Kinetic Particle Theory

Kinetic theory explains matter using tiny particles that are always moving. Their arrangement, motion and energy explain the properties of solids, liquids and gases and changes of state.

Particles in solids, liquids and gases

• Solid: particles packed in a fixed pattern, vibrate only; strong attractions.
• Liquid: particles close, random arrangement, slide past each other; weaker attractions.
• Gas: particles far apart, move quickly and randomly; very weak attractions.

Temperature and energy

Temperature measures average kinetic energy $T \propto \text{average kinetic energy}$. Heating makes particles move faster and further apart. During melting or boiling, energy breaks attractions, so temperature stays constant until the change finishes.

Diffusion

Diffusion is spreading by random motion (for example, perfume across a room). It is faster in gases than in liquids. Lighter gases (lower relative molecular mass) diffuse faster.

Gas pressure and volume

Gas pressure comes from particle collisions with container walls. Higher temperature → faster particles → more frequent, forceful collisions → higher pressure (if volume is fixed). Increasing volume gives fewer collisions, so pressure falls; compressing does the opposite.

Common misconceptions

• Particles in solids do move—they vibrate.
• There are spaces between particles, even in liquids and gases.
• Temperature does not rise during melting or boiling.