Diffusion Edexcel International GCSE Chemistry revision
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In plain words
Open a bottle of perfume and soon the whole room smells of it. Nobody stirred the air: the perfume particles wander about at random, bumping into air particles, until they're spread out evenly. That spreading is diffusion.
Three things to know
- Diffusion is particles spreading out from where there are many (high concentration) to where there are few, because they move at random.
- Lighter particles diffuse faster: ammonia (relative molecular mass 17) is faster than hydrogen chloride (36.5).
- Diffusion is faster at a higher temperature, and much faster in gases than in liquids.
Tips and tricks
- Lighter particles move faster, so they diffuse faster. Compare the relative molecular masses to decide which gas wins the race.
- Use the phrase "from a region of higher concentration to a region of lower concentration". It is usually the marking point.
It lands in your notebook with its questions as flashcards.
Diffusion: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is diffusion?
Random movement spreads the particles out evenly.
Which gas diffuses fastest at the same temperature? (Relative molecular masses: hydrogen 2, oxygen 32, carbon dioxide 44, chlorine 71.)
The lightest molecules move fastest.
What does raising the temperature do to the rate of diffusion?
Hotter particles have more energy and move faster.
What is the white solid formed when ammonia gas meets hydrogen chloride gas?
NH₃ + HCl → NH₄Cl.
Why does diffusion happen?
Random movement naturally spreads particles out.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 7 marks. Write your answers on paper, then check them.
Diffusion
Edexcel International GCSE Chemistry 4CH1 · 7 marks · papermunch.org
Name ______________________________ Date ______________
Explain why the smell of perfume spreads across a room.[2]
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Perfume particles evaporate and move randomly, mixing with the air particles and spreading from high to low concentration.
Cotton wool soaked in ammonia solution is put at one end of a glass tube and cotton wool soaked in hydrochloric acid at the other. A white ring forms nearer the acid end. Explain why.[3]
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Ammonia molecules are lighter than hydrogen chloride molecules, so they diffuse faster and travel further before the two gases meet. Where they meet they form white ammonium chloride.
Explain why diffusion is slower in a liquid than in a gas.[2]
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The particles in a liquid are close together and move more slowly, so they keep colliding and spread out less quickly.
Answers: Diffusion
- 1. Perfume particles evaporate and move randomly, mixing with the air particles and spreading from high to low concentration.
- 2. Ammonia molecules are lighter than hydrogen chloride molecules, so they diffuse faster and travel further before the two gases meet. Where they meet they form white ammonium chloride.
- 3. The particles in a liquid are close together and move more slowly, so they keep colliding and spread out less quickly.