Matter and particles, Year 7

A gas

HintIt spreads out to fill any container.

The whyWith almost nothing holding them together, the particles fly about freely. The large spaces between them are why this state can be squashed.

A liquid

HintIt flows and takes the shape of the bottom of its container.

The whyThe particles are still touching, so this state cannot be squashed, but they are not held in fixed places, so it can flow and be poured.

They vibrate about fixed positions

HintNo swapping places, yet never completely still.

The whyStrong forces hold each particle in its place in a regular pattern, which is why a solid keeps its shape. The particles jiggle on the spot.

Its particles move freely in all directions

HintNothing holds them in place next to their neighbours.

The whyEach particle keeps going in a straight line until it hits something, so the gas soon reaches every corner.

Touching, in regular rows

HintThink of oranges stacked neatly on a market stall.

The whyEach circle touches its neighbours and the rows line up. All the circles should be the same size.

Liquid particles should be touching

HintThis state is almost as hard to squash as a solid.

The whyThe particles of a liquid are close together but jumbled, with no regular pattern. Large gaps belong only in the diagram of a gas.

The particles themselves do not change

HintOnly the arrangement and the movement differ between states.

The whyMelting does not alter the particles. A change of state changes how they are arranged and how they move, nothing else.

They vibrate faster

HintHeating gives them more energy to move.

The whyThe particles stay in their places but shake more and more vigorously. At the melting point the shaking is strong enough to break the regular pattern.

It breaks down and the particles can move past each other

HintThe grip between neighbours is no longer strong enough to hold the pattern.

The whyThe particles are still close together, but they are no longer fixed in place. That is why a liquid can flow and a solid cannot.

0 °C

HintMelting and freezing are one change run in opposite directions.

The whyThe melting point and the freezing point of a substance are equal. Whether it melts or freezes depends on whether energy is going in or coming out.

No — the same particles are held in fixed positions

HintOnly the grip between neighbours changes.

The whyAs the liquid cools, the particles slow down until the forces between them can lock them into a regular pattern. Nothing new is made.

Some surface particles have enough energy to escape

HintNot everything in the liquid is moving equally fast.

The whyParticles in a liquid have a range of speeds. The fastest ones at the top can break away into the air at any temperature.

Water vapour (steam)

HintIt is not air: it is the liquid itself in another state.

The whyIn boiling, the liquid turns to gas throughout, not just at the top. The gas forms bubbles that rise and burst at the surface.

Condensation

HintIt is why a cold window mists up.

The whyWhen gas particles lose energy on meeting something cold, they slow down and come close enough together to form a liquid.

Water vapour in the air has condensed

HintThe can has not leaked.

The whyThe air always contains some invisible water in gas form. It cools on touching the cold metal and turns back into drops of liquid.

Perfume particles spread through the air by diffusion

HintNothing in a gas ever stays still.

The whyPerfume particles and air particles move randomly and collide, so the scent gradually mixes into the air until it has spread through the room.

In hot water

HintThink about how temperature affects the speed of particles.

The whyThe higher the temperature, the faster the particles move, so they mix more quickly. A tea bag colours hot water much faster than cold.

In a gas

HintCompare how much empty space there is to move through.

The whyGas particles move fast and have large gaps to travel through. In a liquid the particles are packed closely and keep bumping into their neighbours.

Evenly coloured all through

HintThe moving particles mix themselves.

The whyThe squash particles and the water particles are all moving, so they slowly spread among each other until the colour is uniform.