2.2.2Bonds, Structure and Properties of Matter

Particle Theory and its Limitations — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions

5 min read 7 parts Lesson 8 of 18

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Part 1 of 5The three states (HT only)

State	Arrangement	Movement	Forces between particles
Solid	Regular, close-packed	Vibrate about fixed positions	Strong
Liquid	Close together, irregular	Move around each other	Weaker than in a solid
Gas	Far apart, random	Move rapidly in all directions	Almost none

1Energy absorbed

Energy absorbed Thermal energy is transferred to the particles and becomes kinetic energy.

2Vibrations increase

Vibrations increase The particles vibrate more strongly, and the solid expands slightly.

3Melting

Melting At the melting point the particles have enough energy to overcome the forces holding them in position, so the solid becomes a liquid.

4Evaporation and boiling

Evaporation and boiling Some surface particles escape as the liquid is heated.
- At the boiling point all the particles have enough energy to break free from one another and the liquid boils.

Stronger forces

Stronger forces The stronger the forces between particles, the more energy is needed to melt or boil the substance,
- so melting and boiling points are higher.

Temperature stays constant

Temperature stays constant On a heating curve the flat sections show a change of state.
- The energy is used to break forces between particles, not to raise the temperature.

Energy and state

Energy and state Changing state is a physical change. The particles themselves are not changed.

Find the melting point and boiling point of the substance in the data given.

If the temperature is below the melting point, the substance is a solid.

If the temperature is between the melting point and boiling point, it is a liquid.

If the temperature is above the boiling point, it is a gas.

Example: a substance melting at −215.6 °C and boiling at −173 °C is a gas at −150 °C because that is above the boiling point.

Particles are all the same

Particles are all the same The model treats every particle as a small, solid, inelastic sphere, whatever the substance.

Types of particle ignored

Types of particle ignored It does not distinguish atoms, ions and molecules, or mixtures of them.

Forces ignored

Forces ignored It does not show the size of the forces between particles, which differ between substances.

Term	Meaning
Particle theory	A model that explains the properties of solids, liquids and gases using the arrangement, movement and energy of particles.
Melting point	The temperature at which a solid changes to a liquid.
Boiling point	The temperature at which a liquid changes to a gas throughout the liquid.
Evaporation	The escape of particles from the surface of a liquid at temperatures below its boiling point.
Intermolecular force	An attraction between separate molecules.
Heating curve	A graph of temperature against time as a substance is heated.

Saying particles themselves melt or expand. They stay the same size; the spacing and energy change.

Writing that the bonds inside molecules break when a simple molecular substance melts. Only the weak forces between molecules are overcome.

Forgetting that the flat part of a heating curve is a change of state, not a pause in heating.

Mixing up the orderbelow melting point is solid, between is liquid, above boiling point is gas.

Giving limitations without detail. State that the model shows particles as identical solid spheres and ignores forces between them.

Last updated: 20 October 2026