Limitations of the Particle Model (OCR Gateway GCSE Chemistry)

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