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

10 marks · Standard Demand difficulty · Extended Answer

Define an exothermic reaction, calculate the percentage by mass of oxygen in calcium hydroxide, and plan an investigation into how the mass of calcium oxide affects temperature change.

Practise this question

Question

Question 2 consists of three parts. Part 02.1 asks what is meant by an exothermic reaction for 1 mark. Part 02.2 gives the chemical formula of calcium hydroxide as Ca(OH)2, relative atomic mass of oxygen as 16, and relative formula mass of Ca(OH)2 as 74, requiring students to calculate the percentage by mass of oxygen in calcium hydroxide for 3 marks. Part 02.3 asks students to plan a method to investigate how changing the mass of calcium oxide affects the temperature change of the reaction mixture for 6 marks.
Question text

02 A student investigated an exothermic reaction.

The student reacted solid calcium oxide with water to produce calcium hydroxide.

02.1 What is meant by an ‘exothermic’ reaction?

[1 mark]

02.2 The formula of calcium hydroxide is Ca(OH)2

Calculate the percentage by mass of oxygen in calcium hydroxide.

Relative atomic mass (Ar): O = 16

Relative formula mass (Mr): Ca(OH)2 = 74

[3 marks]

8 Percentage = %

02.3 When solid calcium oxide is added to water, the temperature of the reaction

mixture changes.

Plan a method to investigate how changing the mass of calcium oxide affects the

temperature change of the reaction mixture.

[6 marks]

Extra space

Mark scheme

Show the mark scheme Mark scheme for Question 2 with a total of 10 marks. Part 02.1 awards 1 mark for a reaction that transfers/releases energy to the surroundings. Part 02.2 awards 3 marks: 1 mark for calculating mass of oxygen as 32, 1 mark for setting up (32 / 74) * 100, and 1 mark for 43.2%. Part 02.3 provides a 6-mark level of response grid and indicative content for Required Practical 10, including measuring water volume, recording initial temperature, adding weighed calcium oxide, stirring, measuring maximum temperature, repeating with different masses, and identifying control variables.

Question 2

AO /

Question Answers Extra information Mark

Spec. Ref.

02.1 allow heat for energy AO1

5.5.1.1

(a reaction) which transfers allow (a reaction) which 1

energy to the surroundings releases energy (to the

surroundings)

AO /

Spec. Ref.

02.2 (mass of oxygen = 2 × 16 =) 32 1 AO2

5.3.1.2

32 allow correct use of an 1

(percentage =) × 100 incorrectly determined mass of

oxygen

= 43.2 (%) allow 43.2432 (%) correctly 1

rounded to at least 2 significant

– figures TRILOGY – 8464/C/1H –

AO /

Question Answers Mark

Spec. Ref.

02.3 Level 3: The method would lead to the production of a valid 5–6 AO1

outcome. All key steps are identified and logically sequenced. 5.5.1.1

RPA10

Level 2: The method would not necessarily lead to a valid 3–4

outcome. Most steps are identified, but the method is not fully

logically sequenced.

Level 1: The method would not lead to a valid outcome. Some 1–2

relevant steps are identified, but links are not made clear.

No relevant content 0

Indicative content

• measure volume of water

o using a measuring cylinder

o pour into a suitable container eg insulated cup

10 • measure initial temperature of water

o using a thermometer

• add known mass of calcium oxide

o measured with a balance

o stir

• measure the highest temperature reached by the mixture

or

measure temperature reached after set time period

o determine the temperature change

• repeat with different masses of calcium oxide

or

add successive masses to the same mixture

• use the same volume of water

o use the same initial temperature of water

for Level 3 all key steps should be described and the control

variable the same volume of water must be given

Total Question 2 10

How to answer it

Exothermic Reactions & Temperature Changes

📌 What this question tests

This question assesses core understanding from AQA GCSE Energy Changes (C5) and Quantitative Chemistry (C3):

  • Key Recall: The precise scientific definition of an exothermic reaction.
  • Quantitative Skills: Calculating the percentage by mass of an element within a chemical compound using formula brackets.
  • Practical Skills (RPA 10): Planning a valid, controlled calorimeter experiment to investigate temperature change versus reactant mass.
Question 02.1 · 1 Mark

Definition of 'Exothermic'

A student investigated an exothermic reaction. What is meant by an 'exothermic' reaction?

✅ Model Answer

A reaction that transfers energy to the surroundings.

1 Mark: Mention of energy (or heat) transferred/released to the surroundings.

💡 Key Knowledge

  • Exothermic: Energy exits the reaction mixture → transfers to the surroundings → surrounding temperature increases.
  • Endothermic: Energy is taken in from the surroundings → surrounding temperature decreases.

