Chemistry: Substances, Equations, Ions and Neutralisation
What this domain tests
The chemistry questions check whether you can classify matter, write and balance equations, and follow ions through a neutralisation. Kagoshima-style items often require that the word equation, the symbol equation, the ionic equation and the observation all agree with each other.
Substances and changes
A pure substance is an element (one kind of atom) or a compound (elements chemically joined in a fixed ratio). Mixtures keep the properties of their parts and are separated physically - filtration, distillation, chromatography. A chemical change makes new substances and is not undone by simple physical means; look for gas, precipitate, colour change or heat.
Chemical equations
Balance by changing coefficients only, never subscripts, because atoms are conserved. Add state symbols (s), (l), (g), (aq). In ionic equations, cancel spectator ions; both atoms and total charge must balance on each side.
Ions and neutralisation
Acids release H⁺ in water; alkalis release OH⁻. Neutralisation is H⁺ + OH⁻ → H₂O, giving a salt and water: HCl + NaOH → NaCl + H₂O. Acid + metal gives hydrogen; acid + carbonate gives carbon dioxide, water and a salt. pH below 7 is acidic, 7 neutral, above 7 alkaline; universal indicator, litmus and phenolphthalein show the change.
Worked example
Sulfuric acid plus sodium hydroxide: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O. Check atoms: H 4, S 1, O 6, Na 2 on both sides. The 2 in front of NaOH is needed because the acid is diprotic. Net ionic equation: 2H⁺ + 2OH⁻ → 2H₂O, simplified to H⁺ + OH⁻ → H₂O; Na⁺ and SO₄²⁻ are spectators.
Traps that cost marks
"Balancing" by rewriting subscripts, for example turning H₂O into H₂O₂. Writing O instead of O₂ for oxygen gas. Forgetting the coefficient 2 for a diprotic acid. Claiming a neutral solution contains no ions - it contains the salt's ions.