Kinetic Theory

Kinetic theory makes a few core claims:

1. All matter is made of tiny particles (atoms, ions or molecules).
2. Particles are in constant, random motion.
3. Temperature is a measure of the AVERAGE kinetic energy of these particles. Higher T = faster average motion.
4. Particles collide with each other and with container walls.
5. In an IDEAL gas, particles have negligible volume and don't attract or repel each other except during collisions.

The way particles move depends on the state:

• Solid: vibrate around fixed positions.
• Liquid: slide past one another while staying close.
• Gas: move freely at high speed in all directions.

Particles in a real sample don't all have the same speed at any moment. Some are slower, some faster — heating shifts the WHOLE distribution to higher speeds.

Imagine a gas in a sealed box. Gas particles move in random directions at high speed and constantly bump into the walls. Each collision pushes the wall a tiny amount.

Gas pressure = the total force per unit area from all these collisions. The more collisions per second (and the harder each collision), the more pressure.

What changes pressure?

• More particles in the same volume (higher concentration) → more collisions per second → MORE pressure.
• Heating → particles move faster → more frequent AND harder collisions → MORE pressure (at constant volume).
• Shrinking the volume → particles cover the wall surface area more often → MORE pressure (at constant temperature). This is the basis of Boyle's law: P × V = constant at constant T.

That's why a tyre pumped up has more pressure (more particles), why a tyre on a hot day has higher pressure (faster particles), and why compressing a gas in a syringe makes it harder to push.

Diffusion is the net movement of particles from a region of HIGH concentration to a region of LOW concentration, caused by random motion. Smelling perfume across a room, dye spreading through water, oxygen reaching the bottom of a fish tank — all are diffusion.

Diffusion is FASTER in gases than liquids:

• In a gas, particles move fast and have lots of empty space.
• In a liquid, particles are closer together and move more slowly — diffusion takes longer.
• In a solid, particles vibrate but don't move past each other — almost no diffusion.

Heat speeds up diffusion in any state. A drop of food colouring spreads through hot water much faster than through cold.

Brownian motion: in 1827, Robert Brown observed pollen grains in water jiggling randomly under a microscope. Einstein explained this in 1905: invisibly small water molecules were colliding with the pollen and giving it tiny random kicks. This was strong experimental evidence for the EXISTENCE of molecules.

You can see Brownian motion with smoke particles in air (they appear lit up in a beam of light and dance about randomly) or with carmine dye in water under a microscope.