Secondary 4 Pure Chemistry Acids Bases Salts Quiz

Secondary 4 Pure Chemistry Quiz - Acids Bases Salts (Answer Key)

Total Marks: 40
Note: Syllabus-first generated content from LLM templates; not past-year derived.

Section A (1 mark each)

Q1. Sulfur dioxide (SO₂).
Teaching note: Burning fossil fuels containing sulfur produces SO₂, which dissolves in rain to form sulfurous acid — a cause of acid rain. Common mistake: writing SO₃ (formed only on further oxidation).

Q2. Acidic oxide (or non-metal oxide).
Teaching note: CO₂ dissolves in water to form carbonic acid; it neutralises NaOH to form a salt, so it acts as an acidic oxide.

Q3. Red.
Teaching note: Litmus is red below pH 7 (acidic), blue above pH 7. pH 3 is strongly acidic.

Q4. KNO₃.
Teaching note: HNO₃ + KOH → KNO₃ + H₂O. Nitrate salt from nitric acid and potassium base.

Q5. CH₃COOH (ethanoic acid / acetic acid).
Teaching note: HCl and H₂SO₄ are strong acids (fully ionised); CH₃COOH is weak (partially ionised).

Section B (2 marks each)

Q6. [2 marks] Add NaOH(aq) dropwise: Al³⁺ gives white ppt of Al(OH)₃ that dissolves in excess NaOH to colourless solution; Pb²⁺ gives white ppt of Pb(OH)₂ that does NOT dissolve in excess.
Marking: 1 mark for correct reagent and Al observation, 1 mark for Pb observation.
Teaching: Al(OH)₃ is amphoteric; Pb(OH)₂ is not.

Q7. [2 marks] ZnCO₃(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂O(l) + CO₂(g); method = excess reactant + filtration + crystallisation.
Marking: 1 eq with states, 1 method name.

Q8. [2 marks] Equivalence point is above pH 7 (~8.5) because weak acid + strong base; phenolphthalein changes at pH 8.3–10, matching the jump.
Marking: 1 for pH reason, 1 for indicator match.

Q9. [2 marks] Observation: vigorous effervescence / explosion / heat released. Unsuitable due to violence, unsafe, hard to control.
Marking: 1 obs, 1 safety reason.

Q10. [2 marks] pH = 7 at equivalence; suitable indicator: methyl orange or phenolphthalein (either, due to sharp jump).
From graph: vertical rise at 25 cm³ crossing pH 7.

Q11. [2 marks] H⁺(aq) + OH⁻(aq) → H₂O(l).
Marking: 1 species, 1 states.

Q12. [2 marks] Effervescence (H₂ gas); magnesium dissolves / disappears; solution may warm.
Marking: any 2 valid obs.

Q13. [2 marks] NH₃(g) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq).
Marking: 1 eq, 1 OH⁻ shown.

Q14. [2 marks] Add calcium hydroxide (lime) / CaCO₃; preferred because less corrosive, safer, supplies Ca²⁺ nutrients.
Marking: 1 substance, 1 reason.

Q15. [2 marks] Heat CuO with dilute H₂SO₄, add excess CuO, filter, crystallise.
Marking: 1 reaction step, 1 crystallisation.

Section C

Q16. [3 marks]
(a) Titration using indicator (1)
(b) Na₂CO₃(aq) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g) (1)
(c) Both soluble, so exact neutralisation via titration needed to avoid excess reactant (1)
Teaching: Soluble + soluble → titration; insoluble + acid → excess method.

Q17. [3 marks]

• Weak acid not fully neutralised at equivalence leaves conjugate base hydrolysing to OH⁻ → pH > 7 (1)
• Phenolphthalein suitable as it changes in pH 8.3–10 region (1)
• Methyl orange unsuitable (1)
  From graph: equivalence at 25 cm³, pH 8.5.

Q18. [4 marks]

• Zn + H₂SO₄ → ZnSO₄ + H₂; method: excess metal + filter + crystallise (2)
• BaCl₂ + Na₂SO₄ → BaSO₄(s) + 2NaCl; method: mix, filter ppt, wash, dry (2)
  Key diff: soluble salt needs crystallisation; insoluble needs filtration of ppt.

Q19. [3 marks]

• Strong acid fully ionises in water: HCl → H⁺ + Cl⁻ (1)
• Weak acid partially ionises: CH₃COOH ⇌ H⁺ + CH₃COO⁻ (1)
• Therefore same concentration of weak acid has lower [H⁺] and higher pH (1)

Q20. [4 marks]
(a) Effervescence, solid dissolves (1)
(b) CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g) (1)
(c) Neutralisation / acid-carbonate reaction (1)
(d) Evaporate filtrate / crystallise to get CaCl₂·xH₂O (1)