The Three States of Matter — Edexcel International GCSE Science Double Award (Modular) - Chemistry Unit 1 Revision Notes & Topical Questions

The three states of matter in solids particles are tightly packed in a regular pattern and only vibrate; in liquids they are close but random and slide past one another; in gases they are far apart and move quickly and randomly.

Heating gives particles more energy.

Diffusion particles in liquids and gases move randomly, so they spread out until evenly mixed. Examples: bromine vapour spreading in a gas jar, ammonia and hydrogen chloride forming a white ring of ammonium chloride in a tube.

The rate of diffusion is greater at higher temperature and for particles of lower relative mass. In the NH₃/HCl tube the ring forms closer to the HCl end because the lighter NH₃ molecules travel further.

Dilution dissolving a potassium manganate(VII) crystal and repeatedly diluting the solution shows the colour remains visible even after many dilutions, which shows that particles are extremely small and very numerous.

Solutions a solute dissolves in a solvent to form a solution. Particles of solute spread out between solvent particles. The mass of the solution equals the mass of solute plus solvent.

Solubility the mass of solute that dissolves in 100 g of solvent at a given temperature. A saturated solution holds the maximum amount. Solubility curves show how solubility changes with temperature; cooling a saturated solution produces crystals, the mass of which can be read from the curve.

Boiling happens at one fixed temperature (the boiling point) and bubbles of gas form throughout the liquid. Evaporation happens only at the surface and at any temperature below the boiling point; it is faster when the liquid is warmer or has a larger surface area.

The stronger the forces between particles, the more energy is needed to separate them, so the melting and boiling points are higher. A change of state is a physical change: the particles themselves stay the same.

For a pure substance, melting and freezing happen at the same temperature, e.g. water at 0 °C. Boiling and condensing also share one temperature, the boiling point.

Diffusion is slower in liquids than in gases because liquid particles are closer together and move more slowly, so they collide more often and spread out less quickly.

Most solids become more soluble as temperature rises, but sodium chloride hardly changes. Curves give values per 100 g of water, so scale for other masses: if solubility is 40 g per 100 g, then 25 g of water dissolves 40 × 25 ÷ 100 = 10 g.

Dissolve a known mass of solid in a small known volume of water by warming, then cool while stirring and note the temperature at which crystals first appear. Add more water and repeat. Each run gives one point on the solubility curve (1 cm³ of water = 1 g).