SKSS 4NA Sci Chem Prelim Mark Scheme

2025 Secondary 4NA Science Chemistry Preliminary Examination Mark Scheme Setter: Ms Tan Ying Rui 1 PAPER 3 [1m for each correct answer; max. 20m] 1 2 3 4 5 6 7 8 9 10 C C B B D D B B C B 11 12 13 14 15 16 17 18 19 20 A D B B A A B A D A PAPER 4 SECTION A [max. 22m] 1 [1m for each correct; max. 5m] [5] (a) oxygen (b) zinc chloride (c) hydrogen (d) brass (e) calcium hydroxide 2 (a) [1m for each correct; max. 2m] argon; nitrogen; carbon dioxide [2] (b) 21 mm [1] (c) green to red / green to orange / green to yellow [1] 3 (a) Alkanes [1] (b) Fractional distillation. reject: distillation [1] (c) White precipitate formed when carbon dioxide is bubbled through limewater. [1] (d) (i) Bioethanol / Biodiesel [1] (ii) Bioethanol is produced from crops like sugarcane or corn that absorb carbon dioxide via photosynthesis, offsetting emissions released during combustion. reject: bioethanol produces less carbon dioxide when combusted. [1] 4 (a) Water / H2O [1] (b) Explanation: Ammonia gas is soluble in water and cannot be collected using the water displacement method. [1] Correction: A gas syringe OR an upward delivery method could be used to collect the gas instead. [1] (c) (i) 3 CuO + 2 NH3 → 3 Cu + 3 H2O + 1 N2 [1] (ii) 46 cm3 [1]

2025 Secondary 4NA Science Chemistry Preliminary Examination Mark Scheme Setter: Ms Tan Ying Rui 2 (iii) [1m for all correct plotted points, 1m for smooth curve; max. 2m] [2] (iv) 4.5 min ± 0.1 (show working with dotted lines) [1] SECTION B [max. 8m] [Only mark the first question if student attempts both questions in this section.] 5 (a) A diatomic molecule has two atoms that are chemically combined/bonded. [1] (b) Group 17. [1] (c) Accept: any value ranging between 0.00290 to 3.10. [1] (d) (i) Liquid [1] (ii) Bromine [1] (e) In aqueous state, NaX has mobile ions to conduct electricity. [1] In solid state, the ions in NaX are held in fixed positions and cannot move to conduct electricity. [1] (f) Element W is more reactive than iodine and can displace iodine from the potassium iodide solution. [1] 0 10 20 30 40 50 60 70 0 1 2 3 4 5 6 volume / cm3 time / min

2025 Secondary 4NA Science Chemistry Preliminary Examination Mark Scheme Setter: Ms Tan Ying Rui 3 6 (a) [1 mark for each correct column; max 2 marks] [2] reaction 1 reaction 2 name of reaction substitution addition / hydrogenation name of reactants chlorine hydrogen (b) [1 mark for correct sharing of electrons] [1 mark for correct number of unshared electrons] [2] (c) [1] (d) (i) H+ [1] (ii) Acid in acid rain reacts away the carbonate to form salt, water and carbon dioxide. [1] (iii) [1 mark for each logical answer to do with either land or water pollution] As non-biodegradable plastics take a long time to decompose, they can accumulate in landfills for long periods, causing land pollution. OR Non-biodegradable plastics can end up in waterways and water bodies, causing harm to marine life.