The big idea
Everything around you is made of tiny particles (atoms, molecules or ions) that are always moving. Whether something is a solid, a liquid or a gas depends on two things: how close together its particles are, and how freely they can move. This simple picture is called the particle model, and it explains a lot of everyday behaviour.
Explained simply
Imagine a school hall. During an exam, everyone sits in neat rows and can only fidget in their seat. That is a solid. At lunchtime, people are still close together but wander around and push past each other. That is a liquid. When the bell goes and everyone runs outside onto the field, spreading out in all directions, that is a gas.
Here is the precise version:
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Shape | Fixed | Takes the shape of the container | Fills the whole container |
| Volume | Fixed | Fixed | Not fixed, it spreads out |
| Can it be squashed? | No | Hardly at all | Yes, easily |
| Arrangement of particles | Regular pattern, touching | Random, mostly touching | Random, far apart |
| Movement of particles | Vibrate about fixed positions | Slide past each other | Move fast in all directions |
Being able to be squashed is called being compressible. Gases are compressible because there is lots of empty space between the particles. In solids and liquids the particles are already touching, so there is nowhere for them to go.
Liquids and gases can both flow, because their particles are not fixed in place. Solids cannot flow.
The melting point is the temperature at which a solid turns into a liquid. The boiling point is the temperature at which a liquid turns into a gas. Every pure substance has its own values.
Worked example
Substance X melts at −7 °C and boils at 59 °C. What state is it in at room temperature (25 °C)?
- Compare room temperature with the melting point. 25 °C is higher than −7 °C, so X has already melted. It is not a solid.
- Compare room temperature with the boiling point. 25 °C is lower than 59 °C, so X has not boiled yet. It is not a gas.
- It is between the two, so X is a liquid at 25 °C.
(Substance X is bromine, one of only two elements that are liquid at room temperature.)
A quick rule:
- Below the melting point: solid.
- Between the melting and boiling points: liquid.
- Above the boiling point: gas.
Common mistakes
- Saying particles in a solid do not move. They do move. They vibrate on the spot.
- Drawing liquid particles far apart. In a liquid the particles are still mostly touching. They are only far apart in a gas.
- Thinking the particles themselves change. A water molecule in ice is the same as a water molecule in steam. Only the spacing and movement change.
- Muddling the order of the temperatures. With negative numbers, −7 °C is colder than 0 °C, so check carefully.
Where you see this in real life
Car tyres and bike pumps work because air can be squashed. Car brakes use a liquid called brake fluid, because liquids barely compress, so pushing the pedal passes the force straight to the brakes. Bridges and buildings rely on solid steel and concrete holding their shape. Even a fizzy drink shows all three states at once: ice cubes, liquid drink and bubbles of carbon dioxide gas.