1. 1
   A shipping company transports liquid nitrogen at very low temperature to keep food frozen during long sea voyages. As the tank warms slightly during the voyage, some of the liquid nitrogen changes state and escapes as a gas through a safety valve.

   (a)

   Using particle theory, which statement best describes the arrangement of particles in the liquid nitrogen inside the storage tank?

   [1 mark]

   • AParticles are held in a fixed, regular arrangement and only vibrate
   • BParticles are far apart and move freely in all directions at high speed
   • CParticles are close together but can move around and slide past each other
   • DParticles have no forces of attraction between them at all

   (b)

   As the liquid nitrogen in the tank warms up and turns into gas, what happens to the energy of the particles and the forces between them?

   [1 mark]

   • AThe particles stop moving completely as they change state
   • BThe particles lose energy and the forces between them get stronger
   • CThe particles gain energy but the forces between them stay exactly the same strength
   • DThe particles gain energy and the forces between them are overcome

   (c)

   Explain, in terms of energy transfer and particle arrangement, why the temperature of the liquid nitrogen does not rise while it is boiling, even though heat is still being supplied to the tank.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A blacksmith heats a solid iron bar in a furnace until it becomes soft enough to bend into shape, without fully melting it. He then hammers the bar into a horseshoe and leaves it to cool in the open air.

   (a)

   Which statement correctly describes what is happening to the iron particles as the bar is heated?

   [1 mark]

   • AThe particles immediately spread far apart as in a gas
   • BThe particles lose energy and vibrate less about their fixed positions
   • CThe particles gain energy and vibrate more about their fixed positions
   • DThe particles stop vibrating entirely once heated

   (b)

   After shaping the horseshoe, the blacksmith leaves it to cool in the open air until it returns to room temperature. Which change of state, if any, occurs during this cooling?

   [1 mark]

   • AFreezing, as the iron changes from liquid to solid
   • BCondensing, as the iron changes from gas to liquid
   • CSublimation, as the iron changes directly from solid to gas
   • DNo change of state occurs; the iron remains solid throughout cooling

   (c)

   Using particle theory, explain why the iron bar must be heated to a very high temperature before it becomes soft enough to bend, compared with a substance such as wax, which softens at a much lower temperature.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A meteorologist explains that clouds form when water vapour in the air rises, cools, and turns into tiny liquid droplets that eventually fall as rain. Later, in freezing winter conditions, some of these droplets freeze into ice crystals before reaching the ground.

   (a)

   Describe, in terms of particle theory, what happens to the water particles during this process and name the change of state taking place.

   [3 marks]

   (b)

   Explain why it is possible to predict the state of water at a given temperature and pressure using melting point and boiling point data, and describe how the state of the ice crystals would change if the surrounding air temperature rose above 0 degrees Celsius.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A company that manufactures dry ice (solid carbon dioxide) sells it to theatres for fog machines and to food suppliers for keeping perishable goods cold during transport without adding extra moisture.

   (a)

   A company that manufactures dry ice (solid carbon dioxide) for use in fog machines and food packaging wants to explain to customers, using particle theory and energy transfer, why dry ice appears to turn directly into a gas ("smoke") without ever forming a visible liquid, and why this makes it useful for creating stage effects. Explain fully what is happening to the carbon dioxide particles as this occurs and why this behaviour differs from that of ice made from water.

   [6 marks]

   (b)

   The food supplier using dry ice for transport asks whether packing food with dry ice instead of ordinary water ice offers any practical advantages, given that dry ice sublimes directly to gas. Using ideas about changes of state and energy transfer, evaluate the advantages and disadvantages of using dry ice rather than water ice to keep perishable food cold during a long delivery journey.

   [6 marks]

   Total for question 4: 12 marks

End of questions