Solids, liquids and gases

All matter consists of particles including atoms and molecules. The arrangement and movement of these particles differ between the different states of matter:

  • solids - particles are close together in a regular structure and vibrate about fixed positions
  • liquids - particles are close together but free to move past each other randomly
  • gases - particles are far apart and move randomly in all directions

The density of a substance changes when it changes state. Its mass does not change. This is because its particles do not disappear - they are just rearranged and occupy a different volume.

Solid cube: side length 1 unit, 64 particles in tightly packed lattice. Liquid cube: 1 unit, contains approx 30 particles. Gas cube: side length 10 units contains 15 scattered particles.

Solids and liquids

A substance melts when it changes from the solid state to the liquid state. Its particles remain close together, so there is usually a relatively small change in volume and density. For example:

  • the density of solid iron is about 7,870 kg/m3
  • the density of liquid iron is about 6,980 kg/m3

Example 1

Calculate the volume of 1,000 kg of iron in the solid state.

Rearrange the equation:

density = \frac{mass}{volume}

volume = \frac{mass}{density}

volume = \frac{1,000}{7,870}

= 0.127~m^3

Example 2

Calculate the volume of 1,000 kg of iron in the liquid state.

volume = \frac{1,000}{6,980}

0.143~m^3

Iron in the liquid state occupies a greater volume than the same mass of iron in the solid state. Iron atoms are further apart in liquid iron.

Liquids and gases

A substance evaporates or boils when it changes from the liquid state to the gas state. Its particles move so there is a large change in volume and density. For example:

  • the density of liquid oxygen is about 1,140 kg/m3
  • the density of gaseous oxygen is about 1.42 kg/m3

Example 3

Calculate the mass of oxygen that occupies 0.250 m3 in the liquid state.

Rearrange the equation:

density = \frac{mass}{volume}

mass = density \times volume

mass = 1,140 \times 0.250

= 285~kg

Example 4

Calculate the mass of oxygen that occupies 0.250 m3 in the gas state.

mass = density × volume

= 1.42 × 0.250

= 0.355 kg

Oxygen in the gas state has a much smaller mass than the same volume of oxygen in the liquid state. Oxygen molecules are much further apart in gaseous oxygen, so they occupy a much larger volume.