Checkpoint 1

• Q1 — C: Rusting forms a new substance (iron(III) oxide, Fe₂O₃) with different properties from iron. Options A, B, and D are all physical changes — no new substance is formed and the change is reversible.
• Q2 — B: Decomposition: one reactant breaks down into two or more products. 2H₂O₂ → 2H₂O + O₂ fits this pattern. Option A is synthesis; C is combustion/synthesis; D is synthesis.
• Q3 — C: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. Check: C 3=3 ✓; H 8=8 ✓; O 10=6+4=10 ✓.
• Q4 — B: BaSO₄ is insoluble (barium sulfate precipitates). All other combinations produce only soluble salts (NaNO₃, KBr, NaBr, CaNO₃, NaCl are all soluble).
• Q5 — C: Lead(II) iodide (PbI₂) is a bright yellow insoluble solid. The net ionic equation: Pb²⁻(aq) + 2I⁻(aq) → PbI₂(s). KNO₃ remains in solution as spectator ions.
• Q6 — B: Complete combustion of any hydrocarbon or alcohol produces only CO₂ and H₂O. C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.
• Q7 — D: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. Products are the salt (NaCl), water, and CO₂. Sodium hydroxide is NOT produced — it is a base, not a product of an acid-carbonate reaction.
• Q8 — B: Acid-base neutralisation: acid + base → salt + water, no gas produced. Option A is acid-carbonate (produces CO₂). Option C is metal-acid (produces H₂ gas). Option D is dissociation, not a reaction.
• Q9 — D: Fe₂O₃ is a metal oxide, which acts as a base when reacting with an acid. Acid + base (metal oxide) → salt + water. No carbonate is present (not acid-carbonate); no precipitation; no combustion.
• Q10 — B: Leaching is a physical process — BMAA dissolves in water (exploiting water-solubility) and diffuses out of the seed into the running water over time. No new substance is formed in the seed or in the water; BMAA is simply diluted and removed. This is a classic physical separation technique.

SA1: (a) BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq). (b) Ba²⁻(aq) + SO₄²⁻(aq) → BaSO₄(s). (c) Precipitate: barium sulfate (BaSO₄, white solid). Spectator ions: Na⁺(aq) and Cl⁻(aq) — they appear on both sides of the molecular equation and are not involved in the precipitation.

SA2: (a) C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. (b) Incomplete combustion occurs when the supply of oxygen is insufficient (limited O₂). The additional product is carbon monoxide (CO) — a toxic colourless gas: 2C₃H₈ + 7O₂ → 6CO + 8H₂O. (c) CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g).

SA3: (i) 2Mg(s) + O₂(g) → 2MgO(s) — synthesis (two reactants combine to form one product). (ii) CaCO₃(s) → CaO(s) + CO₂(g) — decomposition (already balanced; one reactant breaks down into two products). (iii) C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(g) — combustion (check: C 2=2; H 6=6; O 3+1=4 on left; 4+3=7 on right… recheck: left O = 3×2+1=7, right = 4+3=7 ✓).

SA4: A physical change is a change in which no new substance is formed — the identity and composition of the substance remain the same, and the change is often reversible. In cycad preparation, soaking and washing in running water is a physical change: BMAA (a water-soluble amino acid-like toxin) dissolves into the water and is carried away, but no new chemical substance is formed in the seed or the water. Leaching works because BMAA is water-soluble — it dissolves readily and diffuses out of the seed tissue down a concentration gradient into the surrounding water. Replacing the water with running or fresh water maintains a steep concentration gradient, speeding removal. Heat is not ideal as the primary method because: (1) direct heat may not chemically destroy the toxin at low cooking temperatures — BMAA could survive and the products of thermal decomposition are unknown; (2) high heat causes unwanted chemical changes to the seed (gelatinisation, denaturation) altering nutritional quality; (3) the effectiveness of heat in removing a polar, water-soluble toxin is unreliable without extensive testing of the resulting decomposition products for toxicity.