AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Foundation), June 2025: Question 7

✅ Mark Scheme Answer

A reaction which transfers energy to the surroundings.

💡 Key Knowledge

• Exothermic: Energy is transferred to the surroundings (causing the temperature of the surroundings to increase).
• Endothermic: Energy is taken in from the surroundings (causing the temperature of the surroundings to decrease).

❌ Common Errors

• Writing just "it gets hot" or "creates heat" — energy cannot be created, only transferred.
• Forgetting to state "to the surroundings". Always mention where the energy goes!

📐 Step-by-Step Calculation

1. Calculate total mass of oxygen in the formula:
   In Ca(OH)₂, the subscript 2 multiplies everything in brackets: 2 atoms of O.
   Mass of oxygen = 2 × 16 = 32
2. Set up percentage formula:
   Percentage = (Total mass of element / M_r of compound) × 100
   Percentage = (32 / 74) × 100
3. Calculate final percentage:
   (32 / 74) × 100 = 43.2432...% = 43.2% (or 43%)

🧠 Exam Technique: Formula Brackets

• Look carefully at brackets: Ca(OH)₂ contains 1 Ca, 2 O, and 2 H.
• The question already provided M_r = 74, saving you time. Always check if M_r is given in the prompt before spending time calculating it!
• Round reasonably: 43.2% or 43% are both accepted (at least 2 significant figures).

❌ Common Errors

• Using only 1 oxygen atom: (16 / 74) × 100 = 21.6%. This loses 1 mark, though error carried forward (ecf) applies to subsequent steps.
• Inverting the division: (74 / 32) × 100 = 231% (a percentage of an element inside a compound can never exceed 100%!).

✅ Model Method (Level 3: 5–6 Marks)

1. Measure volume of liquid: Use a measuring cylinder to measure a set volume of water (e.g. 25 cm³) and pour it into an insulated container (e.g. polystyrene cup).
2. Measure starting temperature: Place a thermometer into the water and record the initial temperature.
3. Weigh solid reactant: Use a digital balance to weigh a known mass of calcium oxide (e.g. 1.0 g).
4. Mix and observe: Add the solid to the water, stir the mixture thoroughly, and place a lid on the cup to minimise heat loss.
5. Measure final temperature: Record the maximum temperature reached on the thermometer.
6. Calculate temperature rise: Calculate temperature change = maximum temperature − initial temperature.
7. Repeat: Repeat the procedure using different masses of calcium oxide (e.g. 2.0 g, 3.0 g, 4.0 g, 5.0 g).
8. Control variables: Keep the volume of water and the starting temperature the same for each test.

🧠 How to Secure Level 3 (5–6 Marks)

Examiners use level criteria for this question:

• Level 3 (5–6 marks): Method leads to a valid outcome. All key steps are identified and logically sequenced. Must include the control variable: same volume of water.
• Level 2 (3–4 marks): Most steps identified, but sequencing is weak or control variables are missing.
• Level 1 (1–2 marks): Fragmented steps without clear equipment or structure.

Top tip: Always name the measuring instrument for every quantity (measuring cylinder for volume, balance for mass, thermometer for temperature).

💡 Apparatus & Experimental Logic

• Polystyrene cup: Chosen because polystyrene is a good thermal insulator, reducing heat loss to the surroundings so the measured temperature change is accurate.
• Lid: Reduces heat loss via convection and evaporation.
• Stirring: Ensures uniform temperature throughout the solution before recording the highest value.

❌ Common Student Pitfalls

• Missing the control variable: Failing to state that the volume of water must stay the same prevents you from getting Level 3!
• Vague equipment: Writing "add some water" or "weigh with scales" instead of measuring cylinder and balance.
• Forgetting repeats: Changing the independent variable (mass) is essential—you must state that you repeat with different masses.