AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2023: Question 1

12 marks · Standard Demand difficulty · Short Answer

Multi-part question about carbon dioxide: why dissolved CO2 makes water acidic, name an indicator and expected result, interpret a solubility vs temperature graph and pH change with temperature, identify the state symbol in a carbonate + HCl equation, and describe a method to measure CO2 volume produced when varying masses of calcium carbonate react with hydrochloric acid.

Practise this question

Question

Question text

01 This question is about carbon dioxide.

Carbon dioxide is soluble in water and forms an acidic solution.

01.1 Which ion makes the solution acidic?

[1 mark]

01.2 Name an indicator that could be used to test if the solution is acidic.

Give the result of the test.

[2 marks]

Indicator

Result

Figure 1 shows the mass of carbon dioxide that will dissolve in 1 dm3 of water

at different temperatures.

Figure 1

01.3 How does the solubility of carbon dioxide change as the temperature of the

water increases?

*03* [1 mark]

Tick ( ) one box.

The solubility decreases

The solubility does not change

The solubility increases 5

01.4 Carbon dioxide dissolves in water to form an acidic solution.

How does the pH of the solution change as the temperature of the water increases?

Use Figure 1.

[1 mark]

Tick ( ) one box.

pH of the solution decreases

pH of the solution does not change

pH of the solution increases

Calcium carbonate reacts with hydrochloric acid to produce carbon dioxide.

The equation for the reaction is:

CaCO3(s) + 2 HCl(aq) → CaCl2(aq) + CO2(g) + H2O(x)

01.5 What is the state symbol (x) in the equation?

[1 mark]

Tick ( ) one box.

(aq) (g) (l)6 (s)

01.6 Figure 2 shows equipment a student used for an investigation.

Figure 2

The student investigated the volume of carbon dioxide produced when different

masses of calcium carbonate react with hydrochloric acid.

Describe a method the student could use.

[6 marks]

Mark scheme

Show the mark scheme

Question 1

AO /

Question Answers Extra information Mark

Spec. Ref.

01.1 hydrogen / H+ 1 AO1

5.4.2.4

AO /

Spec. Ref.

01.2 MP2 is dependent upon MP1 AO1

being awarded 5.4.2.4

(indicator) universal ignore pH meter 1

(result) (green to) red / orange / 1

yellow

(indicator) (blue) litmus (1)

(result) (turns) red (1)

(indicator) methyl orange (1)

(result) (turns) red (1)

AO /

Spec. Ref.

01.3 the solubility decreases 1 AO3

5.3.2.5

AO /

Spec. Ref.

01.4 pH of the solution increases 1 AO3

5.3.2.5

5.4.2.4

AO /

Spec. Ref.

01.5 (l) 1 AO2

5.2.2.2 7

AO /

Question Answers Mark

Spec. Ref.

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

01.6 AO3

outcome. All key steps are identified and logically sequenced.

5.3.1.3

5.4.2.2

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

outcome. Most steps are identified, but the plan 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 a (stated) mass of calcium carbonate

• use a balance

• add calcium carbonate into a conical flask

• measure a (stated) volume of hydrochloric acid

• use a measuring cylinder

• add hydrochloric acid to the conical flask

• immediately place the stopper (and delivery tube) in the conical

flask

• record the total volume of carbon dioxide gas

record volume of carbon dioxide collected in set time

time taken to collect fixed volume of gas

• collected in the gas syringe

• repeat

• repeat method with different masses of calcium carbonate

Total Question 1 12

How to answer it

Carbon dioxide in water: acidity, indicators, graphs & gas collection

What this question tests

Acids in solution: which ion makes a solution acidic (H⁺).
Choosing a suitable indicator and stating the correct colour change in acid.
Interpreting a graph of solubility vs temperature (trend).
Linking solubility of CO₂ to pH (less dissolved CO₂ → less acidic → higher pH).
State symbols in equations (recognising liquid water as (l)).
Planning a valid method: measuring variables, using equipment, collecting gas, repeats, and control variables.

Part (01.1) — Which ion makes the solution acidic? [1 mark]

✅ Correct answer (1 mark)

Hydrogen ion / H⁺

Marking point: 1 mark for stating hydrogen (H⁺).

💡 Key knowledge

A solution is acidic when it contains H⁺ ions (higher concentration of H⁺ than in neutral water).
CO₂ dissolves in water and forms an acidic solution (because it leads to H⁺ being present).

❌ Common errors

Saying CO₂ or carbonic acid instead of the ion asked for.
Writing H2 or H (must be H⁺ / hydrogen ion).

