States of Matter

1.1 Solids, liquids and gases

All matter is made of tiny particles (atoms, molecules or ions) that are always moving. The kinetic particle theory explains the three states of matter by how far apart the particles are, how they are arranged, and how they move.

Changes of state

A change of state is a physical change: no new substance is made, and the particles themselves do not change. Only their energy, spacing and arrangement change.

When a solid melts or a liquid boils, the thermal energy supplied is used to overcome the forces between the particles, not to make them move faster. That is why the temperature stays constant while the change of state happens.

Heating and cooling curves

A heating curve plots temperature against time as a substance is heated steadily.

• A sloping part: the temperature is rising. The particles gain kinetic energy and move or vibrate faster, in one state only.
• A flat part: a change of state is happening. The energy supplied is used to overcome the forces between the particles, so the temperature stays constant. The first flat part is melting and the second is boiling.
• The flat parts are at the melting point and the boiling point. A pure substance has a sharp melting point and boiling point, but an impure substance melts over a range of temperatures.

A cooling curve is the reverse: the flat parts are at the freezing point (the same temperature as the melting point) and the condensing point.

It is water. At $0\,^\circ\text{C}$ the ice is melting. At $100\,^\circ\text{C}$ the water is boiling. The energy supplied is used to overcome the forces between the particles, so the temperature does not rise.

Gases: temperature, pressure and volume

A gas exerts pressure because its particles are constantly hitting the walls of the container. The more often and the harder they hit, the greater the pressure.

• Temperature. If the temperature rises, the particles move faster. At constant pressure the gas expands (the volume increases). In a sealed container of fixed volume, the faster particles hit the walls more often and harder, so the pressure increases.
• Pressure. If the pressure on a gas is increased at constant temperature, the volume decreases. The same number of particles are in a smaller space, so they hit the walls more often, and the pressure rises to match.

1.2 Diffusion

Diffusion happens in gases and liquids, because their particles move freely. It does not happen in solids, where the particles only vibrate about fixed positions. It is fastest in gases and slower in liquids.

Examples. The smell of cooking spreads through a house. A drop of coloured ink spreads through water, even without stirring. Bromine vapour spreads through a jar of air until the colour is even.

The effect of relative molecular mass

At the same temperature, the particles of a gas with a lower relative molecular mass ($M_\text{r}$) move faster, so the gas diffuses faster. The effect is shown by the experiment with ammonia and hydrogen chloride.

Cotton wool soaked in concentrated ammonia solution is put in one end of a long tube, and cotton wool soaked in concentrated hydrochloric acid is put in the other end, at the same time. The gases diffuse along the tube and a white ring of ammonium chloride forms where they meet:

$\ce{NH3(g) + HCl(g) -> NH4Cl(s)}$

The ring forms nearer the hydrochloric acid end. Ammonia ($M_\text{r} = 17$) is lighter than hydrogen chloride ($M_\text{r} = 36.5$), so its particles travel faster and cover more of the distance before they meet.

Gas X diffuses faster. Its particles have a lower relative molecular mass, so at the same temperature they move faster.

Diffusion is also faster at a higher temperature, because the particles have more kinetic energy.