States of Matter (Kinetic Particle Model)

Matter around us exists mainly as solids, liquids, and gases. The way particles (atoms or molecules) are arranged and how they move explains the properties of each state.

Distinguishing properties

• Solids: fixed shape, fixed volume, not easily compressed.
• Liquids: no fixed shape (take the shape of the container), fixed volume, not easily compressed, flow.
• Gases: no fixed shape or volume, spread out to fill space, easily compressed, flow.

Particle model explanation

• Solids: particles are closely packed in an orderly pattern. They vibrate in fixed positions. Attractive forces are strong.
• Liquids: particles are close together but arranged randomly. They move and slide past each other. Forces are weaker than in solids.
• Gases: particles are far apart and move quickly in random directions. Forces between particles are very weak.

Temperature is linked to particle motion: higher temperature means higher average kinetic energy. In symbols: $\text{temperature} \uparrow \Rightarrow \text{average kinetic energy} \uparrow$. At $-273\,^{\circ}\mathrm{C}$ (absolute zero) particles have the least possible kinetic energy.

Changes of state

• Melting (solid → liquid): particles gain energy and vibrate more until they can move past each other.
• Freezing (liquid → solid): particles lose energy and settle into fixed positions.
• Boiling (liquid → gas): bubbles form throughout the liquid at the boiling point.
• Evaporation (liquid → gas): happens at the surface at any temperature; faster when warmer, with a larger surface area, or with moving air.
• Condensation (gas → liquid): particles lose energy and come closer together.

During a change of state, temperature can stay constant while energy is used to overcome or form attractions between particles.

Real-world links

• Ice melting into water on a warm day (melting).
• Steam from a kettle forming water droplets on a cold window (condensation).
• A puddle disappearing after rain (evaporation).

Common misconceptions

• Particles do not expand when heated; the spaces between particles increase.
• Evaporation is not the same as boiling; boiling happens throughout the liquid at a fixed temperature, evaporation happens at the surface at any temperature.
• Gases are matter: they have mass and can be compressed because particles are far apart.