Chemical Substances and Processes
This domain examines how substances react, how we classify them, and how their properties explain everyday behaviour. BSEH questions usually ask you to balance equations, identify reaction types, use the pH scale, apply the reactivity series, and recognise functional groups in carbon compounds.
Core rules and concepts
- A chemical equation must conserve atoms of every element; state symbols are added after balancing.
- Combination, decomposition, displacement, double displacement and redox reactions each have a characteristic pattern.
- pH below 7 is acidic, 7 neutral, above 7 basic; pH is logarithmic, so pH 3 is ten times more acidic than pH 4.
- Metals above hydrogen in the reactivity series displace it from dilute acids; copper, silver and gold do not.
- Ionic compounds have high melting points because of strong electrostatic forces; covalent compounds are usually molecular and lower-boiling.
- In carbon chemistry, a homologous series differs by CH2 and members share similar chemical properties.
Worked example
Balance: Fe + H2O → Fe3O4 + H2. The product Fe3O4 contains 3 Fe and 4 O, so start with 3Fe and 4H2O. That gives 8 H on the left, so write 4H2 on the right. Final: 3Fe + 4H2O → Fe3O4 + 4H2. Check: Fe 3=3, O 4=4, H 8=8. The reaction is a redox reaction: iron is oxidised and hydrogen is reduced.
Traps that cost marks
- Calling a concentrated acid “strong”: concentration is amount per volume; strength is degree of ionisation.
- Forgetting to balance before adding state symbols.
- Confusing displacement (one element replaces another) with double displacement (ions exchange).
- Writing pH decreases as acidity decreases; it is the reverse.
- Naming a carbon compound without the correct suffix (-ol, -al, -oic acid).