy timing exactly how long the experiment takes

   (b)

   In the sample lane, one spot travels the same distance up the paper as the reference spot of the banned substance. What is the best conclusion the scientists can draw from this alone?

   [1 mark]

   • AThis proves beyond doubt that the banned substance is present
   • BThis provides good evidence that the banned substance may be present, but it should be confirmed with a second solvent or a further test
   • CThis proves the sample is completely pure
   • DThis shows the athlete's sample contains no banned substances at all

   (c)

   Calculate the Rf value of a spot that has travelled 6.4 cm up the paper, given that the solvent travelled 8.0 cm from the same starting line.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   A worker in a metal electroplating workshop notices a hissing sound near an electrolysis cell used to plate metal parts with a thin layer of chromium, and suspects a gas leak. Electrolysis of the aqueous solution used in the cell produces hydrogen gas at one electrode. The worker collects a small sample of the suspected gas in an upturned test tube.

   (a)

   Which test should the worker use to check whether the gas collected is hydrogen?

   [1 mark]

   • ABubble it through limewater and check for a milky colour
   • BHold a lit splint at the mouth of the tube and listen for a squeaky pop
   • CPlace damp litmus paper in the gas and check if it bleaches
   • DInsert a glowing splint and check if it relights

   (b)

   The test gives a positive result for hydrogen. Which safety issue should most concern the workshop, based on a property of hydrogen gas?

   [1 mark]

   • AHydrogen is a dense gas that could pool on the floor
   • BHydrogen is a strong acid that could corrode the equipment
   • CHydrogen is radioactive and poses a long-term health risk
   • DHydrogen is flammable and could ignite explosively if it builds up near a flame or spark

   (c)

   Suggest one immediate action the workshop should take once the leak is confirmed to be hydrogen, and explain why.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   A paint manufacturer produces waste sludge containing dissolved metal ions from its pigment production process. Before the sludge can be safely disposed of, a chemist must identify which metal ions it contains: the two main candidates are aluminium ions and calcium ions, both of which form white precipitates with sodium hydroxide solution.

   (a)

   Describe how the chemist could use sodium hydroxide solution to distinguish between aluminium ions and calcium ions in the sludge, given that both initially form a white precipitate.

   [3 marks]

   (b)

   The chemist also wants to check whether the sludge contains sulfate ions, which would need to be treated differently before disposal. Describe a test for sulfate ions and state the observation that would confirm they are present.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   A sports supplement company is checking a new powder formulation. Their chemist tests its purity by melting point, then checks the metal ion content of a mineral additive that its supplier claims to be pure, using a report from flame emission spectroscopy.

   (a)

   The company's chemist takes a sample of the new powder and finds it melts over a range of temperatures rather than sharply at one exact temperature. Explain what this tells the chemist about the purity of the powder, and describe how paper chromatography could be used to check whether the powder is a single pure compound or a mixture (formulation) of different ingredients.

   [6 marks]

   (b)

   The company also wants to confirm the amount of a specific metal ion (e.g. magnesium) in a mineral additive. Their supplier offers a report from flame emission spectroscopy, an instrumental method, instead of a manual flame test. Explain what flame emission spectroscopy is, and evaluate its advantages compared with a manual flame test for this purpose.

   [6 marks]

   Total for question 8: 12 marks

End of questions