Secondary 3 Combined Science Chemistry Materials Quiz

Secondary 3 Combined Science Quiz - Chemistry Materials

Answer Key

Section A: Multiple Choice Questions

1. (c) Melting point [1]
Marking note: Melting point is a physical property (can be observed/measured without changing the substance chemically). Flammability, reactivity, and ability to rust are chemical properties.

2. (a) W [1]
Marking note: Electrical wires require high electrical conductivity and high ductility (can be drawn into wires). Material W has both. Y has high conductivity but low ductility, making it unsuitable.

3. (b) Ionic bonding [1]
Marking note: NaCl is formed between a metal (Na) and a non-metal (Cl), resulting in transfer of electrons and ionic bonding.

4. (b) Diamond has a giant covalent structure with strong bonds in all directions, while graphite has layers that can slide over each other. [1]
Marking note: Full explanation required. Diamond: each carbon bonded to 4 others in a tetrahedral 3D network. Graphite: each carbon bonded to 3 others in hexagonal layers with weak forces between layers.

5. (c) Fibreglass [1]
Marking note: Fibreglass is a composite made of glass fibres embedded in a polymer resin. Pure copper, NaCl, and water are not composites.

6. (c) Polyethene [1]
Marking note: Addition polymerisation of ethene (C₂H₄) produces polyethene (polyethylene).

7. (c) It sets permanently when heated and cannot be remoulded. [1]
Marking note: Thermosetting plastics undergo irreversible chemical change on heating. Thermoplastics (options a and b) can be remoulded.

8. (b) Calcium carbonate [1]
Marking note: Carbonates react with dilute acid to produce CO₂ gas, which turns limewater milky. This is the standard test for carbonates.

9. (b) A lattice of positive ions surrounded by a sea of delocalised electrons. [1]
Marking note: Metallic bonding involves metal atoms losing outer electrons to form positive cations, with electrons free to move throughout the structure.

10. (d) Oxygen and water [1]
Marking note: Rusting is an electrochemical process requiring both oxygen and water (moisture). Neither alone is sufficient.

Section B: Short Answer and Structured Questions

11. [3 marks]

• Definition of alloy: An alloy is a mixture of a metal with one or more other elements (metals or non-metals). [1]
• Example: Steel (iron + carbon) / Bronze (copper + tin) / Brass (copper + zinc) — any valid example. [1]
• Reason: The alloy has improved properties compared to the pure metal, e.g., greater hardness / greater strength / better resistance to corrosion / lower melting point. [1]
  Marking note: Award the reason mark only if it is linked to a valid improvement. "It is cheaper" is acceptable if justified.

12. [4 marks total]

(a) Material D (or R — note: table uses P, Q, R; highest tensile strength is not directly in table, but R has High tensile strength and High corrosion resistance). [1] for correct choice + [1] for explanation.
Correction based on table: Material P has High tensile strength and High corrosion resistance — most suitable for a bridge cable. [1] Explanation: Bridge cables need high tensile strength to support large loads and high corrosion resistance to withstand weathering. [1]
Marking note: Accept R (High tensile strength, High corrosion resistance, Medium cost) as well. Award explanation mark for linking high tensile strength to load-bearing function.

(b) Material Q [1] — Low cost, Medium flexibility, suitable for disposable packaging. [1] Explanation: Disposable packaging needs to be cheap to produce (low cost) and flexible enough to mould into shape. [1]
Marking note: Award for identifying low cost as the key factor for disposable items.

13. [5 marks total]

(a) Mg + H₂SO₄ → MgSO₄ + H₂ [2]
Marking note: Award [1] for correct reactants and products, [1] for correct balancing. Accept state symbols but not required.

(b) The magnesium ribbon dissolves / bubbles of gas are produced / the test tube becomes warm. [1]
Marking note: Accept any one valid observation.

(c) Hydrogen gas is produced. [1] Test: Insert a lighted splint into the gas; hydrogen burns with a squeaky pop sound. [1]
Marking note: Award [1] for naming hydrogen, [1] for correct test and result.

