1. 1
   A technician labels three unknown samples X, Y and Z. Sample X is a shiny grey solid that cannot be decomposed by heating or by electrolysis; when it is heated in air it forms a white powder whose mass is greater than the mass of X used. Sample Y is a colourless liquid that boils sharply at a single temperature; electrolysis splits it into two gases that are always produced in a volume ratio of 2 : 1. Sample Z is a yellow metallic solid made from copper and zinc; samples bought from different suppliers contain between 60 % and 70 % copper by mass, and Z melts gradually over a range of about 30 °C.

   (a)

   Which row correctly classifies the three samples?

   [1 mark]

   • AX element, Y compound, Z mixture
   • BX compound, Y compound, Z mixture
   • CX element, Y mixture, Z compound
   • DX mixture, Y element, Z compound

   (b)

   Which observation about Z is the best evidence that it is a mixture rather than a compound?

   [1 mark]

   • AIt conducts electricity
   • BIts percentage of copper varies from one supplier to another
   • CIt is a solid at room temperature
   • DIt contains two different elements

   (c)

   A student argues that X must be a compound because its mass changes when it is heated in air. Explain why this reasoning is incorrect.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A chemist keeps two sealed containers of gas. Container 1 holds helium at 27 °C. Container 2 holds argon at 327 °C. Both gases behave as collections of widely spaced particles in rapid random motion.

   (a)

   What is the temperature of the argon in kelvin?

   [1 mark]

   • A54 K
   • B327 K
   • C600 K
   • D−54 K

   (b)

   What is the ratio of the average kinetic energy of the argon particles to that of the helium particles?

   [1 mark]

   • A0.5
   • B1
   • C12.1
   • D2

   (c)

   A student predicts that heating the helium from 27 °C to 54 °C will double the average kinetic energy of its particles. Evaluate this prediction.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student heats a 20 g sample of a pure organic solid at a constant rate and records its temperature every minute. The temperature rises from −20.0 °C to 5.5 °C during the first 2 minutes and then stays at 5.5 °C until 9 minutes. It then rises steadily to 80.1 °C at 17 minutes, and stays at 80.1 °C from 17 minutes until the last of the sample has turned to vapour at 40 minutes.

   (a)

   Deduce the melting point and boiling point of the substance in kelvin, and state the physical state or states present between 2 and 9 minutes.

   [3 marks]

   (b)

   Explain, using the kinetic molecular theory, why the temperature stays constant during both changes of state, and why the second constant-temperature stage lasts more than three times longer than the first.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A food company makes instant coffee by freeze-drying. Brewed coffee, a solution of many flavour and aroma compounds in water, is frozen at −40 °C. The pressure is then lowered so that the ice turns directly into water vapour, leaving dry coffee solids behind. A rival company instead makes its coffee by boiling the brewed coffee until all the water has evaporated.

   (a)

   Describe and explain, using the kinetic molecular theory, the changes in arrangement, movement and energy of the water particles during freezing and then during sublimation. Include an equation with state symbols for the sublimation, and explain why the coffee solids can be recovered in this way.

   [6 marks]

   (b)

   Evaluate freeze-drying compared with boiling off the water as a way of producing instant coffee that keeps the flavour of the brewed coffee.

   [6 marks]

   Total for question 4: 12 marks

End of questions