Particle model of matter · revision map · IGCSE 0625, Topic 2.1

The same particles are arranged and move differently in the three states.

• Solid: fixed shape and volume; particles vibrate in place in a regular pattern.
• Liquid: fixed volume, takes the shape of its container; particles slide past each other.
• Gas: fills the container and is easily compressed; particles move fast and far apart.

Gas pressure comes from particles colliding with the walls of the container.

• Each collision gives the wall a tiny push; billions of collisions give a steady pressure.
• Heating speeds the particles up, so they hit the walls harder and more often and the pressure rises.
• This links to pressure as force per unit area (Topic 1.8).

Evidence that particles are in constant, random motion.

• Large smoke specks are seen to jitter as fast, unseen air molecules strike them.
• Shows molecules move quickly and randomly in all directions.

At constant temperature, squeezing a gas into a smaller volume raises its pressure.

Heating raises the average kinetic energy of the particles, which we measure as temperature.

• Gas temperatures use the Kelvin scale.
• 0 K is absolute zero: particle motion is least.