1. 1
   A jeweller tests a ring advertised as "pure gold" and finds it contains only gold atoms, all with the same proton number, and cannot be broken down into simpler substances by chemical means. A second ring, advertised as "18-carat gold", is found to contain gold atoms mixed with silver and copper atoms, which can be separated by physical means such as melting and density separation.

   (a)

   What type of substance is the ring advertised as "pure gold" made from?

   [1 mark]

   • AAn element
   • BA compound
   • CA mixture
   • DAn alloy

   (b)

   The jeweller's second ring, an "18-carat gold" ring, contains gold, silver and copper atoms that are not chemically bonded together and can be separated by physical methods such as melting and density separation. What type of substance is this second ring?

   [1 mark]

   • AA compound
   • BA mixture
   • CAn element
   • DA single pure substance with one melting point

   (c)

   Explain two differences between an element and a mixture, using the two gold rings as examples.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A chemistry teacher heats a sample of magnesium ribbon in air. The magnesium burns and combines chemically with oxygen to form a new white powder, magnesium oxide, with completely different properties from either magnesium metal or oxygen gas. The teacher then compares this to simply mixing iron filings and sulfur powder together at room temperature without heating, which produces a grey mixture from which the iron can still be extracted using a magnet.

   (a)

   What type of substance is magnesium oxide?

   [1 mark]

   • AAn element
   • BA mixture
   • CA compound
   • DAn alloy

   (b)

   The teacher also mixes iron filings and sulfur powder together at room temperature, without heating them. Which observation shows that this iron–sulfur combination is a mixture rather than a compound?

   [1 mark]

   • AIts properties are completely different from those of iron and sulfur
   • BIt has a single, sharp melting point unlike either iron or sulfur
   • CIt cannot be separated into iron and sulfur by any method
   • DIron can still be extracted from it using a magnet

   (c)

   If the teacher instead heats the iron filings and sulfur powder mixture strongly, a chemical reaction occurs and iron sulfide is formed. Explain why iron sulfide is classified as a compound, and why a magnet can no longer be used to extract iron from it.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An environmental scientist takes a sample of seawater and identifies that it contains water, dissolved sodium chloride, dissolved magnesium sulfate and suspended sand particles.

   (a)

   Explain why seawater is classified as a mixture rather than a single pure compound.

   [3 marks]

   (b)

   The scientist proposes evaporating a measured volume of seawater to dryness and weighing the solid residue, in order to estimate how much dissolved salt the seawater contains. Explain how this evaporation method allows the scientist to separate the dissolved salts from the water, and describe why this method would not work to separate the individual dissolved salts (sodium chloride and magnesium sulfate) from each other.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A quality control chemist at a mining company receives an unlabelled grey metallic sample and must determine whether it is a pure element, a compound, or a mixture (such as an alloy).

   (a)

   Describe a set of practical observations and tests, and the reasoning behind them, that the chemist could use to classify the sample, referring to melting point behaviour, chemical composition and separability by physical methods.

   [6 marks]

   (b)

   Suppose the chemist's tests show that the grey sample melts over a range of temperatures rather than at one sharp temperature, and that a magnet removes some, but not all, of the material, leaving behind a separate non-magnetic grey solid. Explain what conclusions the chemist can draw about the composition of the sample from these results, and outline what further physical separation technique could be used to obtain each component in a purer form.

   [6 marks]

   Total for question 4: 12 marks

End of questions