Mark Scheme

Section A — Structured Questions

Question 1 (8 marks)

(a) Aluminium oxide / Al₂O₃ / alumina (1 mark)

• Accept: "aluminium(III) oxide"
• Reject: "bauxite", "aluminium ore" (without specifying compound)

(b) Electrolysis (1 mark)

• Must use this term
• Reject: "electrolytic reduction" (too vague), "heating"

(c) (i) Al³⁺ + 3e⁻ → Al (2 marks)

• Award 1 mark for correct species
• Award 1 mark for correct balancing of charges and electrons
• Accept: Al³⁺(l) + 3e⁻ → Al(l)
• Reject: equations showing oxidation

(ii) Any TWO of the following: (2 marks, 1 each)

• Lowers the melting point of aluminium oxide / allows electrolysis at lower temperature
• Reduces energy costs / saves electricity / makes process more economical
• Allows the mixture to conduct electricity better / acts as a solvent
• Accept: "makes the ore molten at a lower temperature"
• Reject: "makes it easier to extract" (too vague)

(d) Any TWO appropriate properties: (2 marks, 1 each)

• Good conductor of heat / heats up quickly / distributes heat evenly
• Low density / lightweight / easy to handle
• Does not corrode / rust / resistant to corrosion / forms protective oxide layer
• Non-toxic / safe for food contact
• Malleable / can be shaped easily
• Accept: "high thermal conductivity"
• Reject: "strong" or "shiny" without context

Question 2 (8 marks)

(a) Any TWO of: (2 marks, 1 each)

• Concentration of the acid
• Volume of acid used
• Mass / length / surface area of magnesium ribbon
• Pressure
• Accept: "same piece of magnesium" (for consistency)
• Reject: "temperature" (this is the independent variable being changed)

(b) (i) The rate of reaction at 35°C is faster / greater / higher (than at 25°C) (1 mark)

• Reject: "more" without "faster" or "greater rate"
• Reject: comparative answers without explicit comparison

(ii) Explanation including: (2 marks)

• At higher temperature, particles have more (kinetic) energy (1 mark)
• Particles collide more frequently AND/OR with more energy / more collisions exceed activation energy / more successful collisions (1 mark)
• Accept: "particles move faster at higher temperature" (for first mark)
• Accept: "more particles have energy greater than or equal to activation energy"
• Reject: "particles collide harder" (without mention of energy)

(c) Calculation: (3 marks)

Number of moles of Mg = mass / Mr = 0.05 / 24 = 0.00208(3) mol (1 mark)

From equation: 1 mole of Mg produces 1 mole of H₂

Therefore moles of H₂ = 0.00208(3) mol (1 mark)

Volume of H₂ = moles × 24 = 0.00208(3) × 24 = 0.05 dm³ (or 50 cm³) (1 mark)

Award marks for:

• Correct calculation of moles of Mg (1)
• Recognizing 1:1 mole ratio / correct moles of H₂ (1)
• Correct final answer with unit (1)
• Accept answers: 0.05 dm³, 5.0 × 10⁻² dm³, 50 cm³
• Award 2 marks for correct answer without working if calculation is straightforward

Question 3 (9 marks)

(a) (i) Reactivity decreases down Group VII / halogens become less reactive going down the group (1 mark)

• Must indicate direction
• Reject: "reactivity increases"

(ii) Explanation including: (2 marks)

• Atoms get larger / more shells / increased atomic radius going down the group (1 mark)
• Greater distance between nucleus and outer electrons / increased shielding / weaker attraction for incoming electron / more difficult to gain an electron (1 mark)
• Accept: "outer shell further from nucleus"
• Accept: "nuclear attraction decreases"
• Reject: explanations referring to losing electrons

(b) (i) Any ONE of: (1 mark)

• Perform in fume cupboard / well-ventilated area
• Wear safety goggles / protective gloves
• Do not breathe in the gas / avoid inhalation
• Use small quantities
• Have alkali / sodium hydroxide solution ready to neutralize
• Accept specific, practical safety measures
• Reject: "be careful"

(ii) Test and observation: (2 marks)

