AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2021: Question 3

12 marks · Standard Demand difficulty · Extended Answer

Investigate and explain the temperature change when ammonium nitrate dissolves in water, identify variables and improvements, interpret a temperature-time graph, and draw an exothermic reaction profile.

Practise this question

Question

The question page is headed 'This question is about energy change' and describes an investigation where 10 g of ammonium nitrate is added to 100 cm³ of water, stirred once, and the temperature is measured every minute for 7 minutes. A labelled diagram shows a glass beaker containing water and ammonium nitrate with a thermometer in the beaker, followed by questions asking for the dependent variable, three improvements to increase accuracy, an explanation of the results from a graph, and a reaction profile for an exothermic reaction. The graph plots temperature in degrees Celsius against time in minutes, showing a fall from about 21.4 °C at 0 minutes to about 15.4 °C at 1.5 minutes, then a gradual rise to about 17.0 °C by 7 minutes. The final part provides blank axes labelled Energy on the y-axis and Progress of reaction on the x-axis for students to draw and label an exothermic reaction profile including reactants, products, activation energy, and overall energy change.
Question text

03 This question is about energy change.

A student investigated the temperature change when 10 g of ammonium nitrate was

added to 100 cm3 of water.

This is the method used.

1. Measure the temperature of 100 cm3 of water.

2. Add 10 g of ammonium nitrate.

3. Stir once.

4. Measure the temperature of the solution every minute for 7 minutes.

Figure 3 shows the apparatus.

Figure 3

03.1 What is the dependent variable in this investigation?

[1 mark]

03.2 Give three improvements to the investigation to make the results more accurate.

[3 marks]

03.3 Figure 4 shows the results.

*10* Figure 4

Explain the results.

[4 marks]

03.4 Draw a reaction profile for an exothermic reaction.

You should label:

• the energy level of the reactants and of the products

• the activation energy

• the overall energy change.

[4 marks]

Mark scheme

Show the mark scheme The mark scheme is a table listing answers for parts 03.1 to 03.4 with marks and specification references. It states that the dependent variable is the temperature of the solution; acceptable improvements include insulating the beaker or using a polystyrene cup, adding a lid, stirring more, repeating and averaging, using less water, more ammonium nitrate, a more accurate balance, or a digital thermometer; and for the graph explanation it credits noting the initial temperature decrease because dissolving ammonium nitrate is endothermic, followed by a temperature increase because energy transfers from the surroundings to the solution. For the reaction profile, it awards marks for labelled reactant and product energy levels with products below reactants, a reaction pathway curve, activation energy shown from reactants to the peak, and overall energy change shown from reactants to products, with an example exothermic profile diagram included.

AO /

Question Answers Extra information Mark

Spec. Ref.

03.1 temperature (of solution) allow temperature change 1 AO1

5.5.1.1

03.2 any three from: 3 AO3

5.5.1.1

• insulate the beaker

or

use polystyrene cup

• add a lid

• stir more (times)

• repeat the experiment and

calculate the mean (ignoring

anomalous results)

• use smaller volume (of water)

• use larger mass of ammonium

nitrate

• more accurate balance

• use digital thermometer

or

use a more accurate

thermometer

03.3 (from 0 to 1.5 minutes the) allow the temperature 1 AO3

temperature decreases decreases (from 21.4 ⁰C to 15.4 5.5.1.1

⁰C)

(because) ammonium nitrate 1

dissolving is endothermic

(then) after 1.5 minutes the (then) after 15.4 ⁰C the 1

temperature increases temperature increases

(because) energy transfers to 1

the solution from the – OMBINED SCIENCE: TRILOGY – – JUNE 2021

surroundings

03.4 labelled horizontal lines for 1 AO1

reactants and products, with the 5.5.1.2

product line below the level of

10 the reactant line

reaction pathway allow curve to start / finish 1

anywhere along reactant /

product lines

line from reactants to maximum 1

labelled activation energy

line from reactants to products 1

labelled overall energy change

the diagram below scores 4 marks

Total 12

How to answer it

Ammonium nitrate temperature change

What this question tests

Temperature change in a simple investigation, improving experimental method, explaining an endothermic change, and drawing a reaction profile diagram with reactants, products, activation energy, and overall energy change.

Question overview

💡 Key knowledge

  • Ammonium nitrate dissolving in water is endothermic.
  • In an endothermic process, energy is taken in from the surroundings.
  • This causes the temperature of the solution to fall first.
  • A reaction profile for an exothermic reaction has products lower than reactants.

🧠 Exam technique

  • Give the exact variable asked for: not just “temperature”, but usually temperature of the solution.
  • For “improvements”, each suggestion should clearly make results more accurate.
  • For graph explanations, read the trend first, then link it to the science.

