In short: Matter exists as solids, liquids and gases, which the particle model explains by how close the particles are and how they move. Solids have a fixed shape and volume, liquids have a fixed volume but flow, and gases spread to fill any container. Changing state is a physical change.
The particle model
The particle model (also called kinetic particle theory) says:
- All matter is made of tiny particles (atoms or molecules).
- The particles are always moving.
- There are forces of attraction between particles.
- Heating gives particles more energy, so they move faster.
The three common states of matter differ in how close the particles are and how they move.
Comparing the three states
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Particle arrangement | Tightly packed in a regular pattern | Close together, jumbled | Far apart, random |
| Particle movement | Vibrate on the spot | Slide past each other | Move fast in all directions |
| Forces between particles | Strong | Medium | Very weak |
| Shape | Fixed | Takes the shape of its container | Takes the shape of its container |
| Volume | Fixed | Fixed | Fills the whole container |
| Can it flow? | No | Yes | Yes |
| Can it be squashed (compressed)? | No | Hardly | Yes, easily |
A gas can be compressed because there is lots of empty space between its particles. That is how a scuba tank holds enough air for an hour underwater.
Changes of state
When you heat or cool a substance, it can change state. The particles do not change. Only their energy and arrangement change, so changes of state are physical changes.
| Change | From → to | Energy | Example |
|---|---|---|---|
| Melting | Solid → liquid | Taken in | Ice melting in a drink |
| Freezing | Liquid → solid | Given out | Water in an ice tray |
| Evaporation / boiling | Liquid → gas | Taken in | Puddle drying, kettle boiling |
| Condensation | Gas → liquid | Given out | Fog on a bathroom mirror |
| Sublimation | Solid → gas | Taken in | Dry ice (solid carbon dioxide) |
| Deposition | Gas → solid | Given out | Frost forming on a cold night |
Evaporation happens at the surface at any temperature. Boiling happens throughout the liquid, only at the boiling point.
A fourth state: plasma
When a gas is heated to thousands of degrees, electrons are stripped off its atoms, making a glowing mix of ions and electrons called plasma. The Sun, lightning and neon signs are plasma. It is the most common state of matter in the universe.
- Particles always moving
- Forces between them
- Heat gives more energy
- Fixed shape and volume
- Vibrate in place
- Fixed volume
- Flows, takes container shape
- No fixed shape or volume
- Easily compressed
- Melting, freezing
- Evaporation, condensation
- Sublimation, deposition
- Ions and electrons
- Sun, lightning
Quick summary
- The particle model: all matter is made of moving particles that attract each other.
- Solids have a fixed shape and volume. Liquids have a fixed volume but flow. Gases have no fixed shape or volume and can be compressed.
- Changes of state are physical changes: melting, freezing, evaporation, condensation, sublimation and deposition.
- Evaporation happens at the surface at any temperature. Boiling happens throughout the liquid at the boiling point.
- Pure substances have exact melting and boiling points.
- Plasma is a fourth state made of ions and electrons.
Quiz
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In which state are particles packed tightly in a regular pattern?
- A Solid
- B Liquid
- C Gas
- D Plasma
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A: Solid
Solid particles are in fixed positions, vibrating on the spot.
Which state has a definite volume but no definite shape?
- A Solid
- B Liquid
- C Gas
- D Plasma
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B: Liquid
A liquid keeps its volume but takes the shape of its container.
Which state takes both the shape and the volume of its container?
- A Solid
- B Liquid
- C Gas
- D None of them
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C: Gas
Gas particles spread out to fill any space.