AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), June 2025: Question 1

7 marks · Low Demand difficulty · Short Answer

Complete the molecular formula of dodecane and analyze experimental data investigating how temperature affects the viscosity of dodecane.

Practise this question

Question

Question 01 has five parts. Part 01.1 asks for the formula of dodecane with 12 carbons given the alkane general formula CnH2n+2. Figure 1 illustrates an experiment measuring viscosity, showing a cup with dodecane flowing through a small bottom hole into a 100 cm³ measuring cylinder. Part 01.2 asks why a water bath is used instead of a Bunsen burner. Part 01.3 asks for one control variable. Figure 2 shows a graph of time taken to collect 40 cm³ of dodecane against temperature from 20 to 70 °C, displaying a decreasing curve. Part 01.4 asks to extend the line of best fit to predict the time at 75 °C. Part 01.5 asks to explain how the trend shows the effect of temperature on viscosity.
Question text

01 Dodecane is an alkane.

01.1 The general formula for alkanes is CnH2n+2

A molecule of dodecane has 12 carbon atoms.

Complete the formula of dodecane.

[1 mark]

C H

A student investigated the effect of changing the temperature on the

viscosity of dodecane.

Figure 1 shows the apparatus.

Figure 1

This is the method used.

1. Add 100 cm3 of dodecane at 25 °C to the cup.

2. Immediately start a timer.

*02*3. Stop the timer when 40 cm3 of dodecane has been collected in the

measuring cylinder.

4. Record the time taken.

5. Repeat steps 1 to 4 using dodecane at different temperatures.

01.2 The student used a water bath to heat the dodecane.

Suggest one reason why the student used a water bath instead of a Bunsen burner to

heat the dodecane.

[1 mark]

01.3 Give one control variable used in the investigation.

[1 mark]

Figure 2 shows the results.

Figure 2

01.4 Predict the time taken to collect 40 cm3 of dodecane at 75 °C.

You should extend the line of best fit on Figure 2.

[2 marks]

Time taken = s 5

01.5 The higher the viscosity, the slower dodecane flows.

Explain how the trend in Figure 2 shows the effect of an increase in temperature on

the viscosity of dodecane.

[2 marks]

Mark scheme

Show the mark scheme Mark scheme for Question 1: 01.1 requires C12H26 (1 mark). 01.2 accepts that a water bath controls temperature or will not set dodecane on fire / dodecane is flammable (1 mark). 01.3 accepts volume of dodecane (1 mark). 01.4 awards 1 mark for extrapolating the line to 75 °C and 1 mark for reading the value within ± 0.5 small square. 01.5 awards 1 mark for stating that as temperature increases, time taken decreases, and 1 mark for concluding that viscosity decreases.

Question 1

AO /

Question Answers Extra information Mark

Spec. Ref.

01.1 C12H26 1 AO2

5.7.1.1

AO /

Spec. Ref.

01.2 any one from: 1 AO3

5.7.1.3

• (a water bath) will control the

temperature

• (a water bath) will not set allow dodecane is flammable

dodecane on fire

AO /

Spec. Ref.

01.3 volume of dodecane 1 AO3

5.7.1.3

AO /

Spec. Ref.

01.4 line extrapolated to 75 °C 1 AO3

value at 75 °C read from allow a tolerance of ± ½ a small 1 AO2

extrapolated line square

5.7.1.3

AO /

Spec. Ref.

ignore references to rate

01.5 1 AO3

as the temperature increases 5.7.1.3

the time taken decreases

(so) the viscosity decreases 1

Total Question 1 7

How to answer it

Alkanes & Viscosity Practical Investigation

What this question tests

  • Alkane General Formula: Applying CnH2n+2 to deduce the molecular formula of a given alkane.
  • Practical Safety & Controls: Explaining safe heating apparatus for flammable hydrocarbons and identifying control variables.
  • Graph Skills: Extending a smooth line of best fit (extrapolation) and reading values accurately.
  • Data Interpretation: Explaining how flow time relates to temperature and liquid viscosity.
Question 01.1 • 1 Mark

Deducing the Formula of Dodecane

Alkane general formula calculation

✅ Correct Answer

C12H26

1 mark: Subscripts 12 and 26 correctly written.

