2025 AMKSS EOY 3E SCI CHEM P1 P3 MS

1 END-OF-YEAR EXAMINATIONS 2025 SECONDARY THREE EXPRESS SCIENCE (CHEMISTRY) ANSWER SCHEME Paper 1 1 A 11 D 2 C 12 D 3 C 13 A 4 B 14 B 5 C 15 D 6 D 16 B 7 B 17 D 8 C 18 C 9 A 19 B 10 D 20 C

2 Paper 3 Overall -1M for no unit, -1M for wrong spelling, -1M for presentation eg: using pen for graph, not using ruler Qn Answer Remarks 1a Argon [1] 1b aluminium oxide [1] 1c sulfur dioxide [1] 1d Ammonia OR calcium oxide [1] 1e aluminium oxide [1] 2a The heat taken in is used to overcome the attractive forces between the ethanol particles [1] to change the particles from being closely packed to far apart / from sliding past one another to moving at high speed in any or all directions [1]. R: “particles” were not mentioned in the discussion. 2b The liquid insecticide in the container takes in heat energy from the surrounding [1] through the nozzle to overcome the attractive forces between the liquid insecticide particles [1] to change from closely packed arrangement to being far apart / from sliding past one another to moving at high speed in any or all directions [1]. R: “particles” were not mentioned in the discussion. 2c 1. Gaseous particles spaced are far apart, 2. with big space for them to move closer together, 3. Air can thus be compressed. (credit awarded only if pt 1/2 given) 4. In water, the liquid particles are closely packed, 5. with little space for particles to move closer to one another. 6. Water cannot be compressed. (credit awarded only if pt 4/5 given) 6 points – 3 m 4 points – 2 m 2 points – 1m Reject: particles/space can /cannot be compressed Note: No full credit if idea of “particles can / cannot move closer together” is missing. 3a T [1] It has an electronic configuration of 2,8,8. So its valence shell are fully filled [1], so it does not gain or lose or share electrons. 3b P and R [1] They have the same number of protons, but different number of neutrons.[1`] P has 22 neutrons, whereas R has 20 neutrons.

3 3ci S2+ [1] 3cii SBr2 [1] 4a C [1] 4b E [1] 4c B [1] 4d A [1] 5a stage 1: C + O2 → CO2 [1] stage 2: C + CO2 → 2CO [1] 5bi Average mass [1] of one molecule of a substance when compared with 1/12 of the mass of a carbon-12 atom. [1] 5bii Mr of Fe2O3 = (56x2) + (16x3) = 160 [1] 5biii No. of mol of Fe2O3 = 320 / 160 = 2 mol [1] Allow ecf 5biv Volume of gas = 6 x 24 = 144 dm3 [1] Allow ecf 6ai Fractional distillation [1] 6aii Filtration [1] 6aiii Simple distillation [1] 6bi Paper Chromatography / Chromatography [1] 6bii Four different dyes [1] 6biii 3 correct spots [1m] and correctly labelled [1m] 7a Hydrogen ion / H+ [1]

4 7bi No. of moles of NaOH = 0.06/ 40 = 0.0015 mol [1] 7bii No. of moles of C4H6O5 = (0.0015 ÷ 2) mol = 0.00075 mol [1] Mr of C4H6O5 = [4(12) + 6(1) + 5(16)] = 134 [1] Mass of C4H6O5 = 0.00075 x 134 = 0.1005 g [1] Allow ecf 7biii n C4H4O5Na2 = 0.00075 mol [1] Mr of C4H4O5Na2 = [4(12) + 4 + 5(16) + 2(23)] = 178 [1] mass of C4H4O5Na2 produced = 0.00075 X 178 = 0.1335 g = 0.134 g [1] Allow ecf 8ai Hydrogen chloride: covalent bonding sodium chloride: ionic bonding [1] 8aii [1] for correct sharing of electrons [1] for correct number of unshared electrons Minus [1] if draw all electrons 8aiii [1] for correct arrangement of electrons [1] for correct number of ions and charges Minus [1] if draw all electrons H Cl X X X X X X X Cl X X X X X X X Na + -

5 8bi There are strong electrostatic forces of attraction between the sodium and chloride ions [1] in the crystal lattice. For hydrogen chloride, there are weak intermolecular forces of attraction between the hydrogen chloride molecules [1]. Therefore, more energy is required to overcome [1] the stronger forces of attraction for sodium chloride, compared to that in hydrogen chloride. 8bii In the solid state, the ions are held tightly in their fixed positions [1]. When in molten state, the presence of free, mobile ions [1] allow them to conduct electricity. 9a A: condenser [1] B: fractionating column [1] 9b the glass beads help to increase the surface area for condensation of vapour [1] Vapours with not enough energy / has not yet reached its boiling point will condense and go back into the mixture of liquids [1] OR • large surface area for condensation of vapours [1] • higher boiling vapours condense more readily and flows back into the flask as liquid while the lower boiling vapours continue to rise up the column [1] 9ci The amount of distillate collected will be lesser [1]. 9cii Apparatus A is not fully filled with cold water, so there will not be maximum condensation of vapour [1]. 9d Ensure smooth boiling [1] 9ei CaCO3 + 2 HCl → CaCl2 + H2O + CO2 [1] for correct chemical formula [1] for correct balanced chemical equation 9eii Gas Syringe [1] 10a 4.5 mS [1] Reason: This is because nitric acid has the same concentration of H+ ions concentration as hydrochloric acid. [1] 10b Sulfuric acid has two hydrogen atoms in its formula while nitric acid and hydrochloric acid each has only hydrogen atom in its formula. [1] Hence sulfuric acid produces more /twice the hydrogen ions when dissolved in water. [1] 10ci Ca(OH)2 (aq) + H2SO4 (aq) → CaSO4 (s) + 2H2O (l)

6 1m for formula; 1 m for balanced & ss 10cii Lesser H+ ions present / lesser ions are present. [1] 10di Name: carbon dioxide [1] Test: gas forms white precipitate in limewater [1] 10dii Water [1]