Solid, liquid and gas

Everything around you is matter — it has mass and takes up space. Most matter you meet is in one of three states:

• A solid keeps its own shape and can't be squashed much.
• A liquid flows and takes the shape of its container, but keeps the same volume.
• A gas spreads out to fill any container completely and can be squashed easily.

Same stuff, different state: ice, water and steam are all H₂O. Changing state doesn't change what the substance is — only how its particles are arranged and moving.

Zoom in: the particle model

Imagine zooming in until you can see the tiny particles. Their arrangement, how close they are and how much they move decide the state:

Key idea: the particles themselves stay the same size and mass. Only their spacing, arrangement and movement change between states.

Read the particles

Density: how packed the particles are

Density tells you how much mass is squeezed into a certain volume. It is not the same as "how heavy" — a huge block of foam can weigh more than a tiny steel ball, yet the steel is far more dense.

Because a solid's particles are packed tightly and a gas's are spread out, most substances are densest as a solid and least dense as a gas.

Your turn — density

Dense ≠ heavy

It's tempting to say "steel is heavier than wood" — but a matchstick of steel weighs less than a tree trunk. What we really mean is that steel is more dense: for the same volume, it has more mass because its particles are heavier and more tightly packed.

Watch out: density is about mass per volume, not just weight. Always compare the same volume when you decide which is denser.

Which is more dense?

Changing state (physical changes)

Adding or removing energy can change the state. These are physical changes — no new substance is made and they can be reversed:

• Melting — solid → liquid. Freezing — liquid → solid.
• Boiling / evaporating — liquid → gas. Condensing — gas → liquid.
• Sublimation — solid → gas directly (e.g. dry ice).

Conservation of mass: when ice melts into water, or water boils into steam, no mass is lost. The particles are the same particles — just rearranged. Melt 50 g of ice and you get 50 g of water.

Physical change or not?

A physical change makes no new substance and can be reversed. Tap the correct label for each.

Mass is conserved

In a sealed container, if a solid melts, evaporates or condenses, the total mass stays the same. Nothing is created or destroyed — the particles just spread out or come together.

Why steam looks like it "vanishes": in an open pan the gas escapes into the room, so the pan gets lighter — but the mass isn't lost, it has just spread out into the air.

Your turn — conservation of mass

Heat is not the same as temperature

Temperature tells you how hot something is — really, how fast its particles are moving on average. Heat (or thermal energy) is the total energy transferred, and it always flows from a hotter place to a cooler one.

• A sparkler is at a very high temperature but carries very little heat energy — it barely warms your hand.
• A warm bath is at a lower temperature but holds a huge amount of heat energy because there is so much water.

Direction of flow: energy is transferred by heating from hot to cold until both reach the same temperature (thermal equilibrium) — never the other way round on its own.

Hot vs a lot of energy

The heating curve & latent heat

If you heat ice steadily and record its temperature, the graph has flat plateaus. During a change of state the temperature stops rising even though energy is still going in:

Latent heat: a change of state needs energy without a temperature change. Melting and boiling absorb this energy to break the forces holding particles together.

Reading the plateau

Heating: particles move more, not grow

When you heat a substance, the particles gain energy and move faster, so they push a little further apart. This makes the whole object expand slightly — but the particles themselves do not grow.

Watch out: a common wrong idea is that particles "get bigger" when heated. They don't — they just move faster and spread apart.

Diffusion & Brownian motion

We can't see particles, but their movement leaves clues:

• Diffusion — a drop of ink spreads through still water, and the smell of perfume drifts across a room. The particles move about and mix on their own, with no stirring.
• Brownian motion — tiny smoke or pollen grains seen under a microscope jiggle randomly. They are being knocked about by even smaller, fast-moving particles you can't see.

What does the jiggle show?

Mass vs weight

Mass is the amount of matter in an object, measured in kilograms (kg). It stays the same wherever you take it. Weight is the pull of gravity on that mass, measured in newtons (N), and it changes with the gravitational field strength:

On Earth g ≈ 10 N/kg. On the Moon g is only about 1.6 N/kg, so the same astronaut weighs less on the Moon — but has exactly the same mass.

Your turn — weight

On the Moon

Match the state to its particles

Tap a state on the left, then tap its matching description on the right.

The big ideas to know

You've covered the KS3 Physics — Matter ideas: the particle model, density, changes of state, heat vs temperature, and mass vs weight. Press Finish to see your score.