Particulate Nature of Matter
Core ideas
All matter is made of particles (atoms, ions or molecules) in constant motion. The kinetic particle model explains the three states: in solids, particles are closely packed in fixed positions and only vibrate; in liquids, they are close but can slide past one another; in gases, they are far apart and move rapidly and randomly. Diffusion is the spreading of particles from high to low concentration, and its rate increases with temperature and with lower particle mass. Brownian motion is the random jiggling of visible particles caused by unbalanced collisions with smaller, invisible particles, and it is evidence for the motion of particles.
Traps to watch
- Particles are not 'in' the gaps; gaps between particles are empty space, not air.
- When a gas is compressed, the particles themselves do not shrink - the spaces between them decrease.
- Dissolving is not melting: sugar dissolving in water involves particles separating and mixing, not changing state.
- Heating a solid does not always change its temperature; during melting the energy goes into breaking the forces between particles, so temperature stays constant.
Rule of thumb
For diffusion questions, remember: lighter and hotter = faster. A gas with a lower relative molecular mass diffuses more quickly at the same temperature, and raising the temperature speeds up all particles.
Worked example
A jar of bromine vapour is opened in a room. Explain why the brown colour spreads. Answer: bromine molecules move randomly and collide with air molecules, so they spread from a region of high concentration to low concentration (diffusion); the spreading is faster at higher temperature because the molecules have more kinetic energy.