Particle Theory and States of Matter
The Three States of Matter
All matter is made of particles (atoms, molecules or ions). The physical state of a substance depends on how these particles are arranged and how much energy they have.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Particle arrangement | Regular, close-packed lattice | Random, close together | Random, far apart |
| Particle movement | Vibrate in fixed positions | Move freely; slide past each other | Move rapidly in all directions |
| Shape | Fixed | Takes shape of container | Fills entire container |
| Volume | Fixed | Fixed | Not fixed (expands to fill container) |
| Compressibility | Incompressible | Incompressible | Highly compressible |
| Density | High | High (slightly less than solid) | Very low |
Kinetic Theory
The kinetic theory of matter states that:
- All matter is made of tiny particles (atoms, molecules or ions)
- Particles are in constant motion
- The temperature of a substance is a measure of the average kinetic energy of its particles
- Higher temperature → particles move faster → more kinetic energy
Key implication: When we heat a substance, we transfer energy to its particles, increasing their kinetic energy and speed of motion.
Changes of State
Changes of state occur when energy is added or removed, changing the arrangement and motion of particles — but NOT changing the particles themselves.
Heating curve (solid → gas):
- Melting: solid → liquid at the melting point; particles gain enough energy to break free from fixed positions; temperature stays constant during the change (energy goes into breaking intermolecular forces, not raising temperature)
- Boiling/Evaporation: liquid → gas at the boiling point; particles gain enough energy to escape into the gas phase
Cooling curve (gas → solid):
- Condensation: gas → liquid (temperature stays constant; energy released to surroundings)
- Freezing: liquid → solid (temperature stays constant; energy released)
Key point: During a change of state, temperature remains constant even though energy is being added or removed. This is because all the energy goes into changing the state (breaking or forming intermolecular bonds) rather than changing the temperature.
Melting Points and Boiling Points
- Melting point: temperature at which solid ↔ liquid. Fixed for a pure substance.
- Boiling point: temperature at which liquid ↔ gas. Fixed for a pure substance.
These are used to:
- Identify pure substances (sharp melting point)
- Identify impurities (impure substances melt over a range of temperatures, at a lower temperature than the pure substance)
Diffusion in Gases and Liquids
Particles in liquids and gases are in constant random motion — this causes diffusion (net movement from high to low concentration). Gases diffuse faster than liquids because gas particles move much faster and there are fewer collisions.
Effect of temperature on diffusion rate: Higher temperature → faster particles → faster diffusion.
Example: Bromine gas spreading through air; potassium permanganate dissolving in water (diffusion visible as purple colour spreading).
WJEC Practical Note
WJEC Eduqas required practicals include measuring melting points (e.g., stearic acid cooling curve). You may be asked to draw or interpret a cooling curve and explain the flat section during solidification.
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