Pearson Edexcel International GCSE Science Single Award 6: Chemistry: Principles of chemistry

Topic 1 builds the particle, atomic and bonding models used throughout Chemistry. These notes follow only the Science Single Award points in Pearson 4SS0 Issue 4. They exclude the extra solubility, electronic-configuration, mole, metallic-bonding, electrolysis and extended structure-property points found in broader Pearson routes.

States of matter and particle behaviour

In a solid, particles are closely packed in fixed positions and vibrate. In a liquid, particles remain close but move past one another. In a gas, particles are far apart and move rapidly in random directions. Particle energy and freedom of movement increase from solid to liquid to gas.

Melting changes solid to liquid, freezing changes liquid to solid, boiling or evaporation changes liquid to gas, and condensation changes gas to liquid. Heating transfers energy to particles. During a change of state, the energy changes particle arrangement rather than raising temperature continuously. Cooling reverses the energy transfer and allows attractions to organise particles more closely.

A coloured solution becomes paler when diluted because the same coloured particles occupy a larger total volume. Gas diffusion occurs because particles move randomly and spread from a region of higher concentration to lower concentration. Faster spreading at higher temperature follows from greater particle kinetic energy.

Elements, compounds and mixtures

An element contains one type of atom. A compound contains atoms of different elements chemically bonded in a fixed ratio. A mixture contains substances together without chemical bonding, so its components retain properties and can be separated by physical methods.

A pure substance has a fixed melting point and boiling point at a given pressure. A mixture often changes state over a range. This makes melting or boiling behaviour useful evidence of purity, though measurement uncertainty and pressure must be considered.

Choose separation methods from the property that differs:

• filtration separates an insoluble solid from a liquid;
• crystallisation obtains a dissolved solid by concentrating and cooling a solution;
• simple distillation separates a solvent from dissolved material or liquids with widely different boiling points;
• fractional distillation separates miscible liquids with closer boiling points through repeated evaporation and condensation;
• paper chromatography separates soluble components according to their different attractions to the stationary and mobile phases.

On a chromatogram, one spot can indicate one component under those conditions, while several spots show a mixture. The retention factor is:

$$R_\mathrm{f} = \frac{\text{distance travelled by substance}} {\text{distance travelled by solvent front}}$$