• Test: Place damp/moist blue litmus paper in the gas (1 mark)
• Observation: The litmus paper turns red then bleaches / becomes white / loses colour (1 mark)
• Must have BOTH red AND bleached for second mark
• Accept: "damp universal indicator paper turns red then white"
• Reject: "turns litmus paper white" (without red first)

(c) (i) To kill bacteria / microorganisms / germs / pathogens (1 mark) OR: To disinfect the water / sterilize the water / purify the water

• Accept: "to prevent disease" / "make water safe to drink"
• Reject: "to clean water" (too vague)

(ii) Cl₂(g) + H₂O(l) → HCl(aq) + HClO(aq) (2 marks) OR: Cl₂(g) + H₂O(l) → HCl(aq) + HOCl(aq)

• Award 1 mark for correct species (all four products and reactants)
• Award 1 mark for balanced equation with state symbols
• Accept equations without state symbols for 1 mark only
• Reject: equations showing only one acid product

Question 4 (8 marks)

(a) Alcohols / alkanol (1 mark)

• Accept: "alcohol"
• Reject: "alkane", "alkene", "organic compounds"

(b) (i) Any ONE of: (1 mark)

• Anaerobic conditions / absence of oxygen / no air
• Temperature between 25–40°C / optimum temperature around 37°C / warm temperature
• Aqueous solution / presence of water
• Accept: specific temperature ranges that include 30–40°C
• Reject: "yeast" (already stated in question)

(ii) Calculation: (3 marks)

Moles of glucose = 90 / (6×12 + 12×1 + 6×16) = 90 / 180 = 0.5 mol (1 mark)

From equation: 1 mole glucose produces 2 moles ethanol

Moles of ethanol = 0.5 × 2 = 1.0 mol (1 mark)

Mr of ethanol = (2×12) + (6×1) + 16 = 46

Mass of ethanol = 1.0 × 46 = 46 g (1 mark)

Award marks for:

• Correct moles of glucose (1)
• Correct moles of ethanol using 1:2 ratio (1)
• Correct final answer with unit (1)
• Accept 46, 46.0, 46.00 g
• Award 2 marks for correct answer without full working if clearly systematic

(c) (i) (From) purple to colourless / pink / brown (1 mark)

• Must show initial and final colour
• Accept: "decolourization" / "loses purple colour"
• Reject: "changes colour" (not specific enough)

(ii) C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l) (2 marks)

• Award 1 mark for correct species
• Award 1 mark for correct balancing
• Accept: equations without state symbols for 1 mark only
• Accept: H₂O(g) as product

Question 5 (3 marks)

(a) Calculation: (3 marks)

Mr of urea = 12 + 16 + (2×14) + (4×1) = 60 (1 mark)

Mass of nitrogen in formula = 2 × 14 = 28 (1 mark)

Percentage = (28 / 60) × 100 = 46.7% (accept 46.6 to 46.7%) (1 mark)

Award marks for:

• Correct relative formula mass of urea (1)
• Correct total mass of nitrogen atoms (1)
• Correct percentage calculation (1)
• Accept 46.67%, 46.66%, 47% (rounded)
• Award 2 marks for correct final answer with minimal working if method is clear

Section B — Extended Response

Question 6 (24 marks)

(a) (i) Definition: (2 marks)

• A compound (1 mark)
• Containing hydrogen and carbon only (1 mark)
• Accept: "molecules made of only hydrogen and carbon atoms"
• Must specify "only" or "just" these two elements
• Reject: definitions including other elements

(ii) Fractional distillation (1 mark)

• Must use this term
• Reject: "distillation" alone, "cracking", "refining"

(iii) Description: (4 marks)

Award marks for points including:

• Crude oil is heated / vaporized (1 mark)
• Vapours rise up a fractionating column / tower which is hot at the bottom and cooler at the top / temperature gradient (1 mark)
• Different fractions have different boiling points (1 mark)
• Fractions condense at different heights / temperatures and are collected separately / smaller molecules condense higher up, larger molecules condense lower down (1 mark)

Quality of explanation:

• Accept: "components with lower boiling points rise higher"
• Accept: "separation occurs according to boiling point"
• Must show understanding that it's a physical separation based on boiling points
• Award maximum 3 marks if no mention of temperature variation in column

(b) (i) Definition: (2 marks)

