Chemistry: matter and particles, reactions, ions, acids and bases, equations
What the chemistry section asks
The exam tests matter and particles, chemical reactions, ions, acids and bases, and chemical equations. Expect particle diagrams, word-to-symbol equations, ion formulas, and pH or neutralisation questions; answers must show balanced atoms and correct charges.
Core rules
- Particles and matter: Solids, liquids and gases differ in particle arrangement and motion. Melting and boiling are physical changes; mass is conserved in a closed system. Diffusion is faster at higher temperature because particles have more kinetic energy.
- Reactions and equations: A chemical reaction rearranges atoms; it does not create or destroy them. Balance equations by changing coefficients only, never subscripts. State symbols are (s), (l), (g), (aq).
- Ions: Metal atoms lose electrons to form positive ions; non-metal atoms gain electrons to form negative ions. Ionic compounds are neutral overall, so charges balance, e.g. Mg²⁺ with Cl⁻ gives MgCl₂.
- Acids and bases: Acids release H⁺ in water; bases neutralise acids to form salt and water. pH below 7 is acidic, 7 neutral, above 7 alkaline. Each pH unit is a tenfold change in H⁺ concentration.
Worked example
Balance the combustion of propane: C₃H₈ + O₂ → CO₂ + H₂O. Carbon: C₃H₈ needs 3CO₂. Hydrogen: 8 H needs 4H₂O. Oxygen on the right = 3×2 + 4×1 = 10 O atoms, so write 5O₂. Final: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. Check: 3 C, 8 H and 10 O on each side.
Traps
Forgetting that O₂, N₂, H₂ and Cl₂ are diatomic; changing a subscript while balancing; writing MgCl instead of MgCl₂; saying an acid “contains H⁺” without specifying water; confusing strong with concentrated; and treating pH 3 as three times more acidic than pH 6 rather than 1000 times.