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) (i) Not suitable. pH of the soil is 6.0, which is acidic / not alkaline. 1 (ii) Calcium hydroxide neutralizes the excess acids in the soil, making it alkaline. Both underlined points – 1m 1 (b) (i) 2 NaOH + (NH4)2SO4 → Na2SO4 + 2 H2O + 2 NH3 correct reactants + products – 1m balanced – 1m 2 (ii) Mr = 2(14+4) + 32 + 4(16) = 132 1

(ii) 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑙𝑒𝑠 = 𝑚𝑎𝑠𝑠 𝑀𝑟 = 0.5 𝑔 132 = 𝟎. 𝟎𝟎𝟑𝟕𝟗 𝒎𝒐𝒍 (𝟑𝒔. 𝒇. ) allow ECF from (ii) 1 (c) 1. Filter to remove sand / obtain solution/salt R as filtrate. 2. Evaporate/heat the filtrate/solution to dryness to collect crystal R. [1] OR Carry out crystallization to collect crystal R (all 4 steps for 2nd mark) - heat to saturated. - allow to cool for crystal to form. - filter to collect crystal. - wash with small amt of cold distilled water and dry between sheets of filter paper. 1 1 7 (a) The number of electron shells that an atom has determines its period number. 1 (b) (i) name: hydrogen positive test: lighted splint extinguishes with a ‘pop’ sound 1 1 (ii) Solution changes from green to purple/violet. rej. blue (LiOH is a strong alkali) (c) (i) correct height for each column + same width for each column – 1m (ii) R was coated with plastic, hence it did not rust. S was untreated and hence rusted. 1

(iii) 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑙𝑒𝑠 = 𝑚𝑎𝑠𝑠 𝐴𝑟 = 5.0 𝑔 56 = 𝟎. 𝟎𝟖𝟗𝟑 𝒎𝒐𝒍 (𝟑𝒔. 𝒇. ) 1 (iv) Any 1 from: - conserve finite resources - reduces land / air / water pollution - saves on cost of extracting metal from ores reject. cheaper than extracting metal from ores 1