🔨 Calculation

  1. Identify number of carbons: n = 12
  2. Calculate number of hydrogens: 2n + 2 = (2 × 12) + 2 = 24 + 2 = 26
  3. Combine to give formula: C₁₂H₂₆

❌ Common Errors

  • Writing standard size numbers instead of subscripts (e.g., C12H26).
  • Using the alkene formula (CnH2n) giving C12H24.
  • Arithmetic slip: calculating 2 × 12 as 22 instead of 24.
Question 01.2 • 1 Mark

Heating with a Water Bath vs Bunsen Burner

Laboratory safety and apparatus selection

✅ Correct Answers (Any one)

  • Dodecane is flammable / a water bath will not set dodecane on fire.
  • A water bath allows you to control the temperature more precisely / heat gently and evenly.
1 mark: Either valid safety reason (flammability / no naked flame) OR control reason.

💡 Key Knowledge

Hydrocarbons are fuels and easily ignite when exposed to a naked flame like a Bunsen burner. A water bath provides indirect heat with no open flame.

🧠 Exam Technique

Whenever an organic liquid (hydrocarbon, alcohol) is heated, look for safety first: is it flammable? Always link the hazard directly to the apparatus: Bunsen burner = naked flame = fire hazard.

Question 01.3 • 1 Mark

Identifying a Control Variable

Fair testing in rates and flow experiments

✅ Correct Answer

Volume of dodecane (added to the cup)

1 mark: Specifically identifies the starting volume (100 cm³) of dodecane.

💡 Variables Summary

  • Independent: Temperature of dodecane (°C)
  • Dependent: Time taken to collect 40 cm³ (s)
  • Control: Initial volume (100 cm³), size/shape of the cup and hole.

❌ Common Errors

  • Writing "amount of dodecane" — "amount" is too vague; examiners require volume.
  • Confusing control with dependent variable (e.g. stating "the time taken").
Question 01.4 • 2 Marks

Predicting Flow Time at 75 °C (Extrapolation)

Graph extrapolation and accurate reading

✅ Correct Answer

Time taken: 5 s (typically 4 – 6 s depending on extrapolation)

Mark 1: Extending the smooth line of best fit to 75 °C.
Mark 2: Correct value read from the extrapolated line (± ½ small square tolerance).

🧠 How to Draw the Line

  • Follow the existing curvature smoothly — do not turn the curve upwards or use a ruler to draw a straight line.
  • Continue the curve past the last data point (65 °C) until it crosses the vertical grid line for 75 °C.
  • Draw faint guide lines with a ruler from 75 °C up to your curve, then across to the y-axis to read your value.

❌ Common Errors

  • Drawing a straight horizontal line or dropping the line down sharply to touch 0 seconds.
  • Extrapolating only to 70 °C instead of the requested 75 °C.
  • Reading the y-axis incorrectly (check scale: 1 large square = 20 s, 1 small square = 2 s).
Question 01.5 • 2 Marks

Explaining the Effect of Temperature on Viscosity

Connecting graph trends to chemical concepts

✅ Correct Answer

  1. As the temperature increases, the time taken decreases (or flow is faster).
  2. Therefore, as temperature increases, the viscosity decreases (or it becomes less viscous / runnier).
Mark 1: Correct trend from the graph linking temperature and time.
Mark 2: Deducing that viscosity decreases with increasing temperature.

💡 Key Concept

Viscosity means how thick/sticky a liquid is and how slowly it flows.

The question states: "The higher the viscosity, the slower dodecane flows."
Therefore: Shorter flow time → Faster flow → Lower viscosity.

❌ Examiner Commentary

  • Do not write about rate: The mark scheme specifically states "ignore references to rate". Focus on time and viscosity.
  • Many students forget the second step: they state that time decreases, but fail to explicitly answer what happens to the viscosity.

Topics

Chemistry · C7: Organic Chemistry

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 2 (Higher), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.