Study notes
Particulate Nature of Matter
Why solids, liquids and gases behave differently, explained by how their particles are arranged and how much they move - and what actually happens to those particles during melting, boiling and the other changes of state.
Learn it step by step
The three states, explained by particle arrangement
A solid has particles packed tightly in a fixed pattern, so it keeps its own shape and volume. A liquid has particles close together but free to slide past each other, so it flows and takes the shape of its container, but keeps a fixed volume. A gas has particles far apart and moving freely in all directions, so it spreads to fill whatever container it is in, with no fixed shape or volume.
Particles are always moving, and hotter means faster
Every particle in a solid, liquid or gas is constantly moving, even in a solid, where particles vibrate in place rather than travelling. Heating a substance gives its particles more energy, so they move faster and (in solids and liquids) push slightly farther apart. This is the single idea behind almost every change of state.
Melting and freezing: solid ↔ liquid
Melting is a solid turning into a liquid: heating gives particles enough energy to break out of their fixed positions and start sliding past each other. Freezing is the reverse: cooling a liquid removes energy until particles slow down enough to lock into a fixed pattern again. Both happen at the same fixed temperature for a pure substance, called its melting point.
Boiling and condensation: liquid ↔ gas
Boiling is a liquid turning into a gas throughout the whole liquid, not just at the surface, once particles gain enough energy to escape each other's pull completely and fly apart. Condensation is the reverse: a gas loses energy (usually by cooling) until its particles slow down enough to be pulled back together into a liquid. Water droplets forming on a cold drink can is condensation, not the can "sweating" water from inside.
Evaporation is not the same as boiling
Evaporation is a liquid slowly turning to gas, but only from its surface, and it can happen at any temperature, not just the boiling point (a puddle dries up on a cool day without ever boiling). Boiling happens throughout the whole liquid, only at a fixed temperature, and is much faster.
Sublimation skips the liquid state entirely
A few substances, like dry ice (solid carbon dioxide) or naphthalene mothballs, turn directly from a solid into a gas without ever becoming a liquid. This is called sublimation. The particles gain enough energy to break completely free of the solid's fixed pattern and escape as a gas in one step.
Diffusion happens because particles are always moving and spreading out
Diffusion is particles spreading out on their own from where they are more concentrated to where they are less concentrated, until evenly mixed - no stirring needed. It happens fastest in gases (particles move fastest and are far apart, so little blocks them), slower in liquids, and is effectively too slow to notice in solids. The smell of perfume spreading across a room is diffusion of gas particles.
Worked examples
- Melting turns a solid into a liquid: the particles gain enough energy to start sliding past each other while staying close together.
- Sublimation turns a solid directly into a gas, with no liquid stage at all: the particles gain enough energy to break completely free of each other.
- Ordinary ice melts into liquid water first (it does not sublime under normal room conditions), so "ice melting" and "sublimation" describe two different particle-level changes, not the same process.
- The key distinguishing question is always: does a liquid stage appear in between, or not?
- A common wrong guess is that the water leaks out from inside the can, but the can is sealed.
- Water vapour (a gas) is present in the warm air surrounding the can, with its particles spread far apart and moving freely.
- When those gas particles touch the cold surface of the can, they lose energy and slow down.
- Once slow enough, the particles are pulled back together into liquid water, this is condensation, and it is why the droplets form on the OUTSIDE of the can, from air, not from the drink inside.
- It might seem like drying out should require boiling, since that is the most familiar way to turn liquid water into gas.
- But evaporation is a separate process: it happens only at the liquid's surface, and can occur at any temperature, not just the boiling point.
- Some water particles at the towel's surface always have enough energy, purely by chance, to escape into the air as gas, even at room temperature.
- Over several hours, enough surface particles escape this way that the whole towel dries, without the water ever reaching its boiling point.
Mind map
Mind map for States of Matter.
- Three states
- solid: fixed shape + volume
- liquid: fixed volume, flows
- gas: no fixed shape or volume
- Particle theory
- particles always moving
- heat = more energy = faster movement
- Changes of state
- melting / freezing (solid <-> liquid)
- boiling / condensation (liquid <-> gas)
- evaporation: surface only, any temperature
- sublimation: solid -> gas directly
- Diffusion
- particles spread out on their own
- fastest in gases, slowest in solids