Diffusion in gases
2 marks · HardA laboratory tube is filled at one end with nitrogen gas and at the other end with a coloured gas, chlorine. After some time, the green colour of chlorine spreads along the tube but it takes a few hours until the colour is even throughout.
Explain, using the particle model, why the spreading takes much longer than if chlorine particles could travel in a straight line from one end to the other.
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Also accept: Chlorine particles are always colliding with nitrogen and air particles, which changes their direction, so they do not move in a straight line and take much longer to spread through the tube.; Instead of travelling directly, the chlorine particles randomly change direction due to frequent collisions with other particles; this makes diffusion much slower than straight-line movement.
Common mistakes: Saying particles move in a straight line uninterrupted.; Missing explanation about collisions or random motion.
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Marking points
M1 · 1 mark · Describes that particles undergo many collisions or move randomly rather than straight
A1 · 1 mark · Links random movement/collisions to slow rate of spreading (diffusion)
Worked solution
Chlorine particles do not move in a straight line from one end of the tube to the other. Instead, they collide constantly with nitrogen and other particles. Each time they collide, they change direction and move randomly. Because they are always bouncing off other particles, it takes them much longer to spread throughout the tube than if they could move straight from end to end.