AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2022: Question 3

✅ Correct answer

Sulfur dioxide gas is produced and it escapes from the conical flask.

2 marks: 1 for naming the gas, 1 for saying it leaves the flask.

💡 Key knowledge

• The reaction makes a gas: SO₂.
• Open/flask experiments can lose gas to the surroundings, so mass decreases during the reaction.
• The equation shows a gas product, which helps you spot the answer.

🧠 Exam technique

• State the gas name first.
• Then say it escapes or is lost from the flask.
• Use the equation if you forget: SO₂(g) .

❌ Common errors

• Saying the flask is heavier because the reaction “creates more substance” — this does not explain the mass difference.
• Writing only “gas is produced” may be allowed, but naming sulfur dioxide is stronger and safer.
• Confusing sulfur dioxide with sulfur or sulfate.

✅ Correct answer

Dependent variable

The time changes in response to the independent variable being tested.

💡 Key knowledge

• Independent variable = temperature or concentration, because that is changed.
• Dependent variable = time taken for the cross to disappear.
• Control variables = things kept the same, e.g. volume of acid/thiosulfate.

🧠 Exam technique

Ask yourself: what is measured? That is usually the dependent variable.

❌ Common errors

• Calling time “independent” because it is on the graph axis.
• Mixing up dependent and control variables.

✅ Correct answer

• Plot all points correctly: (19, 82), (32, 48), (45, 43), (52, 15), (63, 7), (73, 3).
• Draw a smooth line of best fit through the pattern of points.

Mark scheme: 2 marks for correct plotting, 1 mark for line of best fit.

💡 Key knowledge

• Temperature is on the x-axis.
• Time in seconds is on the y-axis.
• The graph should show a decreasing curve: higher temperature gives shorter time.

🧠 Exam technique

• Use the correct scale before plotting.
• Plot with a small, neat cross or dot.
• For a best-fit line, do not join dot-to-dot.
• A curve is expected, not a straight line, because the data are not linear.

❌ Common errors

• Swapping the axes.
• Missing one or more points.
• Drawing a line through every point with sharp corners.
• Forgetting the units on axis labels.

📐 Calculations step-by-step

1. Choose two clear points on the tangent, not on the curve. For example, points near (0.10, 15) and (0.60, 7) are consistent with the drawn tangent.
2. Find the y step: 15 − 7 = 8 seconds
3. Find the x step: 0.60 − 0.10 = 0.50 mol/dm³
4. Calculate gradient: gradient = y step ÷ x step = 8 ÷ 0.50 = 16
5. Write the unit: s dm³/mol

Gradient = 16

Unit = s dm³/mol

🧠 Exam technique

• Use the tangent line, not the curve.
• Show your working clearly: one mark is often for the method.
• Always include the unit.
• Use points far apart on the tangent to reduce error.

💡 Key knowledge

• Gradient here means the rate of reaction.
• Because the graph is time against concentration, the slope has units of seconds per concentration.
• Use the same concentration units as the graph: mol/dm³.

❌ Common errors

• Using two points on the curve instead of the tangent.
• Forgetting to subtract correctly: x step and y step must be differences.
• Giving the wrong unit, such as mol/dm³/s .
• Leaving the answer unitless.

✅ Correct answer

As the reaction proceeds, there are fewer sodium thiosulfate particles per unit volume, so the frequency of collisions decreases.

1 mark for fewer particles / lower concentration, 1 mark for fewer collisions.

💡 Key knowledge

• Reactants are used up during the reaction.
• This lowers concentration, so particles are less crowded.
• Fewer collisions per second means a slower rate.

🧠 Exam technique

• Link your answer in a chain:
  fewer particles → fewer collisions → slower rate
• Use the key word concentration if you remember it, but the answer must still mention collisions.

❌ Common errors

• Saying the rate falls because “temperature decreases” — not stated in the question.
• Saying there are “fewer particles” without linking to collision frequency.
• Talking about products instead of reactant concentration.

💡 What to remember

• SO₂ gas is produced in the reaction.
• Time is a dependent variable.
• Higher temperature gives a shorter time.
• Rate can be found from the gradient of a tangent.
• Rate decreases when concentration falls because collisions become less frequent.

🧠 How to score well

• Use scientific vocabulary precisely.
• Show graph and calculation working clearly.
• Include units in every numerical answer.
• For explanation questions, always link cause and effect.