States of Matter and Kinetic Theory
Core ideas
Kinetic theory links the energy of particles to the state of a substance. Melting, boiling, evaporation, condensation and freezing are changes of state, and they are physical changes because no new substance is formed. On a heating curve, temperature rises as kinetic energy increases, then stays flat during a change of state while energy overcomes the forces between particles. Evaporation happens at the surface at any temperature below boiling point, and it cools the remaining liquid. Boiling occurs throughout the liquid at a fixed boiling point.
Gas pressure comes from particles colliding with container walls. Pressure increases with higher temperature (faster, more frequent collisions) and with smaller volume (more frequent collisions). Sublimation is a direct change from solid to gas.
Traps to watch
- During a change of state, temperature does not change even though heating continues.
- Evaporation and boiling are different: evaporation occurs at the surface at any temperature; boiling occurs throughout at a specific temperature.
- A cooling curve is not the reverse of a heating curve in time - but the flat regions occur at the same temperatures.
- Mass is conserved during changes of state, but volume may change.
Rule of thumb
Use particle energy and forces: energy supplied either raises kinetic energy (temperature rises) or overcomes attractive forces (state changes, temperature constant).
Worked example
Ice at −10 °C is heated steadily to steam above 100 °C. Describe the temperature pattern. Answer: temperature rises to 0 °C, stays constant while ice melts, rises to 100 °C, stays constant while water boils, then rises again as steam is heated.