CHIJ 4NA Sci (Chem) Prelim 2023_ANSWER KEY. pdf

4NA Prelim 2023 Mark Scheme Paper 3 1 2 3 4 5 6 7 8 9 10 B B A B A D C C A B 11 12 13 14 15 16 17 18 19 20 C D B D A C D B A C Paper 4 Rounding error / not to 3s.f – 1m deduction for whole paper 1 substance description carbon dioxide B water B bitumen D steel C 1-2 correct – 1m 3 correct – 2m 2 2 (a) MgCl2 1 (b) A chlorine atom has 7 electrons, and gains one electron to obtain a fully-filled valence shell / have 8 electrons in its valence shell. rej. “borrow” 1 (c) • Magnesium chloride has a giant (ionic) lattice structure. • There are strong electrostatic forces of attraction between oppositely-charged ions. • Large amount of energy is needed to overcome these forces. 1-2 points – 1m 3 points – 2m rej. larger/ higher. Need to show the amount of energy (small / large) 2

3 (a) atmospheric pollutant source carbon dioxide combustion of fossil fuels sulfur dioxide volcano activity OR combustion of sulfur-containing fossil fuels 1 (b) (i) sulfur dioxide + water → sulfuric acid rej. Sulfur dioxide + water → acid rain (question asked for COMPOUND) no marks awarded for chemical equation. 1 (ii) Acid rain will corrode the Eiffel Tower. accept corrosion rej. react / destroy / damage (not specific enough) 1 (c) Carbon dioxide. It can be found in clean air. Name + reason – 1m 1 4 (a) alkene(s) reject spelling 1 (b) (i) most common error is that students drew ethene. Remind students that the alkene becomes an alkane after addition reaction. 1 (ii) Reddish-brown solution remains reddish-brown / no visible change. accept brown accept turns reddish-brown accept colourless to reddish-brown 1 (c) water and carbon dioxide / H2O and CO2 1

For Section B, if all 3 questions are done, mark 1st 2 questions (Q5 and Q6). Many students did all 3 questions. Remind students to only do 2 questions. Strike out the 3rd question that they did not do. 5 (a) (i) Mixture of compounds. The chromatogram for orangine shows 4 spots / more than 1 spot. 1 (ii) Chromatography. rej. spelling 1 (iii) The samples (orangine and tartazine) would dissolve in the water and there would be no chromatogram obtained. 1 (iv) No. Orangine contains tartrazine as there is a spot (3rd spot) in orangine that travelled the same distance as tartrazine / there is a spot that matches that of tartrazine. 1 (v) From the label: “1 part orangine with 4 parts water” 75 cm3 is combined with (4 x 75 = 300 cm3) of water. Total volume of orange drink is 75 + 300 = 375 cm3 1 (vi) Mr = 16(12) + 9(1) + 4(14) + 3(23) + 9(16) + 2(32) = 534 No units. No mark awarded if unit is given. (b) Arrangement: (very) closely packed in an orderly arrangement. Movement: vibrating in fixed positions 1m for each. 2 6 (a)