2025 4E Dunman Sec ScChem P3_2025 MS

pg. 1 Dunman Secondary school 2025 Science Prelims Sec 4 express/5NA Answers Paper 3 Question 1 1ai B [1] aii F and G [1] aiii C and F/G [1] aiv A [1] bi Isotopes are atoms of the same element with a different number of neutrons, but same number of protons and electrons. [1] bii D and E [1] Question 2 ai 280°C aii In solids, the geosmin particles are vibrating about their fixed position but in a liquid, geosmin particles are sliding across each other. This sample of geosmin was heated from 25°C to 100°C. Using the axes below, draw the heating curve for this sample of geosmin. • To label 25°C and100°C, and draws correctly shaped graph at these temperatures [1] • substance to melt around 78°C, horizontal line at 78oC(labelled) [1] Temperature / °C Time / min 25 100 78

pg. 2 Question 3 3a • Solid sodium chloride exists as a giant lattice structure with ions vibrating about fixed positions due to strong electrostatic forces of attraction [1] • hence no mobile ions and cannot carry charge [1] b • ammonia gas exist as simple covalent molecules held together by weak intermolecular forces of attraction. [1] • hence, a little heat energy is required to overcome these forces of attraction. [1] 3ci NB: Award 1M for correct charges Award 1M for correct electron arrangement 3cii NB: Award 1M for correct number of atoms Award 1M for correct electron arrangement Question 4 4a A: Zinc nitrate [1] B: Zinc hydroxide [1] C: ammonia gas [1] D: aqueous ammonia [1] 4b [1] equation +balancing, [1] state symbols Possible answers: Zn(NO3)2 (aq)+ 2NaOH (aq)→ Zn(OH)2 (s) + 2NaNO3 (aq) Question 5 5a Initial: 19.0 Final: 28.0 Change: 9.0 NB: 1m- 1 or 2 correct 2m- all 3 correct 5b • An exothermic reaction [1] • occurred as temperature of the surroundings increased. [1] 5c H+ (aq) + OH- (aq) → H2O (l) [1] Question 6

pg. 3 6a 4 6b • Lemongrass is not pure / is a mixture and citronellol oil is pure.[1] • Lemon grass contains citronella and one other component. [1] 6c C10H20O [1] Question 7 7aii Reddish brown liquid 7bi 2KI + Cl2 → 2KCl + I2 NB: 1M for correct chemical formula, 1M for balanced equation 7bii • Solution changes colour from colourless to brown. [1] • A black solid is deposited at the bottom of the test tube. [1] 7biii • Chlorine gas is reduced as it gained one electron to form chloride ions.[1] • Iodide ion is oxidised as it lost one electron to form iodine. Since both oxidation and reduction is occurring at the same time, the reaction is a redox reaction. [1] Question 8 8a No. moles of FeCO3 = 2.32 / [56 + 12 + 3(16)] = 0.02 mol [1] 8b No. moles of CO2 = 0.02 mol [1] Volume of CO2 = 0.02 x 24 = 0.48 dm3 [1] 8c No. moles of FeCl2 = 0.02 mol Mass of FeCl2 = 0.02 mol x [56 + 2(35.5)] = 2.54 g [1] 8d Concentration = 𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒎𝒐𝒍𝒆𝒔 𝒗𝒐𝒍𝒖𝒎𝒆 = 𝟎.𝟎𝟒 𝟓𝟎/𝟏𝟎𝟎𝟎 = 0.8 mol/dm3 [1] Question 9 9a Road vehicles 9b • The high temperature in internal c