Part (a) — 03.1

What is the dependent variable in this investigation?

✅ Correct answer

Temperature of the solution
Accept: temperature change

1 mark for correctly identifying what is measured.

💡 Key knowledge

  • The dependent variable is the thing you measure.
  • Here, the student measures the temperature every minute.

❌ Common errors

  • Writing ammonium nitrate — that is the independent variable/material being added.
  • Writing time — that is controlled by the student, not the main outcome.
  • Giving only temperature with no link to the solution can still be accepted, but “temperature of the solution” is safer.

Part (b) — 03.2

Give three improvements to the investigation to make the results more accurate.

✅ Correct answers — any three

  • Insulate the beaker / use a polystyrene cup
  • Add a lid
  • Stir more / stir repeatedly so the solution is mixed evenly
  • Repeat the experiment and calculate the mean (ignore anomalous results)
  • Use a smaller volume of water
  • Use a larger mass of ammonium nitrate
  • Use a more accurate balance
  • Use a digital thermometer / more accurate thermometer
3 marks total: 1 mark for each valid improvement.

💡 Key knowledge

  • Accuracy means getting a result closer to the true value.
  • Heat loss to the surroundings can hide the real temperature change.
  • Repeating and averaging reduces the effect of random error.

🧠 Exam technique

  • Be specific: “insulate the beaker” is better than “make it better”.
  • If you say “repeat the experiment”, add mean to show how it improves reliability.
  • Any three distinct improvements can score full marks.

❌ Common errors

  • Suggestions like “use more chemicals” without saying larger mass of ammonium nitrate are too vague.
  • “Do it carefully” is not a scientific improvement.
  • “Use a thermometer” is too general — the mark scheme wants digital/more accurate thermometer.

Part (c) — 03.3

Explain the results.

✅ Full-mark explanation

  • From 0 to 1.5 minutes, the temperature decreases.
  • This is because ammonium nitrate dissolving is endothermic.
  • After about 1.5 minutes, the temperature begins to increase.
  • This is because energy transfers to the solution from the surroundings.
4 marks are awarded for four linked points: trend, reason, second trend, second reason.

💡 Key knowledge

  • The graph shows temperature falling from about 21.4 °C to 15.4 °C.
  • An endothermic change takes in thermal energy, so the solution gets colder.
  • After the main dissolving has happened, the mixture starts warming up again because it gains heat from the surroundings.

🧠 Exam technique

  • Use “because” to link the observation to the science.
  • A top answer describes the trend first, then explains it.
  • Use time references from the graph: this shows you have read the data carefully.

❌ Common errors

  • Saying the reaction is exothermic would lose the explanation mark.
  • Saying only “the temperature changes” is too vague.
  • Confusing energy transfer from surroundings to solution with energy being released by the solution.

Part (d) — 03.4

Draw a reaction profile for an exothermic reaction.

✅ What the diagram must show

  • Reactants as a horizontal line at a higher energy level.
  • Products as a horizontal line at a lower energy level than the reactants.
  • A curve that rises to a peak and then falls to the products level.
  • Activation energy labelled from the reactants level up to the peak.
  • Overall energy change labelled from reactants level down to products level.
4 marks: one for reactants/products levels, one for reaction pathway, one for activation energy, one for overall energy change.

💡 Key knowledge

  • For an exothermic reaction, energy is released to the surroundings.
  • That means the products end up at a lower energy than the reactants.
  • The peak shows the activation energy barrier.

🧠 Exam technique

  • If you cannot draw neatly, make sure the labels are still correct and clearly placed.
  • The curve can start/finish anywhere on the reactant/product lines unless the levels are correct.
  • Use clear arrows: one for activation energy, one for overall energy change.

❌ Common errors

  • Putting products above reactants — that would show an endothermic reaction.
  • Forgetting to label the peak or the energy difference.
  • Drawing a straight line instead of a curve with a maximum.
  • Mixing up activation energy and overall energy change.

📐 If you need to sketch it in words

Draw a set of axes with Energy on the y-axis and Progress of reaction on the x-axis. Draw the reactants line first, then a curve rising to a peak, then falling to a lower products line. Label the vertical rise from reactants to the peak as activation energy. Label the vertical drop from reactants to products as overall energy change.

How marks are awarded

💡 Mark-building idea

  • 03.1: name the measured variable exactly.
  • 03.2: list three separate practical improvements.
  • 03.3: describe the trend + explain it using endothermic energy transfer.
  • 03.4: include all four features of the profile diagram.

🧠 Best way to secure full marks

  • Use short, clear scientific statements.
  • Always connect observation to reason.
  • For diagrams, prioritise labels and energy levels over artistic quality.

Topics

Chemistry · C5: Energy Changes

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 1 (Higher), 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.