• Breaking down / decomposition of large / long-chain hydrocarbon molecules (1 mark)
• Into smaller / shorter (more useful) molecules (1 mark)
• Accept: "splitting up of large molecules into smaller ones"
• Must indicate change in molecular size
• Reject: "breaking down oil" without reference to molecules

(ii) C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (2 marks)

• Award 1 mark for C₂H₄ as the other product
• Award 1 mark for balanced equation
• Accept: any correctly balanced equation showing decane → octane + alkene
• Accept: C₁₀H₂₂ → C₈H₁₈ + other alkene (e.g., 2C₂H₄ would require different octene)
• The most common answer should show ethene as the small molecule

(iii) Explanation: (2 marks)

Any TWO of:

• Produces smaller molecules that are more useful / in greater demand (1 mark)
• Produces alkenes which are used to make polymers / plastics / other chemicals (1 mark)
• Produces more petrol / gasoline / fuels for which there is high demand (1 mark)
• Converts less useful fractions into more valuable products / economic benefit
• Accept: "makes better use of crude oil fractions"
• Accept: "supplies molecules needed for chemical industry"

(c) Extended response on carbon dioxide and Caribbean impact: (9 marks)

Level 3 (7–9 marks): Comprehensive evaluation

• Detailed discussion of environmental consequences (greenhouse effect, climate change, specific mechanisms)
• Thorough assessment of multiple impacts on Caribbean nations (sea level rise, hurricane intensity, coral bleaching, tourism, agriculture, economy)
• Clear, relevant suggestion for reduction with explanation
• Coherent structure with effective linking of ideas
• Accurate use of scientific terminology throughout
• Balanced evaluation showing understanding of complexity

Level 2 (4–6 marks): Adequate evaluation

• Sound discussion of environmental consequences with some detail
• Identification of at least two specific impacts on Caribbean nations
• Relevant suggestion for reduction stated
• Generally clear structure
• Mostly accurate scientific terminology
• Some evaluation present but may lack depth or balance

Level 1 (1–3 marks): Limited response

• Basic or incomplete discussion of environmental consequences
• Vague or single impact on Caribbean mentioned
• Suggestion for reduction may be weak or missing
• Limited structure or development
• Limited scientific terminology or inaccuracies present
• Mainly descriptive with little evaluation

0 marks: No creditworthy response

Indicative content:

Environmental consequences:

• Carbon dioxide is a greenhouse gas
• Traps heat in atmosphere / causes global warming / climate change
• Enhanced greenhouse effect
• Rising global temperatures

Caribbean impacts:

• Sea level rise threatening low-lying islands / coastal communities / loss of land
• Increased intensity/frequency of hurricanes/tropical storms
• Coral reef damage / bleaching affecting marine ecosystems
• Impact on fishing industry
• Threat to tourism industry (beaches, reefs)
• Agricultural impacts (drought, changing rainfall patterns, crop damage)
• Economic costs of adaptation / infrastructure damage
• Displacement of populations / climate refugees

Reduction strategies (accept any ONE sensible suggestion):

• Increase use of renewable energy (solar, wind, hydroelectric)
• Improve public transportation / reduce vehicle emissions
• Plant more trees / reforestation / protect forests
• Improve energy efficiency in buildings
• Promote alternative fuels / electric vehicles
• Reduce reliance on fossil fuels
• International cooperation on emissions reduction

Sample Answers with Examiner Commentary

Question 6(c) — Sample Answers

Grade I (Distinction) answer

Increasing carbon dioxide levels in the atmosphere have significant environmental and economic consequences for the Caribbean region. Carbon dioxide is a greenhouse gas that traps infrared radiation in the atmosphere, leading to global warming and climate change. This enhanced greenhouse effect causes rising global temperatures, which have several serious impacts on Caribbean nations.

One of the most critical threats is sea level rise. As global temperatures increase, polar ice caps melt and ocean water expands, causing sea levels to rise. Many Caribbean islands are low-lying, and some nations like the Bahamas and parts of Trinidad could lose significant coastal areas. This threatens homes, infrastructure, and agricultural land. Beach erosion also damages the tourism industry, which is vital to Caribbean economies.

