2025 JYSS Pure Chem Prelim P2 MS

6092 Prelim 2025 Answer Key Paper 1 B 1-5 6-10 11-15 16-20 21-25 26-30 Section A Qn Answer Marks Remarks 1a Al(NO3)3 1 Common mistake: Zn(NO3)2 b NO2 1 c PbO 1 Common mistake: MgO d PbO / NO2 1 Poorly done, lack awareness of amphoteric oxides reactions. e Na2SO4 1 2a Rf = 4/8 = 0.5 1 b Frozen spinach is less fresh compared to spinach from the market. Frozen spinach contains phaeophytin which means the chlorophyll is ‘older’ compared to spinach from the market which does not have this compound. 1 1 Well done c The extract from frozen spinach contains a total of 4 components. The unknown component is the most soluble / Xanthophyl is the least soluble / (students can also list the components in increasing/decreasing order of solubility) 1 1 1st marking point was largely missed out by students even though the question stated the requirement explicitly. Students did not answer the question. Reject: the components have different solubilities (too simplistic) Common mistake for 2nd marking point: The extract from frozen spinach is most/very soluble 3a • Lack of a central positive nucleus. / the positive charges are scattered in the atoms instead of localized in a nucleus. • Absence of protons • Absence of neutrons. • Electrons should orbit in electron shells around the nucleus. Any 3 points above. Accept any other logical answers. 3 Generally well done with a variety of expressions accepted as long as concept of atomic structure is shown.

6092 Prelim 2025 Answer Key Qn Answer Marks Remarks b The positive ion formed should have 10 electrons instead of 11 electrons. The ion should have 11 protons instead 10 protons since the original neutral atom has 11 electrons. 1 1 Many students misinterpreted the question and missed the highlighted words in the question which emphasized that this is an ION formed by the atom in Fig 3.1. Which means that there is nothing wrong with the charge of the ion. However, credit is still given if the student presents a clear argument of how the number of protons and electrons in Fig 3.2 should give rise to a negative charge. Several wrong answers treated the ion as an atom and wrong stated that the number of electrons and protons should be the same. Several students again did not answer the question which stated clearly that the argument should be on the number of subatomic particles. So these students described how the arrangement and placement of the electrons and protons are wrong, which is a repeat of 3(a) and no marks are awarded. c X is negatively charged because it is attracted to the positive plate. Y has no charge/is neutral because it is not affected by the positive and negative plates / remained in the straight path of motion. Z is positively charged because it is attracted to the negative plate. 1 1 1 Well done. 4a Bonding: Both white and black phosphorus consist of strong covalent bonding between the atoms. OR 1 phosphorus atom is covalently bonded to 3 other phosphorus atoms. Structure: 1 Quite well done. Students should avoid using giant lattice structure for giant covalent structure to avoid confusion with giant ionic lattice.

6092 Prelim 2025 Answer Key Qn Answer Marks Remarks White phosphorus exists as simple covalent molecules with weak intermolecular forces of attraction between the molecules. However, black phosphorus exists as a giant covalent structure. 1 1 bi Correct sharing of electrons with 4 hydrogens Correct representation of an ion (square brackets with correct charge) 1 1 Some students drew 2 separate ions and did not draw a parallel from the familiar NH4+ ion. bii true false Sodium phosphide is likely to be insoluble in water. ✓ Phosphorus trioxide is likely to be a poor conductor of electricity. ✓ Phosphine is likely to be a gas at room temperature ✓ 3 correct – 2m 2 correct – 1m 2 Quite well done. 5ai Any alkali. Eg NaOH, KOH, aq ammonia 1 Quite well done. Question asked for a name but some students gave a chemical formula. No

6092 Prelim 2025 Answer Key Qn Answer Marks Remarks deduction this time but it may result in deduction in O’levels. aii The revealer which is an alkali, requires water for ionization to produce hydroxide ions which gives alkaline properties. Without the presence of hydroxide ions released, the pH will start to drop below 10, causing the indicator to revert back to colourless. 1 1 Very poorly done. Only one student remembered that water is required for ionization of alkali. bi ink number invisible ink revealer reason for choice 1 potassium iodide lead(II) nitrate lead(II) iodide which is yellow in colour is formed. 2 potassium iodide aqueous chlorine aqueous iodine which is brown in colour is formed Correct pair (revealer and ink can be interchanged)– 1m Correct reason – 1m 2 Poor understanding of the importance of solubility caused errors here. Many wrongly selected insoluble solids which cannot be used as invisible ink and revealer. Many wrongly selected sodium hydroxide to pair with copper(II) oxide thinking there will be an exchange of ions to produce a blue precipitate of copper(II) hydroxide. But copper(II) oxide is insoluble. It can’t release ions for reaction with sodium hydroxide. There is no reaction between a base and an alkali. bii 2KI + Pb(NO3)2 → PbI2 + 2KNO3 OR 2KI + Cl2 → 2KCl + I2 Correct formula Correct balancing 1 1 Quite well done. 6a Yttrium Sc is in period 4 and Y is in period 5. That means Y has one more electron shell than Sc which gives it a slightly large atomic size. Note: Award 1m for answers that deduce the size from atomic size trend down the Group. However, this answer alone cannot EXPLAIN why Y has a bigger atom size than Sc. 1 1 All can answer that Yttrium has a bigger size but several reasons led to this question being not well done. Phrasing issues: • ‘Down the period’ is a strange phrasing. We usually say ‘down a Group’ or

6092 Prelim 2025 Answer Key Qn Answer Marks Remarks ‘across a period’ because a period is a horizontal row of elements. • It is wrong to say ‘Yttrium has one more valence shell than scandium.’ Because EVERY ELEMENT HAS ONLY ONE VALENCE (outermost) SHELL Conceptual issues: • Many students attributed the increase in atomic size to the increase in the size of nucleus due to the higher number protons and neutrons. However, the size needs to include the electron shells too which are not mentioned in their answers. • Several students also attributed the increase in atomic size to the increase in number of electrons. However, electrons are so tiny, how can their numbers alone account for a change in atom size? b Good conductor of electricity Good thermal conductor Silvery Any 2 of above or any other reasonable answers. 1 1 Students should take note that metals are not merely conductors of electricity. A better answer would be: They are GOOD conductors of electricity. → not penalized this time. ci Redox reactions 1 Poorly done with wrong answers such as oxidation, combustion etc. cii Cerium oxide lowers the activation energy of the reactions by providing an alternative pathway of reaction. More frequent effective collisions occur between reactant particles, leading to a higher rate of reaction. 1 1 Generally well done. ciii The crushed cerium oxide reduces particle size to expose more surface area for reaction to take place even faster. 1 Well done.

6092 Prelim 2025 Answer Key Qn Answer Marks Remarks 7a element oxidation state in NaAu(CN)2 carbon +2 gold +1 nitrogen −3 sodium +1 3 correct = 2m 2 correct = 1m 2 A group of students strangely obtained +4 fo