WDL 4NA Science Chemistry ANSWER KEY :)

1 WOODLANDS SECONDARY SCHOOL PRELIMINARY EXAMINATION 2023 MARKING SCHEME Paper 3 (20 marks) 1 C 11 A 2 B 12 C 3 D 13 D 4 A 14 A 5 A 15 B 6 A 16 C 7 C 17 D 8 C 18 B 9 A 19 A 10 A 20 B Level: Secondary Four Normal (Academic) Marks: 50 Subject: Science (Chemistry) Day: Wednesday Subject code: 5105 and 5107 Date: 2 August 2023 Duration: 1 hour 15 min (for both papers) Time: 1145 – 1300

2 Paper 4 Section A (14 marks) 1 Total = 4 (a) Propene / C3H6 1 (b) Hydrogen / H2 1 (c) potassium hydroxide / KOH 1 (d) sulfuric acid / H2SO4 1 2 Total = 6 (a) Alkanes CnH2n+2 (allow for error carried forward) 1 1 (b) Propane Mr (C3H8) = (12x3)+(1x8) = 44 (allow for error carried forward) 1 1 1 (c) C3H8 + 5 O2 → 3 CO2 + 4 H2O (No mark if any mistake in the equation) (allow for error carried forward) 1 3 Total = 4 (a) atom nucleon number proton number number of protons number of electrons number of neutrons magnesium 24 12 12 12 12 fluorine 19 9 9 9 10 1 1 (b) • electrons are correctly drawn, including the electrons transferred • the charges are correct • correct ratio of ions All 3 points met [2] 1-2 points met [1] 2

3 Paper 4 Section B (16 marks) 4 Total = 8 (a) Burette (no mark for spelling mistake) 1 (b)(i) (b)(ii) • all points correctly plotted [1] • straight line of best fit correctly drawn [1] o must have same number of points on both sides of the line 39oC (±1oC) 2 1 (c) (i) (ii) KOH (aq) + HCl (aq) → KCl (aq) + H2O (l) (No mark if any mistake) H+ + OH- → H2O 1 1 (d) Amphoteric oxide ZnO or Al2O3 or PbO 1 1 5 Total = 8 (a)(i) Group I 1 (a)(ii) Store in oil 1 (a)(iii) 2X + 2H2O → 2XOH + H2 1

4 (b)(i) [1 mark for correct labelling] Name of method: Filtration 1 1 (b)(ii) The ions in solid silver chloride are held in fixed position, so they cannot move around to conduct electricity. 1 (b)(iii) The ions change from vibrating slowly in fixed position to sliding past each other. The ions change from very closely packed and orderly arranged to closely packed and disorderly arranged. 1 1 6 Total = 8 (a)(i) Limestone (accept calcium carbonate) 1 (a)(ii) To remove acidic impurities 1 (b)(i) An alloy is a mixture of a metal and other element(s). 1 (b)(ii) St