Climate change also affects the intensity and frequency of hurricanes. Warmer ocean temperatures provide more energy for storm formation, leading to more destructive hurricanes. Recent examples like Hurricane Maria in Dominica demonstrate the devastating economic and social impacts of severe storms on small island nations. The cost of rebuilding infrastructure and the loss of lives create long-term challenges for development.

Another significant impact is coral reef bleaching. Rising ocean temperatures cause coral to expel their symbiotic algae, leading to bleaching and eventual death. Caribbean coral reefs support marine biodiversity, protect coastlines from erosion, and sustain fishing and tourism industries. Their loss would have cascading effects on food security and livelihoods.

To reduce carbon dioxide emissions, Caribbean countries could increase their use of renewable energy sources such as solar and wind power. The Caribbean has abundant sunshine and trade winds that could be harnessed to generate electricity, reducing dependence on imported fossil fuels. This would not only lower emissions but also improve energy security and create economic benefits through reduced fuel costs. Some islands like Barbados are already investing in solar energy programs, showing that this approach is feasible for the region.

In conclusion, increasing carbon dioxide levels pose severe threats to Caribbean nations through sea level rise, more intense hurricanes, and ecosystem damage. However, transitioning to renewable energy offers a practical way for the region to contribute to emissions reduction while building more sustainable economies.

Mark: 9/9

Examiner commentary: This answer demonstrates comprehensive understanding across all assessment criteria. The candidate provides detailed scientific explanation of the greenhouse effect, identifies multiple specific impacts on the Caribbean with clear examples (sea level rise, hurricanes, coral bleaching), and suggests a well-explained reduction strategy with context-specific detail. The response is coherently structured with effective linking between points, uses accurate scientific terminology throughout, and shows sophisticated evaluation of the complexity of the issue. The inclusion of real examples (Hurricane Maria, Barbados solar program) demonstrates excellent application of knowledge to the Caribbean context.

Grade III (Pass) answer

Carbon dioxide is a greenhouse gas that causes global warming. When carbon dioxide levels increase in the atmosphere, it traps heat and makes the Earth warmer. This is called the greenhouse effect.

The Caribbean region is affected by this in several ways. One major problem is sea level rise. As the Earth gets warmer, ice melts and sea levels go up. This is a big problem for Caribbean islands because many of them are close to sea level. Beaches could be covered by water and people living near the coast might lose their homes. This would also affect tourism because people come to the Caribbean for the beaches.

Another effect is on coral reefs. The warmer water makes the coral turn white and die. This is called coral bleaching. Coral reefs are important for fish and for protecting the coast. If the reefs die, fishermen will catch less fish and the ecosystem will be damaged.

Climate change also makes hurricanes stronger. Warmer oceans give hurricanes more energy so they become more powerful and cause more damage to Caribbean countries.

Caribbean countries can reduce their carbon dioxide emissions by using renewable energy like solar power. The Caribbean has lots of sunshine so solar panels could produce electricity without burning fossil fuels. This would reduce the amount of carbon dioxide released into the atmosphere.

Overall, increasing carbon dioxide is a serious problem for the Caribbean because of rising seas, damaged coral, and stronger storms. Using renewable energy can help reduce emissions.

Mark: 5/9

Examiner commentary: This answer achieves a solid middle band. The candidate demonstrates sound understanding of the greenhouse effect and identifies three specific Caribbean impacts (sea level rise, coral bleaching, hurricanes) with reasonable explanation. A relevant reduction strategy is stated with basic justification. However, the response lacks the depth and sophistication of a top-band answer—explanations are sometimes simplified (e.g., "makes the Earth warmer"), the linkages between impacts and broader consequences could be stronger, and the evaluation is limited. The structure is generally clear but the development of ideas is adequate rather than thorough. To reach higher marks, the candidate needed more detailed scientific terminology, stronger evaluation of the relative importance of impacts, and deeper analysis of the reduction strategy's effectiveness.

Grade V (Near miss) answer

Carbon dioxide is bad for the environment because it causes pollution. When there is too much carbon dioxide in the air, it makes the temperature go up and causes global warming.

This affects the Caribbean because the weather gets hotter. Hot weather is uncomfortable for people and tourists might not want to visit. The beaches might also get dirty from pollution.

