2025 TKGS Chemistry Prelim Paper 1 MS

2025 S4 Preliminary Examination Suggested Solutions Tanjong Katong Girls’ School 1 Answer Key 1 2 3 4 5 6 7 8 9 10 C B C D C A C D C D 11 12 13 14 15 16 17 18 19 20 D B D B B D B B A A 21 22 23 24 25 26 27 28 29 30 A D A B A C C D B C 31 32 33 34 35 36 37 38 39 40 A C A A D C B A B D 1 C Why C: To make a dry acidic gas like SO₂ from solid + acid, you need a delivery tube → drying agent that doesn’t react with SO ₂ (conc. H ₂SO₄ or anhydrous CaCl₂) → downward delivery (SO₂ is denser than air). Why not A/B/D: A: Basic CaO will react with acidic SO2 gas. B: Using the filter funnel will allow the SO 2 gas to escape. D: Upward delivery method is used for gases less dense than air. 2 B Why B: Iodine is a substance that sublimes (changes directly from solid to purple vapour without melting). Sodium chloride (NaCl) does not sublime; it has a very high melting point and remains solid. By heating the mixture gently, iodine vapour can be driven off, collected and cooled back into solid iodine crystals. Sodium chloride stays behind as a solid. Why not A/C/D: A: Both iodine and sodium chloride are solids, not dissolved in a solution, so crystallisation is not suitable. C: Works only to separate insoluble solid from liquid, not two solids. D: Used to separate liquids with different boiling points, not solid mixtures like this. 3 C Step 1: Interpreting statement 1 Z shows four distinct spots on the chromatogram. That means Z contains at least four different components. It cannot be “no more than four” because there could be more that did not separate under the given solvent conditions. So, statement 1 must be “Z contains at least four alcohols”. Step 2: Interpreting statement 2 If spots from X, Y and Z line up at the same Rf values, that suggests those mixtures may contain a common alcohol. Therefore, it is possible they share the same substance. So, statement 2 = “X, Y and Z could contain the same alcohol”. 4 D Statement 1: Molecules gain kinetic energy and temperature increases. Wrong. During melting, the temperature stays constant at 0 ºC. The particles don’t gain kinetic energy; instead, energy goes into overcoming intermolecular forces of attraction. Statement 2: Energy is gained to overcome the intermolecular forces of attraction. Correct. Heat energy supplied during melting is used to overcome intermolecular forces of attraction. Statement 3: Molecules gain sufficient energy to move from fixed positions. Correct. Solid ice particles are in fixed positions. When melting, they break free and can slide past each other. 5 C The lighter the gas, the faster it diffuses. Cl₂: 35.5 × 2 = 71 CO₂: 12 + (16 × 2) = 44 CH₄: 12 + (1 × 4) = 16 NO₂: 14 + (16 × 2) = 46 The smallest Mr = 16 (CH4). Hence, CH4 diffuses the fastest.

Tanjong Katong Girls’ School 2 6 A  A Proton charge = +1 Electron charge = –1 Equal magnitude, opposite signs.  B Neutron has 0 charge, not +1.