1. 1
   A gas jar of air is placed upside down on top of a gas jar containing brown bromine gas, separated by a glass cover. When the cover is removed, the brown colour is seen to spread slowly upward into the jar of air, even though nothing is stirred or shaken.

   (a)

   What does this demonstrate?

   [1 mark]

   • ABromine particles are moving and spreading out among the air particles
   • BBromine is being converted into a new coloured gas
   • CAir particles are heavier than bromine particles
   • DThe air in the upper jar is being cooled by the bromine

   (b)

   What is the name of the process by which particles of one substance spread out and mix with particles of another substance, moving from an area of higher concentration to an area of lower concentration?

   [1 mark]

   • ADistillation
   • BDiffusion
   • CFiltration
   • DCrystallisation

   (c)

   Explain, in terms of particles, why the brown colour spreads upward into the jar of air without any stirring.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A single crystal of purple potassium manganate(VII) is dropped into a beaker of still water and left completely undisturbed, with no stirring, on a laboratory bench.

   (a)

   What is observed over the next hour?

   [1 mark]

   • AThe water turns colourless as the crystal dissolves
   • BThe crystal stays exactly the same size with no colour change in the water
   • CA gas is released and bubbles rise to the surface
   • DA purple colour spreads out gradually through the water from the crystal

   (b)

   Which of the following best explains why the purple colour spreads faster in warm water than in cold water?

   [1 mark]

   • AThe potassium manganate(VII) crystal becomes heavier when warmed
   • BWarm water contains fewer water particles than cold water
   • CParticles have more energy and move faster at higher temperatures
   • DWarm water is less dense than the crystal

   (c)

   Explain, in terms of particles, why the purple colour eventually spreads evenly throughout the whole beaker of water.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A long glass tube is set up horizontally. 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 moment. Both ends of the tube are then sealed.

   (a)

   Describe what is observed inside the tube after several minutes, and explain this observation in terms of particles.

   [3 marks]

   (b)

   The white ring is observed to form closer to the hydrochloric acid end of the tube than to the ammonia end. Explain why this happens, in terms of the relative masses of ammonia and hydrogen chloride particles and their rates of diffusion.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A scientist is comparing several simple diffusion experiments to gather evidence for the existence of particles: bromine gas spreading into air, ammonia and hydrochloric acid gases meeting in a glass tube, and potassium manganate(VII) dissolving and spreading in still water.

   (a)

   A scientist wants to provide evidence for the existence of particles using simple diffusion experiments. Describe how the bromine-in-air experiment and the potassium manganate(VII)-in-water experiment each provide such evidence, and explain what these two experiments have in common in terms of particle behaviour.

   [6 marks]

   (b)

   The scientist repeats the ammonia and hydrochloric acid tube experiment, but this time warms the whole tube gently before sealing it. Explain how you would expect the results to differ from the original room-temperature experiment, and justify your answer using ideas about particle energy and movement.

   [6 marks]

   Total for question 4: 12 marks

End of questions