14. [3 marks]

• In graphite, each carbon atom is bonded to three others in hexagonal layers, leaving one delocalised electron per atom that is free to move and carry charge. [1]
• In diamond, each carbon atom is bonded to four other carbon atoms in a tetrahedral arrangement, so all electrons are involved in covalent bonds and none are free to move. [1]
• Since electrical conductivity requires charged particles (electrons) that are free to move, graphite conducts but diamond does not. [1]
  Marking note: Award marks for identifying delocalised electrons in graphite, absence of free electrons in diamond, and linking mobility of electrons to conductivity.

15. [3 marks]
Advantages (any two):

• Lightweight / low density [1]
• Can be moulded into complex shapes easily [1]
• Resistant to corrosion / do not rust [1]
• Good electrical insulators [1]
• Cheap to produce in large quantities [1]
• Waterproof / do not absorb water [1]

Disadvantage (one):

• Non-biodegradable / cause pollution when disposed of [1]
• Derived from non-renewable resources (petroleum) [1]
• Can release toxic fumes when burned [1]
  Marking note: Award [1] per valid point, maximum [2] for advantages and [1] for disadvantage.

16. [3 marks]

• Galvanising is the process of coating iron or steel with a layer of zinc. [1]
• The zinc acts as a physical barrier, preventing oxygen and water from reaching the iron surface. [1]
• Even when the zinc coating is scratched, zinc is more reactive than iron, so zinc corrodes preferentially (sacrificial protection), protecting the iron underneath from rusting. [1]
  Marking note: Award [1] for describing the coating process, [1] for barrier protection, [1] for sacrificial protection explanation.

Section C: Data Interpretation and Extended Response

17. [4 marks total]

(a) Material D [1] — it has the highest mass at breaking point (30 kg), indicating the highest tensile strength.

(b) Material D [1] — A climbing rope must withstand the weight of a climber and sudden forces without breaking, so the material with the highest tensile strength is most suitable. [1]
Marking note: Award [1] for choosing D, [1] for linking high tensile strength to the need to support weight/withstand force.

(c) Any one of: length of material strip / width of material strip / thickness of material strip / temperature / rate of adding mass [1]
Marking note: Accept any variable that would affect the fairness of comparing tensile strength between materials.

18. [4 marks total]

(a) Ionic compounds have high melting points because the positive and negative ions are held together by strong electrostatic forces of attraction in a giant ionic lattice. [1] A large amount of energy is required to overcome these strong forces. [1]
Marking note: Award [1] for identifying strong electrostatic forces / ionic bonds, [1] for stating that much energy is needed to break them.

(b) In the solid state, ions are held in fixed positions in the lattice and cannot move freely, so they cannot carry charge. [1] When dissolved in water, the ions are free to move and can carry electrical charge through the solution, allowing conduction. [1]
Marking note: Award [1] for fixed ions in solid state, [1] for free/mobile ions in solution enabling conduction.

19. [3 marks total]

(a) In pure copper, the atoms are all the same size and arranged in regular layers that can slide over each other easily, making it soft. [1] In bronze, the tin atoms are a different size from copper atoms, which disrupts the regular arrangement and prevents the layers from sliding, making the alloy harder. [1]
Marking note: Award [1] for describing uniform layers in pure metal, [1] for explaining how different-sized atoms disrupt sliding.

(b) Bronze resists corrosion better than pure iron. Iron rusts easily in the presence of oxygen and water (especially saltwater), while bronze forms a protective layer and does not corrode as readily. [1]
Marking note: Accept any valid reference to corrosion resistance in a marine environment.

20. [5 marks total]

(a) [3 marks — 1 per correct row]

Marking note: Award [1] per correctly completed row. Accept "translucent" or "opaque" for polypropene. Accept "medium" or "high" for aluminium density.

(b) Recommendation: Polypropene [1]
Justification: It has low density (lightweight, easy to carry) and is recyclable (environmentally friendly). [1]
Alternative acceptable answer: Aluminium — recyclable and durable / provides good insulation when double-walled. Glass — transparent and recyclable but heavy and fragile.
Marking note: Award [1] for a valid recommendation, [1] for justification using at least two properties from the table or otherwise valid.

End of Answer Key