AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2019: Question 6

✅ Correct answer

• Test: use a glowing splint.
• Result: the splint relights.

💡 Key knowledge

• Oxygen supports combustion.
• The standard GCSE test is a glowing splint, not a lit splint.
• If oxygen is present, the glowing splint bursts back into flame or relights.

🧠 Exam technique

• Use the exact practical wording: glowing splint .
• Give the observation, not just “oxygen is confirmed”.
• Keep it short and direct: one mark for the test, one mark for the result.

❌ Common errors

• Writing lit splint instead of glowing splint.
• Saying “makes a squeaky pop” — that is the test for hydrogen.
• Writing “burns” without stating that it relights or rekindles.

✅ Correct answer

• The equilibrium shifts to the right-hand side.
• This is towards the products / in the forward reaction.
• This happens because the concentration of SO₃ decreases.

💡 Key knowledge

• Reaction: O₂(g) + 2SO₂(g) ⇌ 2SO₃(g)
• Equilibrium is a dynamic balance.
• If a product is removed, the system responds by making more of that product.
• This is a simple application of Le Chatelier’s principle.

🧠 Exam technique

• Use the phrase shifts to the right or towards products.
• Link the shift to the change: SO₃ is removed, so its concentration decreases.
• If you want to show stronger understanding, say the system tries to re-establish equilibrium.

❌ Common errors

• Saying the equilibrium shifts left because “there is less product” — this is the opposite of the correct response.
• Forgetting to mention the decrease in concentration of SO₃.
• Writing vague answers such as “it changes” or “it balances again” without stating the direction.

📐 Calculations — step by step

1. Find the Mr of each substance:
   • CaO = 40 + 16 = 56
   • CaSO₃ = 40 + 32 + (3 × 16) = 120
2. Use the equation CaO + SO₂ → CaSO₃ . The ratio of CaO : CaSO₃ is 1 : 1.
3. Convert 7.00 g of CaO to moles:
   moles of CaO = 7.00 ÷ 56 = 0.125 mol
4. Because the ratio is 1 : 1, moles of CaSO₃ = 0.125 mol.
5. Calculate mass of CaSO₃:
   mass = moles × Mr = 0.125 × 120 = 15.0 g

Answer: 15.0 g

✅ Correct answer

• Mass of calcium sulfite produced = 15.0 g

💡 Key knowledge

• Relative atomic masses used: Ca = 40, S = 32, O = 16.
• Excess sulfur dioxide means calcium oxide is the limiting reactant.
• The equation shows a 1:1 ratio between CaO and CaSO₃.
• Units matter: mass in g, amount in mol.

🧠 Exam technique

• Write the equation or state the ratio before calculating.
• Show every stage clearly to pick up method marks.
• Use at least 3 significant figures if appropriate: 15.0 g .
• If you make a small earlier mistake but use the correct method, you can still gain marks for the working.

❌ Common errors

• Using the wrong Mr for CaSO₃, often forgetting the three oxygen atoms.
• Using the formula mass of CaO as 40 instead of 56.
• Ignoring the 1:1 ratio in the balanced equation.
• Forgetting to convert grams to moles first.
• Writing 15 g without showing working can lose method marks if the answer is not fully supported.

Oxygen test

Glowing splint → relights

Equilibrium

Removing SO₃ makes its concentration fall, so equilibrium shifts right to make more products.

Calculation

CaO + SO₂ → CaSO₃
7.00 g CaO → 15.0 g CaSO₃