1. 1
   A student sets up a demonstration in a long glass tube: cotton wool soaked in concentrated ammonia solution is placed at one end and cotton wool soaked in concentrated hydrochloric acid is placed at the other end, at the same time. A white ring of ammonium chloride forms inside the tube, closer to the hydrochloric acid end.

   (a)

   A student sets up a demonstration in a long glass tube: a piece of cotton wool soaked in concentrated ammonia solution is placed at one end and a piece of cotton wool soaked in concentrated hydrochloric acid is placed at the other end, at the same time. A white ring of ammonium chloride forms inside the tube closer to the hydrochloric acid end. What is the name of the process that causes the ammonia and hydrogen chloride gases to move along the tube and meet?

   [1 mark]

   • ADiffusion
   • BSublimation
   • CNeutralisation only, with no particle movement
   • DOsmosis

   (b)

   Ammonia (NH3) has a relative molecular mass of 17 and hydrogen chloride (HCl) has a relative molecular mass of 36.5. Which statement correctly explains why the white ring forms closer to the hydrochloric acid end of the tube rather than in the middle?

   [1 mark]

   • AAmmonia particles are negatively charged and are repelled by the hydrochloric acid end
   • BHydrogen chloride particles have a smaller relative molecular mass, so they move faster and travel further
   • CBoth gases move at exactly the same speed, so the ring position is random
   • DAmmonia particles have a smaller relative molecular mass, so they move faster and travel further in the same time

   (c)

   State what is meant by diffusion, and explain in terms of kinetic particle theory why the gases in the tube are able to move from each end towards the middle without any stirring or blowing.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A council worker investigates a gas leak from an underground pipe carrying methane. Several minutes after the leak starts, methane is detected in the air at street level some distance away, even though there is no wind.

   (a)

   Methane has a relative molecular mass of 16. Which process best explains how the methane reached street level?

   [1 mark]

   • AMethane dissolving in rainwater and seeping upward
   • BConvection currents carrying liquid methane upward
   • CDiffusion of methane particles through the air
   • DMethane reacting with soil particles to form a new gas

   (b)

   The council worker also detects traces of a heavier gas, butane (relative molecular mass 58), leaking from a second nearby pipe at the same time as the methane leak. Assuming both gases start diffusing at the same moment from a similar distance below street level, which gas would be expected to reach street level first?

   [1 mark]

   • AButane, because it has the larger relative molecular mass and diffuses faster
   • BMethane, because it has the smaller relative molecular mass and diffuses faster
   • CBoth would arrive at exactly the same time regardless of mass
   • DNeither gas would diffuse because they are both non-polar

   (c)

   Explain, in terms of kinetic particle theory, why methane diffuses faster than butane at the same temperature.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A food scientist opens a fresh loaf of bread in a sealed test kitchen with no ventilation. Within a few minutes, people standing at the far side of the room can smell the bread, even though the air is completely still.

   (a)

   Describe, in terms of kinetic particle theory, how the smell of the bread spreads across the room.

   [3 marks]

   (b)

   The food scientist repeats the test at a much higher room temperature and finds that the smell reaches the far side of the room noticeably faster. Explain, in terms of kinetic particle theory, why increasing the temperature increases the rate of diffusion of the bread's aroma particles.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A laboratory technician sets up two separate diffusion demonstrations at the same room temperature. In demonstration 1, a coloured gas with a small relative molecular mass is released from a point source in a long glass tube. In demonstration 2, an identical mass of a different coloured gas with a much larger relative molecular mass is released from a point source in an identical tube.

   (a)

   Explain fully, using kinetic particle theory, why the gas in demonstration 1 spreads through its tube faster than the gas in demonstration 2, and describe how the rate of diffusion would change if demonstration 2 were repeated at a much higher temperature.

   [6 marks]

   (b)

   The technician then repeats demonstration 1, but this time releases the same coloured gas into a tube that has already been filled with a dense liquid instead of air. Explain, in terms of kinetic particle theory and particle separation, why diffusion of the gas through the liquid would be much slower than diffusion of the gas through air, and outline one everyday application where the rate of diffusion through a liquid or solid is an important practical consideration.

   [6 marks]

   Total for question 4: 12 marks

End of questions