Chemical Substances: Nature and Behaviour
What this domain tests
This is the largest theory block of the paper. Questions ask you to balance equations, classify reaction types, predict products from the reactivity series, connect pH to everyday substances, name carbon compounds and use the periodic table to explain trends.
Core rules
- Every equation must be balanced atom by atom; state symbols help but are not compulsory.
- Reaction types: combination (A + B → AB), decomposition (AB → A + B), displacement (a more reactive metal pushes out a less reactive one), double displacement (ions swap, often forming a precipitate), and redox (oxidation is loss of electrons or gain of oxygen).
- Reactivity series: K Na Ca Mg Al Zn Fe Pb (H) Cu Hg Ag Au. Metals above hydrogen displace H₂ from dilute acids; metals above copper displace Cu²⁺ from solution.
- pH below 7 is acidic, 7 neutral, above 7 basic; one pH unit is a tenfold change in H⁺ concentration.
- Carbon shows catenation and tetravalency; members of a homologous series differ by CH₂; functional groups include –OH (alcohol), –COOH (carboxylic acid), –CHO (aldehyde) and –CO– (ketone).
- Modern periodic law: properties are periodic functions of atomic number. Across a period, atomic radius and metallic character decrease.
Worked example
Balance Fe + H₂O → Fe₃O₄ + H₂. Fe₃O₄ needs 3 Fe, so write 3 before Fe. Four oxygen atoms on the right need 4 H₂O. That supplies 8 H, so write 4 before H₂. Answer: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
Traps
- An unbalanced equation loses the mark even when the products are correct.
- Plaster of Paris is CaSO₄·½H₂O; gypsum is CaSO₄·2H₂O. Do not interchange them.
- Displacement involves one element replacing another; double displacement involves two compounds exchanging ions.
- "Strong" acid means fully ionised, not concentrated; concentration is amount per volume.