Units Used This Topic:

The particle model can be used to explain

  •  The different states of matter (solid, liquid, gas)
  •  Differences in density

The density (measured in kg/m3) of a material is defined by the equation:

density = mass / volume

Question: An iceberg has a mass of 11 200 kg
The volume of the iceberg is 12.0 m3
Calculate the density of the iceberg.

Question: The volume of a balloon is 0.014 m3
The density of the helium in the balloon is 0.180 kg/m3
Calculate the mass of helium in the balloon

States of Matter


Particle arrangement in a solid

Particles are arranged in a regular pattern

Particles vibrate in a fixed position

Particles have strong forces between them

More dense than liquid & gas

Less energy than liquid & gas


Particle arrangement in a liquid

Particles are arranged in an irregular pattern

Particles move freely

Particles have weak forces between them

More dense than gas, less dense than solid

Less energy than gas, more energy then solid


Particle arrangement in a gas

Particles are in a constant, random motion with a range of speeds and directions

Particles have no forces between them

Less dense than liquid & gas

More energy than liquid & gas

The higher the temperature of the gas higher the average kinetic energy of the molecules of the gas.

When substances change state (melt, freeze, boil, evaporate, condense or sublimate), mass is conserved.

Changes of state are physical changes which differ from chemical changes because the material recovers its original properties if the change is reversed.

Question: Describe two ways the arrangement of particles in a solid is different from the arrangement of particles in a liquid. (3 marks)

Question: Explain why heating a gas increases the average speed of the gas particles.