AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Foundation), June 2025: Question 3
10 marks · Low Demand difficulty · Short Answer
Answer questions on screenwash formulations, atomic composition, combustion products, balancing equations, testing for hydrogen, climate change effects, and reversible reactions of cobalt chloride.
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Question text
03 Screenwash is a mixture designed to keep car windscreens clean.
The components in screenwash include glycerol, methanol and water.
03.1 Screenwash is made by mixing the components in carefully measured quantities
so that the product has the required properties.
What name is given to this type of mixture?
[1 mark]
Tick ( ) one box.
Catalyst
Formulation
Polymer
03.2 Glycerol has the formula C3H8O3
How many atoms are there in one molecule of glycerol?
[1 mark]
Tick ( ) one box.
38 12 11 14
03.3 Incomplete combustion happens when glycerol burns in limited oxygen.
Incomplete combustion produces carbon monoxide gas.
Why can the production of carbon monoxide gas be a problem?
[1 mark]
Tick ( ) one box.
Carbon monoxide is a greenhouse gas.
Carbon monoxide is a toxic gas.
Carbon monoxide is an acidic gas.
03.4 Complete combustion happens when methanol (CH4O) burns in excess oxygen.
Balance the equation for the reaction.
[1 mark]
2 CH O + 3 O →13 CO + 4 H O
42 2 2
Methanol can be produced by the reaction of carbon dioxide with hydrogen.
03.5 Describe the test for hydrogen gas.
Give the result of the test.
[2 marks]
Test
Result
03.6 Carbon dioxide is a greenhouse gas.
An increase in greenhouse gases in the atmosphere has led to an increase in average
global temperature.
The increase in average global temperature has caused global climate change.
Give two effects of global climate change.
[2 marks]
Anhydrous cobalt chloride can be used to test for water.
The word equation for the reaction is:
anhydrous cobalt chloride + water ⇌ hydrated cobalt chloride
(blue) (pink)
03.7 Suggest one observation that could be made when water is added to anhydrous
cobalt chloride.
[1 mark]
03.8 The reaction between anhydrous cobalt chloride and water is exothermic.
What type of reaction is the reverse reaction?
[1 mark]
Mark scheme
Show the mark scheme
Question 3
AO /
Question Answers Extra information Mark
Spec. Ref.
03.1 formulation 1 AO1
5.8.1.2
AO /
Spec. Ref.
03.2 14 1 AO2
5.1.1.1
5.8.1.2
AO /
Spec. Ref.
03.3 carbon monoxide is a toxic gas 1 AO1
5.9.3.2
AO /
Spec. Ref.
03.4 2 CH4O + 3 O2 → 2CO2 + 4 H2O 1 AO2
5.1.1.1
5.3.1.1
allow multiples 5.7.1.3
AO /
Spec. Ref.
03.5 (test) AO1
add a burning splint do not accept glowing splint 1 5.8.2.1
(result)
(hydrogen burns with) a pop 1
sound
MP2 is dependent on MP1
– being awarded – –
AO /
Spec. Ref.
03.6 any two from: 2 AO1
5.9.2.3
• melting ice caps
• rising sea levels
• more flooding
• extremes of weather allow increase in frequency of
storms
• increase in droughts allow desertification
• increase in wildfires
• loss of habitats
• reduction in biodiversity
• change in migration patterns
ignore global warming
do not accept acid rain
do not accept global dimming
do not accept deforestation
do not accept natural disasters
do not accept references to
ozone
AO /
Spec. Ref.
03.7 colour change (from blue to 1 AO2
pink) 5.6.2.2
AO /
Spec. Ref.
03.8 endothermic allow (thermal) decomposition 1 AO2
5.6.2.2
Total Question 3 10
How to answer it
Chemical Mixtures, Combustion & Reversible Reactions
This question assesses core recall and foundational chemistry skills across several specification topics:
- Formulations: Understanding mixtures designed with specific properties.
- Formula Counting & Balancing: Counting total atoms in a chemical formula and balancing simple combustion equations.
- Pollutants & Climate Change: Identifying why carbon monoxide is dangerous and stating environmental consequences of global warming.
- Gas Tests: Recalling the standard qualitative chemical test for hydrogen gas.
- Reversible Reactions: Interpreting observations and energy changes in reversible hydration reactions.
Question 03.1: Types of Mixtures
Identifying complex useful mixtures [1 mark]
✅ Correct Answer
Formulation (Tick box 2)
💡 Key Knowledge
A formulation is a mixture designed as a useful product. Each chemical component is measured precisely to ensure the final mixture has the required properties (e.g. paints, cleaning agents, fuels, medicines).
❌ Common Errors & Examiner Commentary
Students sometimes confuse formulation with polymer (large chain molecules made of repeating monomers) or catalyst (substances that speed up reactions without being used up). Screenwash contains multiple separate ingredients mixed in set proportions, which is the definition of a formulation.
