2023_HYSS_Prelim_ScChem_4NA_ANSWER KEY

1 Sec 4N Science (Chemistry) P3 and P4 Prelim Hua Yi Secondary School HYSS Prelim 2023 4NA Science Chemistry Mark scheme Paper 3 Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8 Q9 Q10 A A B D A A C A B A Q11 Q12 Q13 Q14 Q15 Q16 Q17 Q18 Q19 Q20 B C C A D B A B C C Paper 4 Qn Section A Mark 1a E120, E121 and E150 1 1b E121 that is more soluble in the solvent ( will move further up the filter paper than E120 that is less soluble in the same solvent). 1 1c Pen contains dyes which may/can dissolve in the solvent. This will interfere with the the results on the chromatogram. (Accept also: Pencils cannot dis solve in the solvent used in the experiment.) 1 Total 3 2a particle number of protons number of neutrons mass number electronic configuration A 6 6 12 2, 4 B 15 16 31 2,8,5 C 6 8 14 2, 4 D 11 12 23 2, 8, 1 1M for each horizontal row that is entirely correct. 3 b A and C 1 c Carbon 1 Total 5 3a 180; CaCO3 1 1 b Mass of CaCO3 = 0.2 X 100 g = 20 g [1] 1 Total 3 4a haematite / iron ore and coke reject Iron(III) oxide, carbon 1 b To remove (acidic) impurities such as silicon dioxide 1 c Fe2O3 (s) + 3 CO (g) → 2Fe (s) + 3 CO2 (g) 1 Total 3

2 Sec 4N Science (Chemistry) P3 and P4 Prelim Hua Yi Secondary School Section B 5ai S, U, X, T 1 aii Electrolysis R: Electricity 1 aiii Zn + H2O → ZnO + H2 (ignore state symbols) 1 b The unreactive metals were discovered earliest because they are found in uncombined state/ as elements in the ground. Moderately reactive metals are easy to extract as these metal compounds can be reduced by C or by strong heating. Metals that are more reactive tend to form (very) stable compounds and have to be extracted by electrolysis . Hence, these metals are discovered later. Any 2 points. Many students do not answer the question and/or con cept. Common wrong answers include “metals are discovered earli er because they are more common/abundant”. 2 ci 1m - correct points plotted 1m - straight line drawn 2 cii Melting point of potassium: (ans must be read off the graph) 1 Total 8

3 Sec 4N Science (Chemistry) P3 and P4 Prelim Hua Yi Secondary School 6a Y 1 b WZ2 (reject ans that are non expressed as W and Z) 1 ci It has 7 outermost/valence electrons. It has 4 electronic shells. 1 1 cii Note: This question is very poorly done. Question clearly asked for a molecule (i.e more than one atom) 1 di Sodium atom/ Na (2,8,1) loses 1 valence electron to form sodium ion/Na+ (2,8)/achieve a complete valence shell. Chlorine atom/ Cl (2,8,7) gains 1 valence electron to form chloride ion/Cl- (2,8,8) achieve a complete valence shell. 2 dii In the solid state, there are no mobile ions to conduct electricity as they are held in fixed positions in the giant ionic latttice structure. OR In the liquid/aqueous state, there are mobile ions to conduct electricity as the ions are no longer