Particulate Nature of Matter
Particulate nature of matter
The Cameroon GCE O-Level Chemistry paper tests whether you can use the particle model to explain observable behaviour, not just state that matter is made of particles. Questions often ask for explanations of diffusion, Brownian motion, dilution, compression and pressure.
Core rules
- All matter is made of tiny particles: atoms, molecules or ions. The particles have spaces between them and are in constant random motion.
- Particles attract one another. The strength of attraction and the average kinetic energy help determine whether a substance is solid, liquid or gas.
- Heating increases average kinetic energy, so particles move faster and spread further apart. It does not make individual particles expand.
- Diffusion is the net movement of particles from a region of higher concentration to lower concentration because of random motion. Lighter particles diffuse faster at the same temperature.
- Brownian motion is the erratic movement of visible particles, such as smoke or pollen grains, caused by unequal collisions with invisible fluid molecules. It is evidence for random molecular motion.
Worked example
A horizontal tube has cotton wool soaked in concentrated ammonia at one end and concentrated hydrochloric acid at the other. A white ring of ammonium chloride forms nearer the hydrochloric acid end. Explain.
Reasoning: both ammonia and hydrogen chloride vapour diffuse. Ammonia molecules have a smaller relative molecular mass (17) than hydrogen chloride (36.5), so at the same temperature ammonia diffuses faster. The gases meet closer to the HCl end, where the slower HCl has travelled less. The white ring is solid NH4Cl formed by NH3(g) + HCl(g) -> NH4Cl(s).
Traps that cost marks
Do not say particles expand when heated; say they move faster and the spaces increase. Do not describe Brownian motion as the motion of molecules themselves. Diffusion is not air pushing a gas. In explanations, name the particles and state that motion is random.
Key points
- Matter is particulate, with spaces and random motion.
- Heating raises kinetic energy, not particle size.
- Diffusion depends on mass and temperature.
- Brownian motion is evidence for molecular motion.
- Use particle language in explanations.