Question 03.2: Counting Atoms in a Molecule
Total atoms in glycerol, C₃H₈O₃ [1 mark]
✅ Correct Answer
14 (Tick box 4)
📐 Step-by-Step Calculation
Look at each element's subscript:
- Carbon (C): 3
- Hydrogen (H): 8
- Oxygen (O): 3
Total: 3 + 8 + 3 = 14 atoms in one molecule.
🧠 Exam Technique
Make sure to read the question carefully: it asks for atoms, not elements. There are 3 elements (C, H, O), but 14 total atoms.
Question 03.3: Hazards of Carbon Monoxide
Why carbon monoxide production is a problem [1 mark]
✅ Correct Answer
Carbon monoxide is a toxic gas. (Tick box 2)
💡 Key Knowledge
Incomplete combustion occurs in limited oxygen, producing carbon monoxide (CO). CO binds irreversibly to haemoglobin in red blood cells, preventing them from carrying oxygen around the body.
❌ Common Errors
- Calling carbon monoxide a greenhouse gas — that is carbon dioxide (CO₂) or methane (CH₄).
- Calling carbon monoxide an acidic gas — acidic atmospheric gases are mainly sulfur dioxide (SO₂) and nitrogen oxides (NOₓ).
Question 03.4: Balancing a Combustion Equation
Balancing the complete combustion of methanol [1 mark]
✅ Correct Answer
2 CH₄O + 3 O₂ → 2 CO₂ + 4 H₂O
📐 Balancing Step-by-Step
- Count Carbon on the left: In 2 CH₄O , there are 2 × 1 = 2 Carbon atoms.
- Balance Carbon on the right: Since CO₂ contains 1 Carbon atom, you need a coefficient of 2 in front of CO₂ ( 2 CO₂ ).
- Check other elements:
• Hydrogen: Left = 2 × 4 = 8; Right = 4 × 2 = 8 ✓
• Oxygen: Left = (2 × 1) + (3 × 2) = 8; Right = (2 × 2) + (4 × 1) = 8 ✓
🧠 Exam Tip
When only one blank space is provided in an equation, look at the other elements already locked in. Here, the number of carbons on the left (2) immediately gave away the number of CO₂ molecules needed!
Question 03.5: Chemical Test for Hydrogen
Standard test and positive result [2 marks]
✅ Correct Answer
Test: Add / hold a burning splint (or lit splint)
Result: (Hydrogen burns with a) squeaky pop / pop sound
Mark 2: Mentioning a pop sound (dependent on a correct test).
❌ Common Errors & Mark Scheme Traps
- "Glowing splint" gets 0 marks! A glowing splint is the test for oxygen (which relights). You MUST specify a burning or lit splint.
- Mark 2 cannot be awarded if you wrote "glowing splint" or an invalid test.
💡 Quick Reference for Common Gas Tests
- Hydrogen (H₂): Burning splint → squeaky pop
- Oxygen (O₂): Glowing splint → relights
- Carbon dioxide (CO₂): Bubble through limewater → turns cloudy/milky
- Chlorine (Cl₂): Damp litmus paper → bleaches white
Question 03.6: Effects of Global Climate Change
Two distinct consequences of increasing global temperature [2 marks]
✅ Acceptable Answers (Any TWO of the following)
- Melting ice caps / glaciers
- Rising sea levels
- More flooding / coastal erosion
- Extremes of weather / increased frequency of storms
- Increase in droughts / desertification
- Increase in wildfires
- Loss of habitats
- Reduction in biodiversity / extinction of species
- Change in animal migration patterns
❌ What the Examiner Will REJECT
- "Global warming": Ignored because the question already mentions an increase in global temperature.
- "Acid rain": Incorrect (caused by SO₂ and NOₓ, not greenhouse warming).
- "Ozone layer depletion": Incorrect (caused by CFCs, unrelated to global warming).
- "Deforestation": Incorrect (this is a cause of climate change, not an effect).
- "Natural disasters" / "Pollution": Too vague.
Question 03.7: Testing for Water
Observation when water is added to anhydrous cobalt chloride [1 mark]
✅ Correct Answer
Colour change from blue to pink (or turns pink)
🧠 Use the Given Equation!
The exam gave you this clue directly:
anhydrous cobalt chloride (blue) + water ⇌ hydrated cobalt chloride (pink)
Adding water moves the reaction forwards, so it goes from blue to pink. Always check the prompt for free clues!
Question 03.8: Energy Transfer in Reversible Reactions
Deducing the reverse reaction type [1 mark]
✅ Correct Answer
Endothermic (also accept thermal decomposition)
💡 The Golden Rule of Reversible Reactions
If a reversible reaction is exothermic in one direction, it is always endothermic in the opposite direction by the exact same amount of energy.
- Forward reaction: Exothermic (releases heat energy to surroundings)
- Reverse reaction: Endothermic (absorbs heat energy from surroundings)
Topics
Chemistry · C1: Atomic Structure and the Periodic Table · C3: Quantitative Chemistry · C6: The Rate and Extent of Chemical Change · C7: Organic Chemistry · C8: Chemical Analysis · C9: Chemistry of the Atmosphere
Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 2 (Foundation), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.