States of Matter
Principles of Chemistry
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 6% of your exam marks.
Appears regularly as short-answer questions on particle diagrams and state changes.
- Diffusion is the net movement of particles from a region of high concentration to a region of low concentration, by random particle motion
- Happens in gases and liquids only. Solid particles vibrate in place and cannot migrate
- Requires no external energy input
- Faster at higher temperatures (particles carry more kinetic energy)
- Much faster in gases than in liquids (gas particles are further apart and move faster)
Recognise and explain diffusion
What comes up: you name the process by which particles spread and mix (the answer is diffusion), or explain why a coloured gas or solution spreads out evenly.
Write: diffusion is the spreading of particles from where they are more concentrated to where they are less concentrated, because the particles are in constant random motion. It is faster in gases (particles move faster and are further apart) and faster when hotter.
Watch out: don't confuse it with dilution (adding more solvent) — that's the classic wrong answer the mark scheme rejects. Diffusion needs no stirring and no energy input, and still happens even when the gas is denser than the air.
Diffusion in gases: the bromine demonstration
- A gas jar of red-brown bromine vapour is placed beneath an upturned jar of colourless air, separated by a glass cover
- Cover removed → bromine and air particles mix
- After a few minutes both jars show the same uniform pale-brown colour
- Mixing occurs even though bromine vapour is denser than air
Diffusion in liquids: the potassium manganate(VII) demonstration
- A small purple crystal of KMnO₄ is dropped into still water
- The colour spreads outwards through the water with no stirring
- After several hours the beaker is uniformly pale purple
- Much slower than gas diffusion (liquid particles are close-packed)
