Particle diagrams: phase, attraction and energy

Particle diagrams: phase, attraction and energy

Particle diagrams are models for explaining state and physical change. In a solid, particles are close together and vibrate around fixed positions; in a liquid, they remain close but can move past one another; in a gas, they are far apart and move freely. The particles themselves do not expand when a substance boils: the distance between particles changes as attractions are overcome. Temperature is related to average kinetic energy, while a change of state also involves changing the potential energy associated with intermolecular separation. Diagrams are not drawn to scale and should be interpreted with their labels and context.

Work it through

When a liquid is heated to its boiling point, added energy can be used to separate molecules rather than immediately increasing temperature. A suitable particle diagram would show particles becoming much farther apart in the gas, with the same number of molecules before and after. If molecules disappear from the diagram, the model has broken conservation of matter.

Mastery target

Use a particle diagram to distinguish state, kinetic-energy change and intermolecular separation while conserving the number and identity of particles.