Secondary 3 Combined Science Chemistry Materials Quiz

Secondary 3 Combined Science Quiz - Chemistry Materials (Answer Key)

Total Marks: 40

Section A: Multiple Choice Questions [10 marks]

1. Answer: B [1]
Explanation: An alloy is a mixture of two or more elements, at least one of which is a metal. It is not a pure substance (A), a compound (C), or a single element (D).

2. Answer: B [1]
Explanation: In metallic bonding, positive metal ions are fixed in a regular lattice, but the delocalised electrons are free to move throughout the structure. These mobile electrons carry electrical charge, making metals good conductors. Option A is incorrect because metal ions are not free to move in a solid. Option C confuses bonding with conduction mechanism. Option D is incorrect because the regular arrangement alone does not explain conductivity.

3. Answer: B [1]
Explanation: In brass, zinc atoms (different size from copper) disrupt the regular layers of copper atoms. This makes it harder for layers to slide over each other, so brass is harder and less malleable than pure copper. Alloys are generally harder than their constituent pure metals.

4. Answer: C [1]
Explanation: Nylon is a polyamide formed by condensation polymerisation (eliminating water). Polyethene, polypropene, and polystyrene are addition polymers formed from alkene monomers without elimination of small molecules.

5. Answer: B [1]
Explanation: The diagram shows amide linkages (-CO-NH-) in the polymer backbone. This functional group is characteristic of condensation polymers (polyamides or polyesters) formed from monomers with two functional groups, eliminating a small molecule (usually water).

6. Answer: B [1]
Explanation: Chromium in stainless steel reacts with oxygen to form a thin, invisible, adherent layer of chromium(III) oxide (Cr₂O₃) on the surface. This passive layer prevents further oxidation of the iron underneath, making stainless steel corrosion-resistant.

7. Answer: C [1]
Explanation: Ceramics are ionic or covalent network solids with high melting points, high hardness, and brittleness (they fracture rather than deform). They are generally electrical insulators (not conductors) and are not ductile.

8. Answer: B [1]
Explanation: Propene has the formula CH₂=CH-CH₃ (C₃H₆). Poly(propene) is formed by addition polymerisation of propene. Option A is ethene (makes polyethene), C is chloroethene (makes PVC), D is propenenitrile (makes poly(propenenitrile)).

9. Answer: C [1]
Explanation: Starch is a natural polymer (polysaccharide) made of glucose units. Nylon, polyethene, and polyester are all synthetic polymers.

10. Answer: B [1]
Explanation: Limestone (CaCO₃) decomposes to CaO, which reacts with acidic silicon dioxide (sand) impurity to form calcium silicate slag (CaSiO₃). This removes acidic impurities. It does not reduce the ore (coke/CO does), provide heat (coke combustion does), or act as a catalyst.

Section B: Structured Questions [18 marks]

11. (a) Metallic bonding [1]
Marking note: Accept "metallic bonds".

(b) Metals are malleable and ductile because the positive metal ions are arranged in regular layers that can slide over each other when a force is applied. The delocalised electrons act as a "glue" that holds the structure together, maintaining the metallic bonding even when the layers shift. [2]
Mark breakdown: 1 mark for layers sliding; 1 mark for delocalised electrons maintaining bonding/structure.
Common mistake: Saying "atoms slide" instead of "layers of ions slide" or not mentioning the role of delocalised electrons.

(c) As temperature increases, the positive metal ions vibrate more vigorously about their fixed positions. This increased vibration impedes the flow of delocalised electrons through the lattice, increasing resistance and decreasing electrical conductivity. [2]
Mark breakdown: 1 mark for increased vibration of ions; 1 mark for impeded electron flow/increased resistance.

12. (a) Duralumin contains copper, magnesium, and manganese atoms of different sizes from aluminium. These different-sized atoms disrupt the regular arrangement of aluminium ions in the lattice, preventing the layers from sliding easily over each other. This makes the alloy stronger and harder than pure aluminium. [2]
Mark breakdown: 1 mark for different-sized atoms disrupting regular layers; 1 mark for preventing sliding/increasing strength.

(b) Pure aluminium is too soft and not strong enough for structural components in aircraft. [1]
Alternative answers: Low tensile strength; deforms easily under stress.

(c) Low density / lightweight (or: good strength-to-weight ratio, corrosion resistance due to Al₂O₃ layer) [1]
Marking note: Any one relevant property for aircraft use.

13. (a) CH₂=CH-C₆H₅ (styrene/phenylethene) [1]
Marking note: Must show C=C double bond and phenyl group attached. Structural or displayed formula accepted.

(b) Polystyrene (or poly(phenylethene)) [1]

(c) Addition polymerisation [1]

(d) The polymer has a carbon-carbon backbone with strong covalent bonds that are not easily broken by microorganisms. There are no functional groups (like esters or amides) that can be hydrolysed by enzymes. [1]
Marking note: Key idea: non-biodegradable due to inert C-C backbone and lack of hydrolysable groups.

14. (a) -CO-NH- (amide linkage: carbonyl group bonded to nitrogen) [1]
Marking note: Must show C=O and N-H correctly connected. Displayed formula: -C(=O)-NH-

(b) Water (H₂O) [1]

(c) Condensation polymerisation involves monomers with two functional groups each (diamine and dicarboxylic acid) reacting to form a polymer chain with the elimination of a small molecule (water) at each linkage. In Nylon-6,6, the amine group (-NH₂) of one monomer reacts with the carboxylic acid group (-COOH) of the other, forming an amide linkage and eliminating water. [2]
Mark breakdown: 1 mark for two functional groups per monomer / step-growth mechanism; 1 mark for elimination of small molecule (water).

