1Three states
In a solid, particles are packed in a regular pattern, vibrating on the spot. In a liquid they touch but slide past each other. In a gas they're far apart and fast.
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Lesson 29.2 notes
Particles and density
How the particle model explains solids, liquids and gases, their densities, and why ice floats.
2Shape and flow
A solid keeps its shape. A liquid's particles slide past each other, so it flows and takes the shape of its container. A gas spreads out to fill any container.
3Squashing
Only a gas can be compressed (squashed): its particles are far apart, with empty space between them. In solids and liquids the particles already touch.
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4Spacing and density
A box of solid or liquid holds about 1000 times as many particles as the same box full of gas. That's why gases are about 1000 times less dense.
5Boiling takes space
When water boils, the same particles spread out to fill about 1700 times as much space. The mass stays the same, so the density falls hugely.
6Why ice floats
Water is unusual. In ice its particles are held in an open pattern, taking up more space than in liquid water. So ice is less dense, and floats.
7Atoms and molecules
The particles can be single atoms, as in iron or helium, or molecules: groups of atoms joined together, as in water, oxygen and carbon dioxide.
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Explore: Particle lab
Heat or cool a box of particles to melt and boil it, then try squashing it. See how much space the same particles take up as a solid, a liquid and a gas.
Common mistake: Particles in a solid are moving too
Particles in a solid can't move from place to place, but they never stop vibrating on the spot. The hotter the solid, the faster they vibrate.
Remember
- Solid: packed in a regular pattern, vibrating on the spot. Liquid: touching, but sliding past each other. Gas: far apart and fast.
- Particles touch in solids and liquids but are far apart in a gas, so a gas is about 1000 times less dense.
- Water is unusual: ice is less dense than liquid water, so ice floats.
Going further at GCSE