Global warming makes the ice caps melt which makes floods. Caribbean countries have floods that damage houses and buildings. The sea also rises and covers the land.

Carbon dioxide also kills plants and animals. Fish in the ocean die because of pollution in the water. This is bad for fishermen who need to catch fish.

To reduce carbon dioxide, Caribbean countries should stop using cars and use bicycles instead. They could also recycle more and plant more trees. Trees take in carbon dioxide so planting trees would reduce the amount in the atmosphere.

In conclusion, carbon dioxide is harmful to the Caribbean and causes many problems like hot weather, floods, and dead fish. Planting trees and recycling can help solve this problem.

Mark: 3/9

Examiner commentary: This answer remains in the lowest mark band due to several weaknesses. While the candidate shows basic awareness that carbon dioxide causes global warming and identifies some relevant impacts, the response contains common misconceptions—treating carbon dioxide primarily as a pollutant rather than explaining the greenhouse mechanism, conflating different environmental issues, and showing imprecise understanding of climate impacts. The explanation of how carbon dioxide affects the Caribbean lacks specific, accurate detail (e.g., "beaches might get dirty" rather than beach erosion from sea level rise). The suggested solutions are too general and poorly developed. Scientific terminology is limited and sometimes incorrect. To improve, the candidate needs to: clearly explain the greenhouse effect mechanism, distinguish between direct effects (sea level rise, temperature increase) and secondary impacts, use more precise scientific vocabulary, and develop the reduction strategy with specific explanation of how it addresses carbon dioxide emissions.

Question 2(c) — Sample Answers

Grade I (Distinction) answer

Number of moles of Mg = mass ÷ molar mass = 0.05 ÷ 24 = 0.002083 mol

From the equation: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

The mole ratio of Mg : H₂ is 1 : 1

Therefore, moles of H₂ produced = 0.002083 mol

At RTP, 1 mole of gas occupies 24 dm³

Volume of H₂ = moles × molar volume = 0.002083 × 24 = 0.05 dm³

Alternative answer: 50 cm³

Mark: 3/3

Examiner commentary: This is an exemplary calculation answer showing all required steps clearly and systematically. The candidate correctly calculates moles of magnesium, recognizes the 1:1 stoichiometric relationship from the equation, and applies the molar volume correctly to find the final answer. The working is methodical, units are included throughout, and the final answer is given in the correct form with appropriate significant figures. The alternative unit shows good understanding. This represents the standard expected for full marks in quantitative questions at this level.

Grade III (Pass) answer

Moles of Mg = 0.05 ÷ 24 = 0.00208 mol

1 mole of Mg makes 1 mole of H₂

So moles of H₂ = 0.00208 mol

Volume = 0.00208 × 24 = 0.0499 dm³

Answer = 0.05 dm³

Mark: 3/3

Examiner commentary: Despite being less detailed than the Grade I answer, this response still earns full marks because all essential calculation steps are present and correct. The candidate demonstrates understanding of the key concepts (mole calculation, stoichiometry, molar volume) and arrives at the correct answer. While the working is more condensed and lacks some formal mathematical presentation (e.g., explicit statement of formulas), it is sufficiently clear for an examiner to follow the reasoning and award full credit. This illustrates that full marks can be achieved with competent working even if presentation is less polished.

Grade V (Near miss) answer

Moles = 0.05 ÷ 24 = 0.00208

Volume of hydrogen = 0.00208 × 24 = 0.05

Answer = 0.05 g

Mark: 1/3

Examiner commentary: This answer demonstrates a common weakness in quantitative questions—the candidate knows some relevant formulas and performs calculations, but the understanding is incomplete. One mark is awarded for correctly calculating the moles of magnesium. However, the candidate fails to explicitly identify that these moles of magnesium produce an equal number of moles of hydrogen (missing the stoichiometry step, though the calculation accidentally works because of the 1:1 ratio). More significantly, the final answer has the wrong unit (grams instead of dm³), indicating fundamental confusion between mass and volume. This is a typical error at this grade level, where candidates perform arithmetic correctly but misunderstand what the numbers represent. To improve, the candidate should: state what each calculation step represents, check that mole ratios are considered, and ensure the final answer matches what the question asks for (volume, not mass).