Key facts & rules

• Solid: particles close together in a regular lattice, vibrate in fixed positions.
• Liquid: particles close together but random arrangement, free to move past each other.
• Gas: particles far apart, random, move rapidly in all directions.
• Melting: solid → liquid (at melting point). Boiling: liquid → gas (at boiling point).
• Condensation: gas → liquid. Freezing/Solidification: liquid → solid. Sublimation: solid → gas directly.
• Kinetic theory: particles are always moving; higher temperature = higher average kinetic energy.
• Diffusion: particles spread from high to low concentration. Faster in gases than liquids.
• Brownian motion: random movement of particles in a fluid, caused by collisions with invisible molecules.