# States of Matter

> Solids, liquids and gases explained with the particle model, and the changes of state between them.

From HSC Chemistry on WhizzNotes, free study notes for NSW students: https://whizznotes.com/chemistry/states-of-matter/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The disappearing puddle
After a summer storm, Tom noticed a big puddle on the basketball court. By lunchtime it had shrunk. By the end of school it was gone.

"Where did it go?" he asked his science teacher.

"Into the air," she said. "The water did not vanish. The water particles at the surface got enough energy from the sun to escape and become a gas you cannot see. Tonight, when the air cools, some of them will come back as dew on the grass."

The water changed **state**, from liquid to gas and back again, but it was water the whole time. Understanding states of matter starts with picturing the tiny particles inside.

**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:

1. All matter is made of tiny **particles** (atoms or molecules).
2. The particles are **always moving**.
3. There are **forces of attraction** between particles.
4. 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.

*Diagram: Particle diagrams of a solid in a regular packed pattern, a liquid with close but jumbled particles, and a gas with particles far apart*
*Solid: packed in rows. Liquid: close but jumbled. Gas: far apart and moving fast.*

## 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** |

### Key idea: Shape and volume in one line
- **Solid**: definite shape, definite volume.
- **Liquid**: no definite shape, definite volume.
- **Gas**: no definite shape, no definite volume.

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**.

*Diagram: Triangle of changes of state: melting and freezing between solid and liquid, evaporation and condensation between liquid and gas, sublimation and deposition between solid and gas*
*Heating moves to the right and up. Cooling moves the other way.*

| 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**.

### Watch out: Bubbles in boiling water are not air
The bubbles in boiling water are **water vapour** (steam), not air or hydrogen. The water molecules are still $\mathrm{H_2O}$. No new substance forms.

### Exam tip: Melting point and boiling point
A **pure** substance melts and boils at **exact** temperatures. Pure water melts at 0 °C and boils at 100 °C (at sea level). An impure substance melts over a **range** of temperatures. Chemists use this to test purity.

## 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.

**Mind map**

```text
States of matter
  Particle model
    Particles always moving
    Forces between them
    Heat gives more energy
  Solid
    Fixed shape and volume
    Vibrate in place
  Liquid
    Fixed volume
    Flows, takes container shape
  Gas
    No fixed shape or volume
    Easily compressed
  Changes of state
    Melting, freezing
    Evaporation, condensation
    Sublimation, deposition
  Plasma
    Ions and electrons
    Sun, lightning
```

### Year 11: Heating curves
If you heat ice steadily and plot temperature against time, you get a **heating curve**.

The temperature rises, then stays **flat** at 0 °C while the ice melts, rises again, then stays flat at 100 °C while the water boils.

During the flat parts, the energy goes into **breaking the forces between particles**, not into making them move faster. That is why the temperature does not change during a change of state. The energy taken in is called **latent heat**.

The flat part for boiling is **longer** than for melting, because separating particles completely into a gas takes much more energy than loosening them into a liquid.

### Year 12: Temperature, kinetic energy and gas pressure
**Temperature** measures the **average kinetic energy** of the particles. At **absolute zero** (0 K, or $-273\,^\circ\text{C}$) particles have the lowest possible energy.

Gas **pressure** comes from particles hitting the walls of their container. Heating a sealed gas makes particles hit harder and more often, so pressure rises. Squashing a gas into a smaller volume means more hits per second, so pressure rises too.

For a fixed amount of gas, these ideas combine into the **ideal gas law**: $PV = nRT$, where $P$ is pressure, $V$ volume, $n$ moles, $R = 8.314 \text{ J mol}^{-1}\text{ K}^{-1}$ and $T$ temperature in **kelvin**.

## 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

Try these questions from the chapter quiz.

1. In which state are particles packed tightly in a regular pattern?
   - A. Solid
   - B. Liquid
   - C. Gas
   - D. Plasma
   - Answer: A. Solid. Solid particles are in fixed positions, vibrating on the spot.

2. Which state has a definite volume but no definite shape?
   - A. Solid
   - B. Liquid
   - C. Gas
   - D. Plasma
   - Answer: B. Liquid. A liquid keeps its volume but takes the shape of its container.

3. Which state takes both the shape and the volume of its container?
   - A. Solid
   - B. Liquid
   - C. Gas
   - D. None of them
   - Answer: C. Gas. Gas particles spread out to fill any space.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/chemistry/states-of-matter/#quiz