Internal energy: kinetic energy (temperature) plus potential energy (particle positions and forces). During a change of state the temperature (kinetic energy) is constant and the potential energy changes.
Evaporation: surface molecules gain enough kinetic energy to escape into the gas phase. It happens at any temperature below the boiling point, only at the surface, and cools the surroundings.
Boiling: liquid turns to vapour throughout the liquid, with bubbles, at the fixed boiling point. Temperature stays constant while it boils.
Sublimation: solid goes directly to gas without becoming liquid. Deposition: gas goes directly to solid.
Diffusion: spreading and mixing of gas particles from high to low concentration by random motion.
Boyle's law: at constant temperature, volume is inversely proportional to pressure (PV = constant, P₁V₁ = P₂V₂). Charles's law: at constant pressure, volume is directly proportional to absolute temperature (V/T = constant, T in kelvin). Avogadro's law: at constant T and P, volume is directly proportional to moles (V/n = constant). Graham's law: rate of diffusion is inversely proportional to the square root of molar mass.
Must-know points
Melting, boiling and sublimation absorb energy (potential energy rises). Freezing, condensation and deposition release energy.
Heating curve of a substance: A to B solid warming (kinetic energy rises), B to C melting (flat, latent heat of fusion), C to D liquid warming, D to E boiling (flat, latent heat of vaporization), E onward gas warming. A cooling curve is the same in reverse. The flat parts are where the energy changes potential energy only.
Evaporation against boiling: evaporation is slow, at the surface, at any temperature below the boiling point. Boiling is fast and vigorous, throughout the liquid, at one fixed temperature, with bubbles.
Pressure and boiling: higher external pressure raises the boiling point (pressure cooker cooks faster). Lower pressure (high altitude) lowers the boiling point, so water boils at a lower temperature on mountains. Lower pressure raises the rate of evaporation, higher pressure lowers it.
Gas laws by kinetic theory: smaller volume means more wall collisions per second and more pressure (Boyle). Higher temperature means faster particles, so volume must rise to keep pressure constant (Charles). More particles need more volume at constant pressure (Avogadro).
Diffusion: lighter gas diffuses faster, because at the same temperature all gases have the same average kinetic energy (½mv²), so smaller m means larger v. Higher temperature raises the rate because particles move faster.
Sublimation substances: dry ice (solid CO₂), camphor, naphthalene. Uses: solid air fresheners, dye-sublimation printing (T-shirts, mugs), freeze-drying of food.
Diffusion in medicine: the diffusion rate decides how fast a drug reaches the blood and cells. A high concentration gradient and a liquid form act faster than a tablet. Controlled-release forms keep the effect steady.
Why perfume on the palm feels cold, and why sweat cools us: the fastest molecules leave, taking energy from the surface, so the average kinetic energy left behind falls.
Past-paper note
2026 MCQ: in sublimation a substance changes from solid to gas. No long question from this unit in 2026. Likely short questions: why temperature stays constant during melting; evaporation against boiling; effect of external pressure on boiling point; effect of temperature and molar mass on diffusion.