Kinetic Particle Theory
Section: 1. States of Matter | Syllabus: Cambridge IGCSE Chemistry (0620)
The Kinetic Particle Theory
The kinetic particle theory is the model scientists use to explain the properties of solids, liquids and gases in terms of the motion of tiny particles.
- All matter is made up of extremely small particles (atoms, molecules or ions)
- These particles are in constant, random motion
- Particles move faster as temperature increases
- There are forces of attraction between particles, and the strength of these forces is different in solids, liquids and gases
- When particles collide with each other or with the walls of a container, they bounce off rather than losing energy
Kinetic Energy and Temperature
Temperature is a measure of the average kinetic energy of the particles in a substance.
- Higher temperature → particles have more kinetic energy → particles move faster on average
- Lower temperature → particles have less kinetic energy → particles move slower on average
- The theoretical temperature at which particles have the minimum possible kinetic energy is called absolute zero (0 K, equal to -273°C)
Evidence for the Kinetic Particle Theory
Particles are far too small to see, even with a light microscope, so the kinetic particle theory is supported by observations that only make sense if particles are constantly moving.
Brownian Motion
- If smoke particles suspended in air are viewed under a microscope, they can be seen moving in a random, jittery, zig-zag path
- The smoke particles themselves are too large to be moved by their own energy alone
- They are constantly struck, unevenly and from random directions, by fast-moving air molecules that are too small to see
- This uneven bombardment is what pushes the smoke particles along their random path
The random, jerky path is caused by unequal collisions from invisible air molecules, not by the smoke particle moving on its own
Worked Example: Explaining Brownian Motion
- Question: smoke particles in a smoke cell are observed under a microscope. They are seen to move in continuous, random, jerky paths. Explain this observation in terms of the kinetic particle theory. [2]
- Answer: the smoke particles are being hit/bombarded by fast-moving air molecules [1]; because these collisions come from random directions and are unequal in number/force at any instant, the smoke particles are knocked off course, producing a random path [1]
Common Mistakes
- ❌ Thinking Brownian motion is the movement of the air molecules themselves
- ✓ Brownian motion is the visible random movement of much larger particles (e.g. smoke or pollen grains), caused by invisible, faster-moving molecules colliding with them
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