Why can't

Solids and liquids be compressed (squashed).

• Because there is lots of space between the particles, so we say they are very dense/have a high density.

• Solids and liquids have the ability to expand and contract easily.
• Solids and liquids have no fixed shape or volume.
• Because there are no spaces between the particles, so we say they are very dense/have a high density.

Liquids and gases can ____ because the particles are free to move/have no fixed point and so can move over one another.

Gases ___be compressed because the particles have large spaces between them and move around. They will take the shape of any container they are placed in.

Mercury is a metal but is a liquid at room temperature. Describe the arrangement of the particles of mercury as a liquid, compared to at –38°C when it becomes a solid.

• Mercury cannot change its state of matter.

• At –38°c when mercury becomes a solid, the forces between the particles are weak.

• As a liquid, the particles can flow. As a solid, the forces between the particles are stronger.

• As a liquid the particles are very close together.

Why do scientists make models?

• Scientists use models to show a fair test.

• Scientists use models to help describe and explain phenomena that are difficult to observe.

• Scientists use models to help describe how states of matter change.

• Scientists use models to ensure they are runway ready.

We used an analogy (a comparison) for density comparing the idea to packing lots of clothes into a suitcase. Can you think of two other analogies for density?

• People going one a bus.

• The school gate at 3pm compared to 2pm

• People at a football match.

• Students going to the canteen at lunchtime compared to other times of the day.