LVSS PRELIM 2023 4NA SCI CHEM ANSWER KEYY

Mark Scheme for LVSS PRELIM 4NA Science Chem 2023 Paper 3 Paper 4 Qns Ans Marks Comments 1 a B 1 b H has full valence/outermost shell OR noble gas configuration OR octet configuration 1 c G 1 2 a salt formula relative formula mass, Mr copper(II) sulfate CuSO4 160 magnesium nitrate Mg(NO3)2 24 + 2 x (14 + 16 x 3) = 148 [1] sodium sulfate Na2SO4 23 x 2 + 32 + 16 x 4 = 142 [1] Working is not required. 2 b 0.5 x 148 = 74 1 c i Acid: sulfuric acid Base: sodium hydroxide / sodium oxide Both answers must be correct to be awarded mark. 1 ii Heat to form saturated solution [1] Cool to allow crystals to form (Filter crystals), wash and dry crystals [1] 2 iii 7 1 3 a description name of substance a greenhouse gas carbon dioxide causes acid rain nitrogen dioxide a noble gas argon 2 1 2 3 4 5 6 7 8 9 10 A B D C B B D C C A 11 12 13 14 15 16 17 18 19 20 B B B B A C B A C C

3 correct: [2] 1-2 correct: [1] b i Combines with haemoglobin, Prevents red blood cell from carrying oxygen to body 1 ii Shortage of oxygen, incomplete combustion of carbon-containing fuel 1 4 a i To remove silicon dioxide / sand impurities 1 ii 1 Fe2O3 + 3 CO → 2 Fe + 3 CO2 1 b i oxygen, water / moisture ii Plastic coat [1] mass of nail after 10 weeks remain the same / no change in mass / no additional mass, nail did not rust [1] must mention data from table for 2nd mark 2 iii In steel, carbon atom is smaller / different size than iron [1] disrupts the regular arrangement of (iron) atoms, more difficult for layers (of atoms) to slide over each other [1] In pure iron, only iron atoms present, easy for layers to slide over each other Award 1 mark as long as comparison words are used. 2 c Limited amount left in nature / finite resource / cheaper / less wastage 1 5 a Two atoms chemically combined in a molecule 1 b Group VII: 7 electrons in its outermost / valence shell [1] Period 4: contains 4 electron shells [1] 2 c 1 pair of shared electrons: [1] total of 8 electrons in each outer-shell: [1]

d i 2 ✓ to get [1] halogen reaction with sodium chloride reaction with sodium bromide reaction with sodium iodide chlorine ✓ ✓ 1 ii Cl2 + 2 NaBr → Br2 + 2 NaCl OR Cl2 + 2 NaI → I2 + 2 NaCl correct formulae: [1] balanced equation: [1] 2 6 a All points plotted correctly: [1] Straight line passing through all the points: [1] 2 b -32 oC dotted lines must be shown on graph 1 c Same general formula; Each successive member differs by a -CH2 group; Same functional group; Similar chemical reactions Any one of the above 1 d i catalyst: nickel temperature: 200 oC Both must be correct 1

ii Test: add aqueous bromine to propane and propene (separately) [1] Observation with propane: reddish-b