1. 1
   A jeweller wants to check whether a customer's ring is made of pure gold or an alloy containing gold mixed with other metals. The jeweller carefully heats a small sample removed from the ring and records the temperature at which it melts. Pure gold has a sharp, known melting point of 1064°C.

   (a)

   If the ring sample melts sharply at exactly 1064°C, what does this suggest?

   [1 mark]

   • AIt is pure gold, since pure substances melt sharply at a single, fixed temperature
   • BIt is a mixture, since mixtures always melt at exactly their listed value
   • CThis cannot be determined from melting point alone
   • DIt means the ring contains only silver

   (b)

   If the sample instead starts to soften and melts gradually over a range of temperatures below 1064°C, what does this suggest?

   [1 mark]

   • AIt is pure gold that has been damaged
   • BIt is a mixture (alloy) containing gold, since impurities generally lower the melting point and cause melting over a range rather than a single sharp temperature
   • CIt is a completely different pure metal
   • DThe thermometer used must be faulty

   (c)

   Explain why the presence of another substance mixed with the gold lowers its melting point and causes it to melt over a range of temperatures.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A chemist is given a sample of rock salt, a mixture of insoluble sand and soluble salt (sodium chloride), and needs to obtain a sample of pure, dry salt crystals from it.

   (a)

   What should the chemist do first to begin separating the mixture?

   [1 mark]

   • AHeat the solid rock salt directly until it melts
   • BFilter the solid rock salt without adding any water
   • CDissolve the rock salt in water, since the salt dissolves but the sand does not
   • DAdd the rock salt directly onto chromatography paper

   (b)

   After the mixture has been filtered to remove the sand, how can pure, dry salt crystals be obtained from the remaining salt solution?

   [1 mark]

   • AAdd more water to the filtrate
   • BFilter the solution a second time
   • CCool the solution rapidly in a freezer without heating
   • DGently heat the salt solution to evaporate off the water, leaving solid salt crystals behind

   (c)

   Describe the full procedure the chemist should follow to obtain pure, dry salt crystals from the rock salt.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A group of students carries out paper chromatography to check whether a sample of ink from an 'invisible ink' pen matches the ink from a specific commercial marker pen. Spots of both inks are placed on the same baseline of a chromatography paper, which is then placed upright in a beaker with a shallow layer of solvent, and left until the solvent front has travelled 8.0 cm up the paper. The invisible ink produces a single spot that has travelled 5.6 cm from the baseline.

   (a)

   Calculate the Rf value of the invisible ink spot, showing your working, and state what an Rf value represents.

   [3 marks]

   (b)

   The marker pen ink separates into three distinct spots with Rf values of 0.3, 0.5 and 0.7, while the invisible ink shows only the single spot described above. Explain what this suggests about the two inks, and state whether they can be considered a match.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A pharmaceutical company has recrystallised a batch of a drug to purify it. Quality control tests a sample by measuring its melting point and comparing it with the published melting point of the pure drug, which is 158°C. The test sample begins to melt at 152°C and is not fully melted until 161°C.

   (a)

   Explain what these results suggest about the purity of the drug sample, referring to both the melting point range and how it compares to the published value.

   [6 marks]

   (b)

   Describe the technique of recrystallisation, explaining how it works to purify the drug further.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   A student sets up the electrolysis of water, acidified with a small amount of dilute sulfuric acid to help it conduct electricity, using two inert electrodes connected to a battery, and collects the gas produced at each electrode in separate upturned test tubes.

   (a)

   Which test confirms that the gas collected at the negative electrode is hydrogen?

   [1 mark]

   • AA lighted splint is held near the gas, and it burns with a squeaky pop
   • BA glowing splint is held near the gas, and it relights
   • CThe gas turns limewater cloudy
   • DThe gas turns damp litmus paper red

   (b)

   Which test confirms that the gas collected at the positive electrode is oxygen?

   [1 mark]

   • AA lighted splint produces a squeaky pop
   • BA glowing splint relights
   • CThe gas smells sharp and choking
   • DThe gas forms a white precipitate with silver nitrate solution

   (c)

   Explain why roughly twice the volume of gas collects at the negative electrode compared with the positive electrode.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   A safety trainer shows new lab technicians two unlabelled gas jars, X and Y, each containing a different gas with a sharp, choking smell, and demonstrates simple tests to identify each gas safely. Gas X turns a piece of damp red litmus paper blue. Gas Y turns a piece of damp blue litmus paper red, and then bleaches it white.

   (a)

   Based on the litmus test described, what is gas X?

   [1 mark]

   • AChlorine
   • BHydrogen
   • CAmmonia
   • DCarbon dioxide

   (b)

   Based on the litmus test described, what is gas Y?

   [1 mark]

   • AAmmonia
   • BOxygen
   • CHydrogen
   • DChlorine

   (c)

   Describe an additional confirmatory test the trainer could use for gas X, and state the expected observation.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   A jewellery recycler receives a sample of melted metal salt residue and wants to identify which metal ion it contains. A small sample is first tested using a flame test, producing a lilac flame. A separate sample is dissolved in water, and sodium hydroxide solution is added dropwise, but no precipitate forms even in excess.

   (a)

   Identify the metal ion present based on the flame test result, and briefly describe how a flame test is carried out.

   [3 marks]

   (b)

   Explain what the result of the sodium hydroxide test confirms about the sample, and explain why this result is consistent with the metal ion identified from the flame test.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   A school laboratory analyses an unknown white crystalline salt through a systematic series of qualitative tests, in order to identify both the metal ion and the negative ion (anion) present in the compound.

   (a)

   Describe a full sequence of tests that could be carried out on the salt to identify its metal ion and its anion, explaining what each possible result would indicate.

   [6 marks]

   (b)

   The salt gives a lilac flame, forms no precipitate with sodium hydroxide, produces no fizzing with dilute acid, but forms a yellow precipitate when dilute nitric acid and silver nitrate solution are added. Identify the full identity of the salt, explaining your reasoning at each step.

   [6 marks]

   Total for question 8: 12 marks

End of questions