AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Foundation), November 2020: Question 6

9 marks · Low Demand difficulty · Short Answer

Balance equations and state chemical tests and results for carbon dioxide, hydrogen, and alkenes related to propane and propene.

Practise this question

Question

Question text

06 This question is about the Earth’s resources.

When most fuels burn carbon dioxide is produced.

Propane (C3H8) is a fuel.

06.1 Balance the equation for the combustion of propane.

[1 mark]

C3H8 + O2 → 3CO2 + 4H2O

06.2 Describe the test for carbon dioxide.

Give the result of the test.

[2 marks]

Test

Result

06.3 Propane can be cracked to produce propene and hydrogen.

Complete the symbol equation for the reaction.

[1 mark]

C3H8 → + H2

propane propene hydrogen

06.4 Describe the test for hydrogen.

Give the result of the test.

[2 marks]

Test

Result

06.5 Propene is an alkene.

*22* Describe the test for alkenes.

Give the colour change in the test.

[3 marks]

Test

Colour change to

Mark scheme

Show the mark scheme

AO /

Question Answers Extra information Mark

Spec. Ref.

06.1 C H + 5 O → allow multiples 1 AO2

38 2

5.1.1.1

3 CO2 + 4 H2O 5.7.1.3

06.2 MP2 is dependent upon correct AO1.2

response in MP1 5.8.2.3

(bubble gas through) lime water allow (bubble gas through) 1

calcium hydroxide (solution)

turns milky / cloudy / white 1

white precipitate forms

06.3 C H 1 AO2

5.1.1.1

5.7.1.4

06.4 MP2 is dependent upon correct AO1

response in MP1 5.8.2.1

burning / lit splint allow flame 1

do not accept glowing splint

burns with a (squeaky) pop allow pops 1

sound

06.5 bromine (water) do not accept bromide 1 AO1

5.7.1.4

(colour change) orange 1

allow 1 mark for colourless

(to) orange

(to) colourless 1

ignore clear

Total 9

How to answer it

Combustion, Cracking & Qualitative Chemical Tests

WHAT THIS QUESTION TESTS

Core knowledge and skills assessed in this question:

Balancing Equations: Balancing complete combustion reactions involving hydrocarbons.
Gas Tests: Specific laboratory tests and positive results for carbon dioxide (CO₂) and hydrogen (H₂).
Cracking Reactions: Writing symbol equations for thermal decomposition of alkanes into alkenes and hydrogen.
Testing for Unsaturation: Using bromine water to distinguish saturated alkanes from unsaturated alkenes.

QUESTION 06.1 • 1 MARK

Balancing Hydrocarbon Combustion

Balance the equation: C₃H₈ + ___ O₂ → 3CO₂ + 4H₂O

✅ Correct Answer

The missing balancing number is 5.

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

1 Mark: Correct coefficient 5 (multiples such as 2 C₃H₈ + 10 O₂ → 6 CO₂ + 8 H₂O are also accepted).

📐 Step-by-Step Balancing

Count oxygen atoms on the right side:
(3 × 2 in CO₂) + (4 × 1 in H₂O) = 6 + 4 = 10 oxygen atoms.
Oxygen gas is diatomic (O₂), meaning each molecule has 2 atoms.
Divide total oxygen atoms needed by 2:
10 ÷ 2 = 5.

QUESTION 06.2 • 2 MARKS

Test and Result for Carbon Dioxide

Describe the test for carbon dioxide and give the result.

✅ Correct Answer

Test: Bubble the gas through limewater (or calcium hydroxide solution).
Result: Turns cloudy / milky (or forms a white precipitate).

Mark 1: Limewater.
Mark 2: Turns cloudy / milky / white precipitate forms (dependent on Mark 1).

🧠 Dependent Marks

In AQA mark schemes, Mark 2 is dependent on Mark 1.

If you name the wrong testing reagent (e.g., universal indicator or lit splint), you score 0/2 even if you mention a valid colour change.

❌ Common Errors

Saying it turns "clear" — limewater is already colourless and clear; it turns cloudy.
Confusing limewater with lime juice or bromine water.

QUESTION 06.3 • 1 MARK

Cracking Equation: Propane to Propene

Complete the symbol equation: C₃H₈ → ________ + H₂

✅ Correct Answer

Formula required: C₃H₆

C₃H₈ → C₃H₆ + H₂

1 Mark: C₃H₆ only.

💡 Key Knowledge

Conservation of mass: The number of atoms on both sides must be equal.

Carbon atoms on left = 3 → must be 3 on right (C₃).
Hydrogen atoms on left = 8 → (8 - 2 from H₂) = 6 on right (H₆).
Alkene general formula: CₙH₂ₙ (when n = 3, H = 2 × 3 = 6).

❌ Common Trap

Writing the name "propene" instead of the chemical formula C₃H₆. The question specifies a symbol equation.

QUESTION 06.4 • 2 MARKS

Test and Result for Hydrogen Gas

Describe the test for hydrogen and give the result.

✅ Correct Answer

Test: Hold a burning / lit splint (near the mouth of the test tube).
Result: Burns with a (squeaky) pop sound.

Mark 1: Burning / lit splint (allow flame).
Mark 2: Squeaky pop / pops (dependent on Mark 1).

❌ The #1 Student Error

Writing a glowing splint.

Glowing splint = Test for Oxygen (it relights).
Lit / burning splint = Test for Hydrogen (squeaky pop).

Examiners strictly do not accept "glowing splint" for hydrogen.

QUESTION 06.5 • 3 MARKS

Test for Alkenes (Unsaturation)

Describe the test for alkenes and give the colour change.

✅ Correct Answer

Test: Add bromine water (or bromine).
Colour change: orange to colourless.

Mark 1: Bromine (water).
Mark 2: Orange (starting colour).
Mark 3: Colourless (final colour).

💡 Why does this happen?

Alkenes contain a C=C double bond (unsaturated). They undergo an addition reaction with bromine across the double bond to form a colourless dibromoalkane.

Alkanes (saturated) do not react rapidly with bromine water, so the mixture stays orange/brown.

❌ Common Errors & Mark Losses

"Bromide" vs "Bromine": Writing bromide or bromide water loses Mark 1. Bromide is the ion (Br⁻), not the reagent!
"Clear" vs "Colourless": Writing "goes clear" instead of "colourless" will lose the final mark. A solution can be clear and orange (see-through); "colourless" means having no colour.
Reversing colours: Writing "colourless to orange" only gets 1 partial mark for both colours.

Topics

Chemistry · C1: Atomic Structure and the Periodic Table · C7: Organic Chemistry · C8: Chemical Analysis

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 2 (Foundation), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.