🧠 Exam Technique

Always state where the energy goes! Writing simply "it gives off heat" is borderline. Using the exact exam-board phrasing: "transfers energy to the surroundings" guarantees the mark every single time.

❌ Common Errors

  • Writing "it gets hot" (describes an observation, not what the reaction means in terms of energy).
  • Mixing up exothermic and endothermic.
  • Forgetting the phrase "to the surroundings".
Question 02.2 · 3 Marks

Percentage by Mass Calculation

Calculate the percentage by mass of oxygen in calcium hydroxide, Ca(OH)₂.
Ar: O = 16 | Mr: Ca(OH)₂ = 74

📐 Step-by-Step Calculation

  1. Step 1: Calculate total mass of oxygen in the formula
    Look at the brackets: (OH)₂ means there are 2 oxygen atoms.
    Mass of oxygen = 2 × 16 = 32
    ✔ Mark 1: Correct total mass of oxygen (32)
  2. Step 2: Set up the percentage formula
    % mass = (Total mass of element ÷ Total Mr of compound) × 100
    % mass = (32 ÷ 74) × 100
    ✔ Mark 2: Correct substitution: (32 / 74) × 100 (allow ecf if oxygen mass was wrong)
  3. Step 3: Calculate and round the final answer
    32 ÷ 74 × 100 = 43.2432...%
    Final Answer = 43.2% (or 43%)
    ✔ Mark 3: Correct value rounded to at least 2 significant figures (43.2% or 43%)

❌ Common Calculation Traps

  • The Bracket Trap: Using 16 instead of 32 because the subscript "2" outside the bracket was overlooked. (16/74 gives 21.6% — loses Mark 1).
  • Inverting the fraction: Dividing 74 by 32. The percentage must always be less than 100%!
  • Premature rounding: Always keep precision in your calculator before writing down the final rounded percentage.

🧠 Exam Technique

The question provides the Mr (74), so you do not need to calculate it yourself! Always check given values carefully before spending time working them out from scratch.

Question 02.3 · 6 Marks

Extended Response: Planning a Calorimetry Method

Plan a method to investigate how changing the mass of calcium oxide affects the temperature change of the reaction mixture.

✅ Model Answer (Full 6/6 Marks — Level 3)

  1. Measure water volume: Use a measuring cylinder to measure a set volume of water (e.g. 25 cm³) and pour it into an insulated polystyrene cup placed inside a glass beaker for stability.
  2. Initial temperature: Place a thermometer into the water and record the initial starting temperature.
  3. Measure reactant mass: Use a digital balance to weigh out a known mass of solid calcium oxide (e.g. 1.0 g).
  4. Mix and stir: Add the calcium oxide to the water, stir the mixture continuously using the thermometer or stirring rod to ensure thorough reaction.
  5. Record maximum temperature: Observe the thermometer and record the highest temperature reached by the mixture.
  6. Calculate change: Calculate the temperature change = (highest temperature − initial temperature).
  7. Repeat across a range: Repeat the entire procedure using different masses of calcium oxide (e.g. 2.0 g, 3.0 g, 4.0 g, and 5.0 g).
  8. Control variables: Keep the volume of water constant (e.g. 25 cm³) and ensure the initial starting temperature of the water is the same for each trial.

💡 Equipment & Variables Summary

  • Independent Variable: Mass of calcium oxide (measured using a balance).
  • Dependent Variable: Temperature change (measured using a thermometer).
  • Control Variables: Volume of water (measuring cylinder), initial water temperature, type of cup (polystyrene insulation).

🧠 Examiner Criteria for Level 3 (5–6 Marks)

  • Logical Sequencing: Steps must follow a sensible chronological order (measure water → initial temp → add mass → stir → highest temp).
  • Essential Control Variable: The mark scheme specifically states that to achieve Level 3, keeping the volume of water the same MUST be explicitly included.
  • Range: Mention at least 4–5 different masses or state "repeat with different masses".

❌ Why Students Lost Marks on this 6-Marker

  • Vague equipment: Writing "put some water in a cup" without naming a measuring cylinder or polystyrene cup.
  • Missing initial temperature: You cannot find the temperature change without recording the starting temperature before adding the powder!
  • No repetition with different masses: The question asks about the effect of changing the mass. Failing to state that you repeat with different masses caps your response to Level 1 or 2.
  • Omitting the volume control: Forgetting to specify that the volume of water remains constant immediately prevents you from getting into Level 3.

Topics

Chemistry · C3: Quantitative Chemistry · C5: Energy Changes

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 1 (Higher), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.