1.1 States of MatterEdexcel IGCSE Chemistry: Revision notes

Matter exists as solid, liquid or gas, depending on how the particles are arranged, how they move and how much energy they have.

Particles in a solid are held by strong forces of attraction; in a gas the forces are so weak they are almost negligible.

The six interconversions between states have specific names:

• Melting: solid → liquid (particles gain energy, vibrate more until the lattice breaks down)
• Freezing: liquid → solid (particles lose energy, arrangement becomes fixed)
• Boiling/evaporating: liquid → gas (particles gain enough energy to overcome attractive forces completely)
• Condensing: gas → liquid (particles lose energy, forces pull them closer together)
• Subliming: solid → gas directly (e.g. solid carbon dioxide)
• Deposition: gas → solid directly

In every case where energy is gained, particle movement and separation increase; where energy is lost, they decrease.

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration, caused by the random movement of particles.

• Gas diffusion (e.g. a coloured gas spreading through air) shows that gas particles move quickly and randomly in all directions, mixing without stirring.
• Dilution of a coloured solution (e.g. potassium permanganate spreading through water without stirring) shows that particles in a liquid also move and spread out on their own, though more slowly than in a gas because liquid particles are closer together and experience stronger attractive forces.

Both experiments are explained by particles constantly moving and colliding randomly.

• Solute: the substance that dissolves
• Solvent: the liquid that the solute dissolves in
• Solution: the mixture formed when a solute dissolves in a solvent
• Saturated solution: a solution in which no more solute will dissolve at that temperature, with undissolved solute (usually) present in the mixture

Solubility is the mass of solute that will dissolve in 100 g of solvent at a stated temperature, given in units of g per 100 g of solvent.

A solubility curve plots solubility (g per 100 g solvent) on the y-axis against temperature (°C) on the x-axis.

1. Solubility generally increases with temperature for most solid solutes.
2. To read a solubility curve: find the temperature on the x-axis, trace up to the curve, then across to read the solubility.
3. Points above the curve represent an unsaturated amount that will fully dissolve; points below represent excess solid that will remain undissolved.

To investigate the solubility of a solid at a specific temperature: dissolve the solid in a measured mass of water at that temperature by adding it in small portions with stirring until no more dissolves (saturation), then calculate solubility by scaling the mass dissolved up to 100 g of water.