Diffusion in gases
3 marks · NormalTwo cotton wool plugs, one soaked in concentrated ammonia solution and the other in concentrated hydrochloric acid, are placed at opposite ends of a long glass tube.
After some time, a white ring of ammonium chloride forms closer to the hydrochloric acid end.
Explain why the white ring forms closer to the hydrochloric acid end rather than in the middle. Use ideas about particles and diffusion.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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Also accept: Ammonia particles move faster than hydrogen chloride particles, so travel further in the same time.
Common mistakes: Saying ammonia is simply lighter without relating to speed.; Not clearly linking speed and position of the ring.
Show mark scheme and worked solution
Marking points
M1 · 1 mark · States both gases diffuse through air.
M2 · 1 mark · States ammonia particles move faster (as they are lighter/have lower relative mass) than hydrogen chloride particles.
A1 · 1 mark · States the ring forms closer to the hydrochloric acid end because ammonia particles travel further in the same amount of time.
Worked solution
Both ammonia gas and hydrogen chloride gas diffuse through air inside the tube.
Ammonia particles are lighter and move faster than the heavier hydrogen chloride particles.
Therefore, the ammonia particles travel further along the tube in the same time, so the white ring forms closer to the hydrochloric acid end.