Secondary 1 Science Chemistry Materials Quiz

Secondary 1 Science Quiz - Chemistry Materials (Answer Key)

Total Marks: 40

Section A: Multiple Choice Questions (10 marks)

1. Answer: C [1]

• Explanation: Gold (Au) is a pure element found in the Periodic Table. Air is a mixture of gases, carbon dioxide (CO₂) is a compound, and steel is an alloy (mixture of iron and carbon).

2. Answer: A [1]

• Explanation: Box X shows particles closely packed in ordered rows — this represents a solid with a regular structure, typical of metals. Box Y shows closely packed but random arrangement (liquid), and Box Z shows widely spaced particles (gas).

3. Answer: B [1]

• Explanation: Iron is magnetic while sulfur is not. Magnetic separation uses a magnet to attract iron filings, leaving sulfur behind. Filtration separates insoluble solids from liquids, distillation separates liquids by boiling point, and evaporation separates dissolved solids from solution.

4. Answer: D [1]

• Explanation: At room temperature (25°C), a substance is a gas if its boiling point is below 25°C. Substance S sublimes at -78°C (boiling point -78°C), so it is a gas at 25°C. P (ethanol) and Q (water) are liquids; R (NaCl) is a solid.

5. Answer: B [1]

• Explanation: Compounds have a fixed composition by mass (law of definite proportions). They cannot be separated by physical methods (A is false), do not retain properties of constituent elements (C is false), and are formed by chemical combination (D is false).

6. Answer: A [1]

• Explanation: The plateau at 0°C represents melting/freezing where solid and liquid coexist. At 100°C, liquid and gas coexist. The substance is water.

7. Answer: B [1]

• Explanation: Sand is insoluble in water and will be trapped as residue on the filter paper. Salt dissolves in water and passes through as filtrate.

8. Answer: A [1]

• Explanation: In solids, particles vibrate in fixed positions (least movement). In liquids, particles slide past each other. In gases, particles move freely and rapidly (greatest movement).

9. Answer: C [1]

• Explanation: Alloys like brass are mixtures, not compounds (A, D false). They cannot be separated by filtration (B false). Alloys have properties different from their constituent metals (e.g., brass is harder than copper or zinc).

10. Answer: D [1]

• Explanation: Rf = distance moved by spot / distance moved by solvent front. Z travels 8 cm (highest), so it has the highest Rf value (8/10 = 0.8).

Section B: Structured Questions (20 marks)

11. (a) Substance A [1] — It does not conduct electricity in solid or molten state, has a low melting point (-114°C), and is miscible in water. These are properties of a covalent molecular substance (e.g., ethanol). [1]

(b) Substance C [1] — It does not conduct in solid state but conducts when molten, has a high melting point (801°C), and is soluble in water. These are properties of an ionic compound (e.g., NaCl). [1]

(c) Copper has a metallic structure with a lattice of positive ions in a sea of delocalised electrons [1]. These mobile electrons can carry charge in both solid and molten states [1].

12. (a) Inks P and Q [1] — Mixture M shows spots at 2 cm, 3 cm, and 5 cm. These match the spots of Q (2 cm, 5 cm) and P (3 cm). R has a spot at 5 cm only, but M has three spots, so P and Q are present.

(b) Rf = 5/8 = 0.625 [2]

• Distance moved by spot = 5 cm [1]
• Distance moved by solvent front = 8 cm [1]
• Rf = 5/8 = 0.625 (no units) [1 for correct calculation]
• Accept 0.63 or 5/8

(c) Pencil is insoluble in the solvent [1]. Ink from a pen would dissolve and travel up the paper, contaminating the chromatogram.

(d) Ethanol is flammable [1]. Keep away from naked flames / use a water bath for heating / work in a well-ventilated area.

13. (a) Fractional distillation [1]

(b) To provide a large surface area for repeated condensation and vaporisation [1], allowing better separation of components with close boiling points.

(c) At 78°C, ethanol is boiling [1]. The temperature remains constant because the heat energy supplied is used to overcome intermolecular forces during the change of state from liquid to gas (latent heat), not to increase kinetic energy of particles [1].

(d) Water [1] — Water boils at 100°C and distils over after ethanol has been collected.

