States of matter — the simple particle model
The simple particle model treats every substance as made of tiny round particles with forces between them. It explains the three states (solid, liquid, gas) and the transitions between them.
The three states
| State | Particle arrangement | Movement | Energy | Shape | Volume |
|---|---|---|---|---|---|
| Solid | Regular, packed close | Vibrate about fixed positions | Low KE | Fixed | Fixed |
| Liquid | Close, irregular | Slide over each other | More KE | Takes shape of container | Fixed |
| Gas | Far apart, random | Move quickly in all directions | High KE | Fills container | Fills container |
The amount of energy each particle has = its kinetic energy. The forces between particles are intermolecular forces in molecular substances, or ionic/metallic/covalent bonds in giant lattices.
State symbols
- (s) solid
- (l) liquid
- (g) gas
- (aq) aqueous (dissolved in water)
Used in equations: NaCl(s) + H₂O(l) → NaCl(aq) — note no chemical change here, just dissolving.
Changes of state
| Process | Direction | What happens |
|---|---|---|
| Melting | s → l | Particles gain enough energy to break some forces; flow as liquid |
| Freezing | l → s | Particles lose energy; settle into fixed positions |
| Boiling / evaporating | l → g | Particles gain enough energy to overcome forces and escape |
| Condensing | g → l | Particles lose energy; forces pull them back together |
| Sublimation | s → g | Direct (e.g. solid CO₂ "dry ice") |
Changes of state are physical changes — particles are unchanged, only their arrangement and energy change. Mass is conserved.
Predicting state from melting and boiling points
Compare room temperature (typically 20–25 °C) with the substance's m.p. and b.p.:
- If room temp < m.p. → solid
- If m.p. < room temp < b.p. → liquid
- If room temp > b.p. → gas
Worked example: bromine has m.p. −7 °C, b.p. 59 °C. At 25 °C, m.p. < 25 < b.p. → liquid.
Limitations of the simple particle model
The simple model treats every particle as a hard sphere of identical size with no internal structure. Real-world limits:
- Particles are not actually solid spheres — atoms have nuclei + electron shells; molecules have shapes (linear, bent, tetrahedral).
- The model ignores forces between particles quantitatively; it can't predict melting points without more information.
- It treats all particles the same, but molecules differ in size and shape, affecting properties like viscosity.
⚠Common mistakes
- Saying particles in a gas have no forces. Forces exist but are very weak compared to KE — particles move freely.
- Saying molecules melt. During melting, intermolecular forces break; the molecules themselves stay intact (no chemical change).
- Saying boiling makes particles bigger. Particles only spread further apart and move faster.
- Drawing solid particles touching with no gaps. They are close-packed but vibrate, so spacing varies.
Links
Builds on C2.3 (covalent — explains why simple molecules have low b.p.). Connects to C2.6 (linking properties to bonding) and C6 (rates and collision theory).
AI-generated · claude-opus-4-7 · v3-deep-chemistry