1.1.2—Kinetic molecular theory
- Syllabus
- First assessment 2025
- Objective
- 1.1.2
- Level
- SL
Explaining States with Particles
The kinetic molecular theory is a model that explains observable state properties using particle arrangement, movement, spacing, and attractions. Solid particles are closely packed and vibrate about fixed positions. Liquid particles remain close but change neighbours and flow past one another. Gas particles are far apart relative to their size and move freely, so a gas fills its container and is much more compressible.
| State | Particle model | Fixed volume? | Fixed shape? | Symbol |
|---|---|---|---|---|
| Solid | Close, ordered or fixed positions; vibrate | Yes | Yes | (s) |
| Liquid | Close, disordered; move past neighbours | Yes | No | (l) |
| Gas | Widely spaced; rapid random motion | No | No | (g) |
(aq) does not name a fourth state of pure matter. It means the stated species is dissolved in water.
| Change | Direction | Energy transfer for the substance |
|---|---|---|
| Melting | solid → liquid | absorbed |
| Freezing | liquid → solid | released |
| Vaporization | liquid → gas | absorbed |
| Condensation | gas → liquid | released |
| Sublimation | solid → gas | absorbed |
| Deposition | gas → solid | released |
Heating increases particle motion until a transition begins. During the transition, supplied energy changes the extent of intermolecular attraction and particle arrangement; the chemical identity remains the same. Evaporation can occur at a liquid surface below the boiling point, whereas boiling occurs throughout a liquid when its vapour pressure matches the external pressure.
Do not say that particles themselves melt, expand, or become a different substance. In a physical state change, the same particles adopt different motion and arrangement. For molecular substances, the energy change is associated with intermolecular attractions; it does not generally break the covalent bonds inside each molecule.
State Change Reasoning
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Particulate Matter Summary
Retrieve the progression: classify matter as an element, compound, or mixture; use particle movement and spacing to explain states and state changes; then connect absolute temperature in kelvin with average kinetic energy.
When checking an answer, ask three questions: Is the composition fixed or separable physically? Which direction do the state symbols show? Am I using kelvin when the claim concerns average kinetic energy?