The Particle Model
The particle model explains the properties of solids, liquids, and gases in terms of the arrangement, separation, and motion of their particles.
Comparison of Solids, Liquids and Gases
| Property | Solids | Liquids | Gases |
|---|---|---|---|
| Arrangement | Particles arranged in a fixed, regular pattern. | Particles not arranged in any fixed pattern. | Particles arranged randomly. |
| Separation | Particles very close together. | Particles close together. | Particles far apart. |
| Motion | Particles vibrate about fixed positions. | Particles slide past each other. | Particles move freely and rapidly. |
Summary by State
- Solid: Molecules form a 3D lattice; vibrate about fixed positions. When heated, they gain kinetic energy and may melt or sublime.
- Liquid: Molecules stay in contact but move freely; forces of attraction are weaker than in solids, allowing flow but maintaining volume.
- Gas: Forces between molecules are negligible; particles move freely and collide with each other and container walls, producing gas pressure.
Temperature and Motion of Particles
Temperature indicates how hot or cold a substance is and is related to the average kinetic energy of its particles.
Key Relationship
As temperature increases → particle motion increases.
As temperature decreases → motion decreases.
At absolute zero (0 K) → particles cease all motion.
Temperature Scales
- Celsius scale: 0 °C = melting point of ice, 100 °C = boiling point of water.
- Fahrenheit scale: 32 °F = melting point, 212 °F = boiling point.
- Kelvin scale: 273 K = freezing point, 373 K = boiling point.
T(K) = T(°C) + 273
Absolute zero = 0 K or −273 °C → particles have minimum kinetic energy.
Gas Pressure and Particle Collisions
- Gas pressure results from collisions between gas particles and container walls.
- Each collision exerts a force on the wall.
- Increasing the number of particles or their temperature increases collision frequency → higher pressure.
Brownian Motion
Brownian motion is the random, zig-zag motion of microscopic particles suspended in a fluid, caused by collisions with fast-moving molecules of the fluid.
Evidence for the Kinetic Model
Observed by Robert Brown using pollen grains in water.
It provides direct evidence that particles of matter are in constant motion.
Exam Tip
When describing Brownian motion, distinguish between the microscopic particles (visible under a microscope) and the molecules of the fluid causing the collisions.
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