Secondary 3 Combined Science Chemistry Materials Quiz

Secondary 3 Combined Science Quiz - Chemistry Materials

Answer Key

Section A: Multiple Choice Questions (10 marks)

1. C [1]

• Distilled water (H₂O) is a pure substance (a compound). Air, salt water, and milk are mixtures.

2. A [1]

• Sodium chloride has the chemical formula NaCl. Sodium has a valency of +1 and chlorine has a valency of -1.

3. B [1]

• Covalent bonding involves the sharing of electrons between atoms. Ionic bonding involves transfer of electrons; metallic bonding involves a sea of delocalised electrons.

4. B [1]

• Carbon dioxide (CO₂) is a compound made of two different elements chemically bonded. O₂, Au, and N₂ are elements.

5. B [1]

• A neutral solution has a pH of 7 at 25°C. Values below 7 are acidic; values above 7 are alkaline.

6. B [1]

• Hydrochloric acid (HCl) is an acid. Sodium hydroxide is a base; distilled water and sodium chloride solution are neutral.

7. B [1]

• The reaction between an acid and a base is called neutralisation. It produces a salt and water.

8. C [1]

• Metals are malleable (can be hammered into shape). They are also good conductors of electricity and heat, and have a shiny appearance.

9. B [1]

• The atomic number of carbon is 6, which equals the number of protons in the nucleus.

10. C [1]

• Sulfur is a non-metal. Iron, copper, and aluminium are all metals.

Section B: Structured Questions (25 marks)

11. (a) An element is a pure substance that cannot be broken down into simpler substances by chemical means. [2]

• Award 1 mark for "pure substance" and 1 mark for "cannot be broken down by chemical means."

(b) Two differences between a compound and a mixture: [2]

• Difference 1: A compound is formed by chemical bonding, while a mixture is formed by physical mixing. [1]
• Difference 2: A compound has a fixed composition, while a mixture can have variable composition. [1]
• Alternative acceptable answers: Compounds can only be separated by chemical methods; mixtures can be separated by physical methods. Compounds have properties different from their constituent elements; mixtures retain the properties of their components.

12. (a) 11 protons [1]

• Sodium has an atomic number of 11, so it has 11 protons.

(b) 11 electrons [1]

• In a neutral atom, the number of electrons equals the number of protons.

(c) Number of neutrons = Mass number − Atomic number = 23 − 11 = 12 neutrons [2]

• Award 1 mark for the correct formula/substitution and 1 mark for the correct answer.

13. (a) An ionic bond is the electrostatic force of attraction between oppositely charged ions (positive and negative ions). [2]

• Award 1 mark for "electrostatic force of attraction" and 1 mark for "between oppositely charged ions."

(b) Sodium chloride has a high melting point because it has a giant ionic lattice structure with strong electrostatic forces of attraction between the Na⁺ and Cl⁻ ions. A large amount of energy is required to overcome these strong forces. [2]

• Award 1 mark for identifying strong electrostatic forces/ionic bonds and 1 mark for explaining that a lot of energy is needed to overcome them.

14. (a) Solution A [1]

• Solution A has the lowest pH (pH 1), making it the most acidic.

(b) Solution B [1]

• Solution B has a pH of 7, which is neutral.

(c) Alkaline [1]

• Solution C has a pH of 10, which is above 7, so it is alkaline.

(d) C → B → D → A [1]

• Increasing acidity means decreasing pH: pH 10 → pH 7 → pH 4 → pH 1.

15. (a) Mg + 2HCl → MgCl₂ + H₂ [2]

• Award 1 mark for correct reactants and products and 1 mark for correct balancing.

(b) Hydrogen gas [1]

(c) Test: Hold a lighted splint near the gas. [1] Observation: The gas burns with a 'pop' sound. [1]

16. (a) Two physical properties of metals: [2]

• Property 1: Good conductor of electricity [1]
• Property 2: Malleable / ductile / good conductor of heat / high melting point / shiny appearance (any two valid properties, 1 mark each)

(b) Metals are good conductors of electricity because they have delocalised (free-moving) electrons that can carry electrical charge through the metal. [2]

• Award 1 mark for "delocalised/free electrons" and 1 mark for explaining that these electrons carry charge/current.

17. (a) Calcium carbonate + Hydrochloric acid → Calcium chloride + Water + Carbon dioxide [2]

• Award 1 mark for correct reactants and 1 mark for correct products.

(b) Step 1: Calculate the relative formula mass of CaCO₃ [1]

• CaCO₃ = 40 + 12 + (16 × 3) = 100

Step 2: Calculate moles of CaCO₃ [1]

• Moles = mass / RFM = 10 / 100 = 0.10 mol

Step 3: From the equation, 1 mol CaCO₃ produces 1 mol CO₂, so 0.10 mol CaCO₃ produces 0.10 mol CO₂.

• Mass of CO₂ = moles × RFM = 0.10 × (12 + 32) = 0.10 × 44 = 4.4 g [1]

Section C: Application and Analysis (15 marks)

18. (a) [3]

• Substance X: Ionic bonding [1] — high melting/boiling points, conducts electricity when liquid but not when solid.
• Substance Y: Covalent (simple molecular) bonding [1] — low melting/boiling points, does not conduct electricity in any state.
• Substance Z: Metallic bonding [1] — very high melting/boiling points, conducts electricity in both solid and liquid states.

(b) Substance X has a high melting point (801°C) and conducts electricity only when liquid (not solid). This indicates a giant ionic lattice with strong electrostatic forces between ions. In the solid state, ions are fixed in position and cannot move to carry charge. In the liquid state, ions are free to move and conduct electricity. [2]

• Award 1 mark for identifying strong ionic bonds/high melting point and 1 mark for explaining conductivity in liquid state due to mobile ions.

(c) Substance Z is most likely a metal. [1] Reason: It conducts electricity in both solid and liquid states and has very high melting and boiling points, which are characteristic properties of metals with metallic bonding. [1]

19. (a) NaOH + HCl → NaCl + H₂O [2]

• Award 1 mark for correct reactants and products and 1 mark for correct balancing.

(b) Moles of NaOH = concentration × volume (in dm³) [1]

• Moles = 0.10 × (25.0 / 1000) = 0.10 × 0.025 = 0.0025 mol [1]

(c) From the balanced equation, the mole ratio of NaOH to HCl is 1:1. [1]

• Therefore, moles of HCl needed = 0.0025 mol
• Volume of HCl = moles / concentration = 0.0025 / 0.10 = 0.025 dm³ = 25.0 cm³ [1]

20. (a) Two advantages of using plastics: [2]

• Advantage 1: Lightweight [1]
• Advantage 2: Durable / can be moulded into various shapes / inexpensive / good insulator (any two valid advantages, 1 mark each)

(b) One environmental problem: [1]

• Plastics are not biodegradable, so they accumulate in the environment and cause pollution. / Plastics harm wildlife when ingested. / Plastics take up landfill space.

(c) Biodegradable plastics made from plant materials are more environmentally friendly because they can be broken down naturally by microorganisms in the environment, reducing pollution and waste accumulation. [2]

• Award 1 mark for "broken down by microorganisms/decomposers" and 1 mark for "reduces pollution/environmental impact."