Everything around you is made of tiny particles. This mini-lesson uses the particle model to explain the three states of matter, how substances change state, why mass is conserved, what causes gas pressure, and how gases and liquids spread out by diffusion.
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The particle model is a simple, powerful idea: all matter is made of huge numbers of tiny particles that are always moving. The particles are far too small to see, even with a normal microscope.
We picture each particle as a small ball. What makes a solid, a liquid or a gas different is not the particles themselves — it is three things:
Watch out: the particles are all the same in ice, water and steam. Heating does not change the particles into something new — it only changes how they are arranged and how fast they move.
This is why a solid has a fixed shape and a fixed volume, and cannot be squashed (compressed).
This is why a liquid has a fixed volume but takes the shape of its container, and can be poured.
This is why a gas fills any container (no fixed shape or volume) and can be squashed into a smaller space.
Watch out: in a gas the particles are still the same tiny particles — there is just a lot of empty space between them. Air is not "nothing"; it is particles you cannot see.
Tap a state on the left, then tap the description on the right that matches it.
Heating gives particles more energy so they move more and break free; cooling takes energy away so they settle closer together. Each change has its own name:
Common mix-up: boiling happens all through the liquid at its boiling point; evaporation happens from the surface at any temperature. Both turn liquid into gas.
Every change of state either needs heat added or heat taken away. Tap a change, then tap the box it belongs in.
When you heat a solid, you give its particles more energy. They vibrate faster and faster until they have enough energy to break away from their fixed positions — the solid melts into a liquid.
Keep heating and the liquid particles move faster still, until the fastest ones escape completely and fly off as a gas.
Big misconception alert: the particles themselves do not melt, expand or change size. Melting is about particles gaining energy and moving apart — each particle stays exactly the same. It is the arrangement and movement that change, not the particles.
When a substance changes state, the number of particles does not change — none are created and none are destroyed. So the total mass stays exactly the same. This is conservation of mass.
Watch out: if an open beaker of water seems to "lose mass" as it boils, the mass isn't destroyed — the water particles have escaped into the air as a gas. Seal the container and the mass stays the same.
Gas particles fly about quickly in all directions. They keep colliding with the walls of their container. Each tiny collision gives a tiny push. Billions of collisions every second add up to a steady outward force — this is gas pressure.
If you heat the gas, the particles move faster and hit the walls harder and more often, so the pressure rises. If you squash the gas into a smaller space, the particles hit the walls more often too — again the pressure rises.
Diffusion is the spreading of particles from a region where they are crowded (high concentration) to where there are fewer of them (low concentration), until they are evenly mixed. It happens because gas and liquid particles are moving randomly all the time.
Diffusion is faster in gases than in liquids, because gas particles move faster and have more empty space to travel through. It does not happen in solids, where particles are locked in place.
• In a gas the particles are far apart with lots of empty space between them.
• Squashing pushes those particles closer together into the empty space.
• In a liquid the particles are already close together and touching, so there is almost no space to squash them into.
Solid: regular pattern, close, vibrating on the spot → fixed shape & volume.
Liquid: random, close, sliding past each other → fixed volume, takes shape of container.
Gas: random, far apart, fast in all directions → fills the container, can be squashed.
Heating gives particles energy to move more / break free; cooling lets them settle closer.
Gas pressure = particles colliding with the walls. Diffusion = particles spreading by random motion.
Remember the three myths to avoid: particles do not melt or expand (the particles themselves never change); mass is conserved on any change of state; and a gas is particles far apart in mostly empty space — not a solid block of "air".
Three states: solid, liquid, gas — same particles, different arrangement, closeness & movement.
Changes of state: melting, freezing, boiling, evaporating, condensing, sublimation.
Conservation of mass: mass stays the same on any change of state.
Gas pressure: caused by particles hitting the container walls.
Diffusion: particles spreading from high to low concentration by random motion.
You've covered the whole of the KS3 topic the particulate nature of matter. Press Finish to see your score.
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