Matter and particles, Year 7

A gas

提示It spreads out to fill any container.

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

A liquid

提示It flows and takes the shape of the bottom of its container.

为什么The 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

提示No swapping places, yet never completely still.

为什么Strong 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

提示Nothing holds them in place next to their neighbours.

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

Touching, in regular rows

提示Think of oranges stacked neatly on a market stall.

为什么Each circle touches its neighbours and the rows line up. All the circles should be the same size.

Liquid particles should be touching

提示This state is almost as hard to squash as a solid.

为什么The 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

提示Only the arrangement and the movement differ between states.

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

They vibrate faster

提示Heating gives them more energy to move.

为什么The 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

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

为什么The 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

提示Melting and freezing are one change run in opposite directions.

为什么The 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

提示Only the grip between neighbours changes.

为什么As 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

提示Not everything in the liquid is moving equally fast.

为什么Particles 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)

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

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

Condensation

提示It is why a cold window mists up.

为什么When 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

提示The can has not leaked.

为什么The 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

提示Nothing in a gas ever stays still.

为什么Perfume 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

提示Think about how temperature affects the speed of particles.

为什么The 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

提示Compare how much empty space there is to move through.

为什么Gas 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

提示The moving particles mix themselves.

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