15. (a) High carbon steel. It has the highest carbon content (0.60–1.50%), which makes it the hardest and strongest, allowing it to maintain a sharp cutting edge. [2]
Mark breakdown: 1 mark for identifying high carbon steel; 1 mark for linking high carbon content to hardness/cutting ability.

(b) Stainless steel contains chromium (12–18%) which forms a thin, protective, adherent layer of chromium(III) oxide (Cr₂O₃) on the surface. This passive layer prevents oxygen and water from reaching the iron underneath, preventing rusting. Mild steel lacks chromium, so it forms porous iron(III) oxide (rust) that flakes off, exposing fresh iron to further corrosion. [2]
Mark breakdown: 1 mark for chromium forming protective oxide layer; 1 mark for contrast with mild steel rusting.

Section C: Data-Based and Extended Response Questions [12 marks]

16. (a) Graph plotting: [2]

• 1 mark: All 7 points plotted correctly (± half a small square)
• 1 mark: Smooth curve of best fit showing increase, plateau, then slight decrease
  Expected curve: Hardness increases steeply at first, then gradient decreases, plateauing around 1.25% C, then slight drop at 1.50% C.

(b) As carbon content increases from 0.10% to 1.25%, the hardness of steel increases. The rate of increase is greatest at low carbon contents and gradually decreases. Above 1.25% carbon, the hardness plateaus and then decreases slightly. [2]
Mark breakdown: 1 mark for overall increasing trend; 1 mark for describing plateau/decrease at high carbon content.

(c) At very high carbon content (>1.25%), excess carbon forms iron carbide (cementite, Fe₃C) which is very hard but brittle. The increased brittleness leads to microcracking, which reduces the measured hardness. Additionally, the formation of coarse pearlite and cementite networks can create stress concentration points. [2]
Mark breakdown: 1 mark for formation of brittle cementite/iron carbide; 1 mark for microcracking/brittleness reducing measured hardness.

(d) Approximately 0.60–0.65% [1]
Marking note: Accept 0.58–0.68% (reading from graph). Must show evidence of using graph (e.g., horizontal line from 280 to curve, then down to x-axis).

17. (a) C(s) + O₂(g) → CO₂(g) [1]
Marking note: State symbols not required but accepted. This is coke combustion at the bottom of the furnace (~1500°C).

(b) Fe₂O₃(s) + 3CO(g) → 2Fe(l) + 3CO₂(g) [1]
Marking note: Accept Fe(s) or Fe(l). This is the main reduction step in the middle zone (~700°C).

(c) CaCO₃(s) → CaO(s) + CO₂(g) [1]
Marking note: Thermal decomposition of limestone in the upper-middle zone.

(d) CaO(s) + SiO₂(s) → CaSiO₃(l) [1]
Marking note: Slag formation. Accept CaSiO₃(s) or (l). This occurs in the lower-middle zone.

(e) Molten iron is denser than molten slag, so it sinks to the bottom of the furnace while the less dense slag floats on top. [1]
Marking note: Key concept: density difference causes separation.

18. (a) HDPE has a linear structure with little branching, allowing polymer chains to pack closely together in a regular arrangement. This close packing increases the number of contact points between chains, resulting in stronger intermolecular forces (van der Waals forces) per unit volume. LDPE has many branches that prevent close packing, so chains are further apart with weaker intermolecular forces, giving lower density. [3]
Mark breakdown: 1 mark for linear vs branched structure; 1 mark for close packing / more contact points; 1 mark for stronger intermolecular forces / higher density.

(b) HDPE is more rigid and has higher tensile strength, so it holds its shape well and can support the weight of milk without deforming – suitable for rigid milk bottles. LDPE is more flexible and softer, so it can be easily squeezed and returns to shape – suitable for squeeze bottles that need to be deformed by hand. [2]
Mark breakdown: 1 mark for linking HDPE properties to rigid bottle use; 1 mark for linking LDPE properties to squeeze bottle use.

19. (a) Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s) [1]
Marking note: State symbols preferred. Spectator ion SO₄²⁻ omitted. Accept full equation with state symbols.

(b) Magnesium > Zinc > Copper (most reactive first) [1]

(c) A metal cannot displace itself from a solution of its own salt / no displacement reaction occurs because magnesium is not more reactive than magnesium. [1]
Marking note: Key idea: no driving force for reaction; same element on both sides.

(d) Fe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s) [1]
Marking note: Iron in steel displaces copper. State symbols preferred.

20. (a) Plastic bottles in landfill take hundreds of years to decompose, occupying valuable land space. They can also fragment into microplastics that leach into soil and groundwater. [1]
Acceptable answers: Non-biodegradable / long decomposition time; land use; microplastic pollution; leaching of additives.

(b) Incineration of plastics releases carbon dioxide (a greenhouse gas) and potentially toxic gases such as hydrogen chloride (from PVC) or dioxins (from incomplete combustion). [1]
Acceptable answers: CO₂ emissions; toxic gases (HCl, dioxins, furans); air pollution; ash disposal.

(c) Recycling plastic bottles means the polymer is melted and reformed into new products, reducing the need to produce new polymer from crude oil. Since crude oil is the raw material for making monomers (like ethene) for polymerisation, recycling conserves this finite resource by keeping the carbon chain in use. [2]
Mark breakdown: 1 mark for recycling reduces need for new polymer production; 1 mark for crude oil as raw material for monomers/polymers.

(d) Contamination of bottles (food residue), mixed plastic types that are difficult to separate, collection and sorting costs, lack of consumer participation, or degradation of polymer quality after multiple recycling cycles. [1]
Acceptable answers: Any one valid reason: contamination, sorting difficulty, economic viability, polymer degradation, collection logistics.

End of Answer Key