The Particle Model and States of Matter
The Particle Model
All matter is made of tiny particles that are too small to see. The particle model describes the three states of matter in terms of how particles are arranged, how they move, and how strongly they are held together by attractive forces.
In a solid, particles are packed closely together in a regular arrangement. Strong forces of attraction hold them in fixed positions. Particles cannot move from place to place — they only vibrate about their fixed positions. This gives solids a definite shape and a fixed volume. In a liquid, particles are still close together but in an irregular arrangement. The forces of attraction are weaker, allowing particles to slide past each other. Liquids have a fixed volume but no fixed shape — they flow to fill the bottom of a container. In a gas, particles are far apart, in no fixed arrangement. The forces of attraction are very weak, so particles move rapidly in random directions. Gases have no fixed shape or volume and expand to fill any container.
Solid: regular arrangement, vibrate in fixed positions, close together, strong forces. Liquid: irregular, slide past each other, close together, weaker forces. Gas: random, fast movement in all directions, far apart, very weak forces.
Changes of State
When matter 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 their arrangement, spacing and movement change. Changes of state are physical changes: they are reversible and no new substances are produced.
Melting (solid → liquid): particles gain energy, vibrate more vigorously and overcome the forces holding them in fixed positions, so they can flow past each other. Boiling or evaporation (liquid → gas): particles gain enough energy to escape the remaining attractive forces and move freely. Condensing (gas → liquid): particles lose energy, slow down, and forces of attraction pull them closer. Freezing (liquid → solid): particles lose more energy and lock into fixed positions. Sublimation (solid → gas directly, without passing through the liquid state). Deposition (gas → solid directly).
During melting and boiling, energy is absorbed from the surroundings to break forces between particles. During condensing and freezing, energy is released to the surroundings as new forces form. Crucially, the temperature stays constant during a change of state — all the energy input goes into breaking or forming intermolecular forces rather than increasing particle kinetic energy.
During a change of state, temperature remains constant because energy is used to break (or form) forces between particles rather than raising kinetic energy. Changes of state are physical — reversible, no new substances formed.
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 (solid warming up), a flat section at the melting point (solid converting to liquid at constant temperature), another rising section (liquid warming up), and a flat section at the boiling point (liquid converting to gas at constant temperature). A cooling curve is the reverse: it shows the same flat sections at boiling point (condensation) and melting point (freezing).
A pure substance shows sharp, flat plateaux at precise melting and boiling points. An impure substance (a mixture) melts and boils over a range of temperatures rather than at a single fixed point. This is why melting point measurement is used to test the purity of a compound — a pure substance has a sharp, narrow melting range.
Physical and Chemical Changes
A physical change does not produce any new substances. The particles themselves stay the same — only their arrangement or energy changes. Physical changes are reversible under normal conditions. A chemical change forms new substances with different properties. Atoms are rearranged into new combinations; old bonds are broken and new ones form. Chemical changes are usually irreversible — for example, burning magnesium in air produces magnesium oxide, which cannot easily be turned back into magnesium and oxygen under normal conditions.
The simple particle model represents all particles as identical, inelastic, solid spheres (like tiny billiard balls). This is useful for explaining states of matter and changes of state at a basic level, but it has important limitations:
• The model does not account for the forces of attraction between particles — these are what actually hold matter together, yet the simple model treats particles as though they repel or are neutral.
• It treats all particles as having the same size and mass, when in reality different atoms and molecules have different sizes and masses.
• It does not accurately represent the actual spacing between particles in gases — particles in a gas are very much further apart than diagrams typically show.
• The model cannot explain why different substances have different melting and boiling points, because it treats all particles as identical spheres. To explain differences in properties, we need to consider the nature of the particles themselves (atoms, ions, molecules) and the bonds or intermolecular forces between them.
Limitations of the particle model: it treats all particles as identical spheres; it ignores forces of attraction between particles; it does not account for different particle sizes; it cannot explain why different substances have different melting and boiling points.