At both SL and HL, the three principal states of matter are solid, liquid, and gas. They change state when energy is transferred between the substance and its surroundings, altering particle movement and the separation between particles without changing their chemical identity.
The Particle Mechanism
The kinetic molecular theory explains states and changes of state in terms of particle arrangement, movement, and energy.
- In a solid, particles are closely packed in fixed positions and vibrate.
- In a liquid, particles remain close together but can move past one another.
- In a gas, particles are widely separated and move rapidly in random directions.
Temperature in kelvin measures the particles' average kinetic energy. Heating generally increases particle movement, while cooling decreases it. During a change of state, however, transferred energy is used to overcome or establish intermolecular forces, so the temperature of a pure substance remains constant until the change is complete, provided pressure is constant.
| Change of state | Direction and energy transfer | Particle-level explanation |
|---|---|---|
| Melting | Solid to liquid; endothermic | Particles gain energy and overcome some intermolecular attractions. |
| Freezing | Liquid to solid; exothermic | Particles lose energy and become fixed in an ordered arrangement. |
| Vaporization | Liquid to gas; endothermic | Particles overcome intermolecular attractions and separate widely. |
| Condensation | Gas to liquid; exothermic | Particles lose energy and move closer together. |
| Sublimation | Solid directly to gas; endothermic | Particles gain enough energy to escape the solid without forming a liquid. |
| Deposition | Gas directly to solid; exothermic | Gas particles lose energy and form a solid directly. |
A common misconception is that boiling breaks covalent bonds within molecules. It does not: boiling overcomes intermolecular forces, while the molecules remain chemically intact.
Exam Technique
For an IB explain question, link the macroscopic change to particles and energy: state the direction of energy transfer, describe particle movement or separation, and refer to intermolecular forces. Do not merely name the change of state.
