Limitations of the Particle Model
Particle Theory
- Particle theory explains how matter changes state depending on the energy and forces present between the particles in the substance
- The amount of energy needed to change from a solid to a liquid and from a liquid to a gas depends on the relative strength of the forces acting between the particles
- The stronger the forces between the particles, the higher the energy needed for melting and boiling to occur
- When substances are heated, the particles absorb thermal energy which is converted into kinetic energy
- Heating a solid causes its particles to vibrate more and as the temperature increases, they vibrate so much that the solid expands until the bonds break and the solid melts
- On further heating, the now liquid substance expands more and some particles at the surface gain sufficient energy to overcome the intermolecular forces and evaporate
- When the boiling point is reached, all the particles gain enough energy for the intermolecular force to break and the molecules to escape as the liquid boils
- How particles behave when a substance changes state can be summarised on the following graph:
Particles and state changes
Limitations of the Particle Theory
- Particle theory considers all particles, irrespective of their state or chemical identity, to be small, solid and inelastic
- It doesn’t consider the difference caused by different particles, such as atoms, ions or molecules or mixtures of all three
- The theory also fails to consider the intermolecular forces that exist between different particles in different substances
Examiner Tip
Remember that the horizontal sections of a heating / cooling curve represent phase changes during which the temperature doesn’t change as the heat energy is absorbed during the process of changing state.
