The Particle Model
States of Matter
All matter exists in one of three states of matter: solid, liquid and gas. The particle model explains the differences between these states by describing how particles are arranged, how they move, and how strongly they are held together.
In a solid, particles are held in fixed positions by strong forces of attraction. They vibrate about these fixed positions but cannot move from place to place. This is why solids have a definite shape and a fixed volume. In a liquid, particles are close together but can move past each other. The forces of attraction are weaker than in a solid, giving liquids a fixed volume but no definite shape — they flow to fill the bottom of a container. In a gas, particles are far apart and move quickly in random directions. The forces of attraction are very weak, so gases have no fixed shape or volume and will spread out to fill any container.
Solids — particles in a regular arrangement, vibrate in fixed positions, close together. Liquids — particles close together but irregularly arranged, can slide past each other. Gases — particles far apart, move randomly at high speed.
Changes of State
When a substance is heated or cooled, it can change from one state to another. These are called changes of state. During a change of state, the particles themselves do not change — only the arrangement and movement of the particles change. This makes changes of state physical changes, not chemical changes, because they are reversible and no new substances are formed.
Melting — solid to liquid. Particles gain energy and overcome some of the forces holding them in fixed positions, so they can slide past each other. Boiling (or evaporation) — liquid to gas. Particles gain enough energy to overcome the remaining forces of attraction and escape into the air. Condensing — gas to liquid. Particles lose energy, slow down, and forces of attraction pull them closer together. Freezing — liquid to solid. Particles lose energy and lock into fixed positions. Sublimation — solid directly to gas without becoming a liquid first. Deposition — gas directly to solid.
During melting and boiling, energy is absorbed from the surroundings to break the forces between particles. During condensing and freezing, energy is released to the surroundings as new forces form between particles. Importantly, the temperature stays constant during a change of state — all the energy goes into breaking or forming intermolecular forces, not raising the temperature.
During a change of state, the temperature remains constant because the energy supplied is used to break (or form) forces between particles rather than increasing kinetic energy.
Heating and Cooling Curves
A heating curve is a graph of temperature against time for a substance being heated steadily. It shows: a rising section as the solid heats up, a flat section at the melting point (solid turning to liquid), a rising section as the liquid heats up, and a flat section at the boiling point (liquid turning to gas). The flat sections show that the substance is changing state — energy is being used to break intermolecular forces rather than raising the temperature.
A cooling curve shows the reverse process. The flat sections now represent condensation and freezing. A pure substance has sharp, flat plateaux at precise temperatures, while an impure substance changes state over a range of temperatures.
Physical Changes vs Chemical Changes
A physical change (such as a change of state) does not produce any new substances. The particles themselves stay the same — only their arrangement, spacing and energy change. Physical changes are reversible: ice can melt to water and then freeze back to ice. A chemical change forms new substances with different properties. Atoms are rearranged into new combinations. Chemical changes are usually irreversible under normal conditions — for example, burning magnesium in air produces magnesium oxide, which cannot easily be turned back into magnesium and oxygen.
The simple particle model represents particles as inelastic solid spheres (like bowling balls). This is useful for explaining states of matter and changes of state, but it has important limitations. The model does not account for the forces of attraction between particles, which are what actually hold substances together. It does not show that different particles have different sizes and masses. It does not represent the space between particles accurately — for example, gas particles are much further apart than diagrams usually show. It also fails to explain why different materials have different melting and boiling points, because it treats all particles as identical spheres. To explain differences in properties, we need to consider that the particles themselves (atoms, ions, molecules) and the bonds or forces between them are different in different substances.
The simple particle model (inelastic spheres) does not account for: forces of attraction between particles, different particle sizes, or the actual spacing between particles. It cannot explain why different materials have different properties.