NAS 2026 Chem Prelim P1 + P2 MS

3 Ngee Ann Secondary 2026 4E Chemistry 6092 Prelim Answers for 2026 4E Chemistry Paper 1 1 2 3 4 5 6 7 8 9 10 B B D C B D A C A B 11 12 13 14 15 16 17 18 19 20 C D B D B A B A B C 21 22 23 24 25 26 27 28 29 30 B B D C C D C B B A 31 32 33 34 35 36 37 38 39 40 C C B B C A D A B D Paper 2: Structured [70 marks] 1a trend down the group only true for Group 1 only true for Group 17 true for both The number of electron shells increases.  The density increases.  The reactivity increases.  The colour of the elements darken.  1 mark for every 2 ticks [2] 1b Fluorine is more reactive than iodine [1], and displaces iodine / iodide ions from potassium iodide , forming brown aqueous iodine / iodine solution [1] and potassium fluoride. OR Fluorine oxidises potassium iodide to form aqueous iodine while it is reduced to form potassium fluoride. [1] The aqueous iodine produced is a brown solution. [1] [2] Marker Comments for Question 1: Part (a) is not done well. Most students had the misconception that only Group 17 elements experience an increase in density down the group. A similar trend is observed for Group 1. Part (b) is also not well done. Many students provided incorrect statements such as “fluorine is more reactive than iodide”. Mark is not awarded if student say that aqueous iodine is red-brown. 2ai Both H11 and H12 have the same number of protons / 1 proton but have different number of neutrons / H11 has 0 neutron while H12 has 1 neutron. No need for students to list the number of protons and neutrons. [1] 2aii They have the same number of valence electrons / They have 1 valence electron. [1] 2bi Add aqueous sodium hydroxide and warm / heat gently. [1]

4 Gas evolved turns moist red litmus blue. 2bii The relative molecular mass / Mr of ND3 is higher than that of NH3, hence ND3 diffuses slower than NH3. ND3 would still diffuse faster than HCl. ND3 would react with HCl in a position nearer to the left compared to the position of ammonium chloride. 1 mark for correct position of ND4Cl on diagram [shifted to the left, but not beyond the middle of the tube] 1 mark for explanation [2] Marker Comments for Question 2: For part (a)(i), some students are still unable to define isotopes. For part (a)(ii), many are unable to identify that the number of valence electrons are responsible for the chemical properties of elements. Many incorrectly gave protons and electrons as their answer. Part (b)(i) is poorly done. Many either fail to add sodium hydroxide or warm the reaction mixture before testing for the gas. Part (b)(ii) is generally ok. Many students are able to predict the position of ND4Cl and correct explain the reason. 3a Magnesium is more reactive than zinc, which is more reactive than copper. [1] Show correct ranking of reactivity: Mg > Zn > Cu The more reactive a metal, the higher the thermal stability of / the more thermally stable, the metal carbonate , hence the metal carbonate took a longer time to decompose OR [2]

5 The less reactive a metal, the lower the thermal stability of / the less thermally stable, the metal carbonate , hence the metal carbonate took a lesser time to decompose [1] 3bi Mass of carbon dioxide = 2.00 – 1.29 = 0.71 g [1] 3bii 2.00 g Sodium carbonate is thermally stable and does not decompose. [1] Marker Comments for Question 3: For part (a), many students, in some part of their answers, interchange metals and metal carbonates. For instance, they incorrect mention that one metal carbonate is more reactive than another. Or incorrectly stating that one metal is more thermally stable than another. Hence, they were not awarded the mark. For 3(b)(i), most students were able to calculate the mass of carbon dioxide. Those who got this question wrong used the values for magnesium carbonate or zinc carbonate instead of copper(II) carbonate. Part 3(b)(ii) is poorly done. Only a small handful of students remembered that sodium carbonate is stable to heat and will not decompose. 4a Any two: • same general formula • same functional group • same chemical properties • show a gradual change in physical properties as the number of carbon atoms per molecule increase • each member in the series differs from the earlier member by a CH2 group. [2] 4b As molecular size of aldehyde increases, the strength of intermolecular forces of attraction between the molecules increases. [1] More energy is required to overcome the stronger intermolecular forces of attraction between bigger molecules, resulting in a higher boiling point. [1] [2] 4ci Acidified potassium manganate(VII) [1] 4cii Purple solution turns decolourises / colourless [1] 4d Condensation reaction [1] 4ei Propanal and Compound 1 are isomers. explanation They have the same molecular formula (C3H6O), but different structural formula. [2] 4eii • Add magnesium metal / carbonate (accept any suitable metal or carbonate) Compound 2 will show effervescence. [1] • Add aqueous bromine Compound 1 will decolourise red-brown aqueous bromine / turn colourless [1] [3]

6 • Propanal will have no visible change with either magnesium / carbonate or aqueous bromine. [1] Do not accept: Use of acidified potassium manganate( VII) as both propanal and compound 1 will be oxidised to acid. Marker Comments for Question 4: Part (a) is not well done. Many students did not study for this simple recall question and did not get the mark. Many give molecular formula instead of general formula or simply mentioned the increase in boiling point instead of talking about a gradual change in physical properties as the number of carbon atoms increases. Part (b) is also not well done. Most students were unable to identify that the increasing size of the molecule was the reason for the increase in boiling point of aldehydes as the number of carbon atom increases. Also, a number of students incorrectly mentioned that the increase in boiling point is due to the increase in number of covalent bonds to be broken as number of carbon atoms increases. Part (c)(i) is also not well done. Many students forgot the need for an acidified solution. They also provided incorrect naming for the oxidising agent used. Part (d) is poorly done. Most students gave incorrect answers, where the most popular was condensation polymerisation. For 4(e)(i), many students misread the question and attempted to name the unknown compounds. Also, many did not know the definition for isomers. Part 4(e)(ii) is not well done. Some students were able to test using metal / carbonate and aqueous bromine but were unable to describe that propanal will show no visible change for both tests. Also, many used KMnO4 to test of propanal. 5a When the particle size is small [], the total exposed surface area in contact with reactant particles is larger. [] A catalyst provides an alternative pathway with a lower activation energy. [] This results in increased number of reactant particles colliding with energy greater than or equal to the activation energy. [] Together, this increases the frequency of effective collisions between particles, therefore rate of reaction increases. [] 2 [] – 1 mark 3 to 4 [] – 2 mark 5 [] – 3 marks [3] 5b A temperature of 800°C has higher percentage yield of NO. OR A temperature of 900°C has lower percentage yield of NO. [1] However, a temperature of 900 °C has a higher / faster rate of reaction OR A temperature of 800 °C has a lower / slower rate of reaction [1] [2]

7 5c 1 mark for correct shape and formula of reactants and products 1 mark for correct labelling of activation energy and enthalpy change Marks penalised for double-sided arrows, or arrows that were “floating” in the middle. [2] 5d Nitrogen monoxide reacts with oxyge