Limitations of the Particle Model (OCR GCSE Chemistry A (Gateway))

Limitations of the Particle Model

Higher Tier Only

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