Particle Theory & its Limitations (AQA GCSE Chemistry: Combined Science)

Higher tier only

What is 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 a solid is heated:
  • The particles absorb thermal energy which is converted into kinetic energy
  • The particles 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 forces to break and the molecules to escape as the liquid boils

Limitations of 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

• The physical state of a substance under certain conditions can be predicted from a given set of data
• Normally you are given melting and boiling point data for a substance and asked to predict its physical state in specified conditions.
• At temperatures below the melting point:
  • The substance will be in the solid state

• At temperatures between the melting point and the boiling point:
  • The substance will be in the liquid state

• At temperatures above the boiling point:
  • The substance will be in the gas state