States of Matter & Particle Theory
All substances exist in one of three states of matter: solid, liquid, or gas. The state of a substance depends on how strongly its particles are held together and how much energy they have.
| State | Shape | Volume | Can be compressed? | Particle arrangement |
|---|---|---|---|---|
| Solid | Fixed | Fixed | No | Regular, close-packed — particles vibrate about fixed positions |
| Liquid | Not fixed — takes the shape of container | Fixed | No (virtually) | Close but irregular — particles can move past each other |
| Gas | Not fixed — fills the container | Not fixed | Yes — easily | Widely spaced, random movement — particles move rapidly in all directions |
Changes of State
When a substance is heated or cooled, it can change state. Changes of state are physical changes — they are reversible and no new substance is formed.
| Change of state | Direction | Energy change |
|---|---|---|
| Melting | Solid → Liquid | Energy absorbed |
| Freezing | Liquid → Solid | Energy released |
| Evaporation / Boiling | Liquid → Gas | Energy absorbed |
| Condensation | Gas → Liquid | Energy released |
| Sublimation | Solid → Gas (directly) | Energy absorbed — e.g. dry ice (solid CO2) |
Melting: particles gain enough energy for their vibrations to overcome the forces holding them in fixed positions. They break free and can move past each other.
Boiling: particles gain enough energy to completely overcome the forces holding them close together and escape into the gas phase.
During melting and boiling, the temperature stays constant — all the energy supplied is used to break the forces between particles, not to raise the temperature.
Heating Curve
Heating curve for a pure substance. The temperature stays constant during melting and boiling because the energy supplied is used to break forces between particles rather than raise the temperature.
Students often think energy is not being supplied during the flat sections of a heating curve. Energy is still being supplied — it is just being used to break the forces between particles rather than to raise the temperature. The flat sections occur at the melting point and the boiling point.
State Symbols
In chemical equations, the physical state of each substance is shown using a state symbol in brackets after the formula:
| State symbol | Meaning | Example |
|---|---|---|
| (s) | Solid | NaCl(s) |
| (l) | Liquid | H2O(l) |
| (g) | Gas | CO2(g) |
| (aq) | Aqueous solution (dissolved in water) | NaCl(aq) |
When state symbols are asked for, include them carefully. They are especially important in ionic equations, precipitation reactions and electrolysis. They also often explain observations of a reaction e.g. bubbles may be caused by a gas, or a precipitate may appear if a product is solid.
The Particle Model and its Limitations
The simple particle model represents particles as small solid spheres. It is a useful model, but it is important to recognise that atoms themselves do not have the bulk properties of the materials they make up. For example, a single water molecule is not wet, does not flow, and does not have a surface tension — these are properties that only emerge when vast numbers of molecules interact together.
Limitations of the Simple Particle ModelH
The simple particle model treats particles as identical, solid spheres with no forces between them. In reality:
- Particles are not all the same size — atoms, molecules and ions vary in size.
- Particles are not solid spheres — atoms are mostly empty space.
- Forces do exist between particles (even in gases, intermolecular forces affect behaviour at high pressure and low temperature).
- The model does not explain energy changes during changes of state at the atomic level.
A student heats a pure, solid substance and measures its temperature every minute. The temperature rises steadily, then stays constant for several minutes, then starts rising again.
(a) What change of state is taking place during the flat section?
(b) Explain, in terms of particles, why the temperature does not change during this time.
Show answer
- (a) Melting — the substance is changing from solid to liquid at its melting point.
- (b) During melting, the energy supplied is used to break the forces holding particles in their fixed positions, rather than increasing the temperature. In this topic we will be learning the different possibilities for what these forces could be.
🧪 Exam-style questions
The table shows the melting and boiling points of three substances.
| Substance | Melting point (°C) | Boiling point (°C) |
|---|---|---|
| A | −219 | −183 |
| B | 801 | 1465 |
| C | −39 | 357 |
State the physical state of each substance at 25 °C, and explain how you used the data to decide for substance A.
Show answer
- A is a gas 1 mark
- B is a solid 1 mark
- C is a liquid 1 mark
- For A: 25 °C is above its boiling point (−183 °C), so it has already boiled and is a gas 1 mark
Rule: below the melting point → solid; between melting and boiling point → liquid; above the boiling point → gas.
When a pure solid is heated, its temperature stays constant while it is melting. Explain why.
Show answer
- The energy supplied is used to overcome / break the forces of attraction between the particles 1 mark
- rather than increasing the kinetic energy of the particles (so the temperature does not rise) 1 mark
Which state symbol shows that a substance is dissolved in water? Tick (✓) one box.