Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 6% of your exam marks.
stable
Low
Stable6%
Appears regularly as short-answer questions on particle diagrams and state changes.
The particle model
- All chemical substances exist as a solid, a liquid or a gas
- The state of a given substance depends on temperature (and, for gases, pressure)
- The particles inside (atoms, molecules or ions) are the same in every state
- What differs between states is the arrangement, motion and energy of the particles
- The particle model pictures every particle as a small hard sphere
- Used to compare arrangement, spacing and motion between the three states
Comparing the three states
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Arrangement | Regular arrangement | Randomly arranged | Randomly arranged |
| Spacing | Very close together | Close together (still touching) | Far apart |
| Motion | Vibrate in fixed positions | Move around each other | Move freely in all directions |
| Average energy | Lowest | Intermediate | Highest |
| Fixed shape? | Yes | No (takes the shape of its container) | No (fills its container) |
| Fixed volume? | Yes | Yes | No |
| Compressible? | No | Almost not at all | Yes |
Why state changes need energy
- Particles are held together by attractive forces (intermolecular forces, or ionic / covalent / metallic bonds in giant structures)
- Changing state means giving the particles enough kinetic energy to weaken or overcome those forces
- Stronger forces between particles → higher melting and boiling points
- e.g. iron melts at 1538 °C, sodium chloride at 801 °C
- Weaker forces between particles → lower melting and boiling points
- e.g. oxygen at −183 °C, methane at −161 °C
- A change of state is a physical change, not a chemical one
- The particles themselves are unchanged
- Only the forces holding them in place change