14. (a) Sublimation [1]

(b) Diagram [3] — Award marks for:

• Inverted funnel over watch glass/evaporating dish containing mixture [1]
• Cotton wool plug in funnel stem [1]
• Gentle heat source below, with ammonium chloride depositing on cool funnel sides and sodium chloride remaining in dish [1]

(c) Ammonium chloride sublimes (changes directly from solid to gas) when heated [1]. The vapour rises and deposits as solid on the cool funnel [1]. Sodium chloride does not sublime (high melting point) and remains in the dish.

15. (a) Substance Y [1] — It shows a metallic lattice with a 'sea of delocalised electrons' [1], which allows metals to conduct electricity and be malleable.

(b) Substance X [1] — It shows two different atoms in a regular alternating lattice [1], characteristic of an ionic compound with oppositely charged ions.

(c) Substance Z has a simple molecular structure [1]. Weak intermolecular forces (van der Waals forces) exist between molecules [1]. Little energy is needed to overcome these forces, so it has a low melting point.

Section C: Data-Based and Application Questions (10 marks)

16. (a) Graph [3] — Award marks for:

• Axes labelled with units: Temperature (°C) and Solubility (g/100g water) [1]
• Appropriate scale covering all data points [1]
• All 5 points plotted correctly and smooth curve drawn [1]

(b) ~87 g/100g water [1] — Accept 85–90 g (read from graph at 50°C).

(c) Mass crystallised = 112 g [2]

• At 80°C: 170 g dissolves in 100 g water → in 200 g water: 170 × 2 = 340 g [1]
• At 20°C: 32 g dissolves in 100 g water → in 200 g water: 32 × 2 = 64 g [1]
• Crystallised = 340 - 64 = 276 g? Wait, let me recalculate.
• Actually: 170 g/100g at 80°C, 32 g/100g at 20°C. For 200 g water:
  • At 80°C: 170 × 2 = 340 g dissolved
  • At 20°C: 32 × 2 = 64 g remains dissolved
  • Crystallised = 340 - 64 = 276 g [2]
• Award 1 mark for correct method if arithmetic error

(d) Higher temperature gives particles more kinetic energy [1], allowing solvent particles to overcome solute-solute forces more effectively and pull more solute into solution.

17. (a) 11 [1] — Proton number = number of electrons = 2 + 8 + 1 = 11.

(b) 2,8,1 [1] — Electrons in shells: 1st shell 2, 2nd shell 8, 3rd shell 1.

(c) Sodium (Na) [1] — Element with proton number 11.

(d) Dot-and-cross diagram [3] — Award marks for:

• Na⁺ ion with 2,8 configuration (empty 3rd shell or 8 crosses in 2nd shell) and + charge [1]
• Cl⁻ ion with 2,8,8 configuration (7 crosses + 1 dot in outer shell) and - charge [1]
• Clear indication of electron transfer (dot from Na to Cl) [1]

18. (a) In pure metals, atoms are arranged in regular layers [1]. These layers can slide over each other when force is applied, making metals malleable and ductile [1].

(b) Carbon atoms are smaller than iron atoms [1]. They disrupt the regular layers of iron atoms, preventing them from sliding easily over each other [1].

(c) High strength-to-weight ratio (strong but light) [1], corrosion resistance / durability [1]. (Accept: high strength, low density, good fatigue resistance)

(d) Bronze is harder and stronger than pure copper [1], making it more suitable for tools and weapons that need to hold an edge and withstand impact.

19. (a) Distillation [1] (or simple distillation)

(b) Seawater is heated in a flask [1]. Water evaporates/boils, leaving salts, sand, and microorganisms behind in the flask [1]. Water vapour enters the condenser, cools, and condenses into pure liquid water collected in a receiver [1].

(c) Percentage yield = 80% [2]

• Yield = (Volume collected / Initial volume) × 100% [1]
• = (80 / 100) × 100% = 80% [1]

20. (a) 80°C [1] — The plateau on a cooling curve represents the freezing point.

(b) At the plateau, particles are arranging into a regular, fixed structure (solid) [1]. Both liquid and solid states coexist; particles in the solid vibrate in fixed positions while liquid particles still move freely [1].

(c) During freezing, heat energy is released as particles form bonds and lose potential energy [1]. This latent heat of fusion released compensates for heat lost to surroundings, keeping temperature constant [1].

(d) A pure compound has a sharp, distinct plateau (constant freezing point) [1]. A mixture freezes over a range of temperatures (sloped cooling without a sharp plateau).