🧠 Exam technique

Underline the command word: it asks which ion, so answer must be an ion.
For GCSE acids: the “acidity ion” is almost always H⁺.

Part (01.2) — Name an indicator & give the result [2 marks]

✅ Correct answers (2 marks)

You must give an indicator and the correct colour in acid.

Universal indicator → green to red/orange/yellow
Blue litmus → turns red
Methyl orange → turns red

Mark breakdown:
• 1 mark for a correct indicator
• 1 mark for the correct result (depends on the indicator you chose)
Examiner note from mark scheme: the result mark is dependent on the indicator mark being awarded.

❌ Common errors (from examiner guidance)

Writing pH meter: the mark scheme says to ignore pH meter for this indicator question.
Giving an indicator but no colour change (or the wrong colour).
Universal indicator: saying only “it goes red” can be too vague if you don’t show it changed from the starting colour; best is “green to yellow/orange/red”.

🧠 Exam technique: how to secure both marks

Pick a standard indicator you know well (universal or litmus are safest).
Write it as two lines (as the answer space suggests): Indicator: … Result: …
Match your colour change to “acidic”:
- Litmus: blue → red
- Methyl orange: orange → red (in acid it is red)
- Universal: green (neutral) → yellow/orange/red (acid)

💡 Key knowledge

Indicators show acidity by colour because they respond to the concentration of H⁺ ions.

Part (01.3) — Solubility change with temperature [1 mark]

✅ Correct answer (1 mark)

The solubility decreases as temperature increases.

Why: From Figure 1, the curve goes downward as temperature goes up.

🧠 Exam technique (graph reading)

Don’t guess using general “solids dissolve more when warm”. This is a gas in water, and gases usually dissolve less in warmer water.
Use the direction of the line: if y-value goes down as x-value increases → decreases.

❌ Common errors

Choosing “increases” because of confusion with solids dissolving.
Saying “does not change” without checking the graph slope.

💡 Key knowledge

For gases (like CO₂) dissolved in water: increasing temperature generally makes the gas less soluble.

Part (01.4) — How does pH change as temperature increases? [1 mark]

✅ Correct answer (1 mark)

pH of the solution increases as temperature increases.

Link to Figure 1: Higher temperature → less CO₂ dissolves → solution is less acidic → higher pH.

💡 Key knowledge (the chain of reasoning)

More dissolved CO₂ → more acidic solution (more H⁺ present) → lower pH.
Figure 1 shows dissolved CO₂ decreases with temperature.
So acidity decreases → pH increases.

❌ Common errors

Thinking “higher temperature makes it more acidic” without using the solubility graph.
Mixing up pH direction: more acidic means lower pH, not higher.

🧠 Exam technique

For 1-mark multiple choice, you still need the logic: use Figure 1 to justify the tick.
Remember: pH up = less acidic; pH down = more acidic.

Part (01.5) — State symbol (x) in the equation [1 mark]

✅ Correct answer (1 mark)

The state symbol (x) for water in the equation is (l).

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l)

Marking point: 1 mark for (l).

💡 Key knowledge: state symbols

(s) = solid
(l) = liquid
(g) = gas
(aq) = aqueous (dissolved in water)

❌ Common errors

Putting (aq) for water: pure water in this reaction is written as (l).
Mixing (l) and (aq): (aq) is used for solutions, not for the solvent written as a product.

🧠 Exam technique

Check the equation: CO₂ is (g), calcium carbonate is (s), acid is (aq). That leaves water as a liquid product.

Part (01.6) — Required practical-style method: volume of CO₂ vs mass of CaCO₃ [6 marks]

💡 What the examiner is looking for

This is a level of response question. Top answers give a method that would produce a valid outcome, with steps in a clear logical order, using the given equipment (balance, measuring cylinder, conical flask, gas syringe).

Level descriptors (from mark scheme):
Level 3 (5–6): Valid outcome likely; all key steps identified and logically sequenced.
Level 2 (3–4): Most steps identified, but not fully sequenced / may not guarantee valid outcome.
Level 1 (1–2): Some relevant steps, but unclear links; not likely to lead to a valid outcome.

✅ Level 3 model method (5–6 marks)

Use a balance to measure a stated mass of calcium carbonate (e.g. 1.00 g). Record the mass.
Put the calcium carbonate into a conical flask.
Use a measuring cylinder to measure a stated volume of hydrochloric acid (e.g. 50 cm³). Keep the acid concentration the same each time.
Add the hydrochloric acid to the conical flask.
Immediately fit a stopper and delivery tube connected to the gas syringe (so gas is not lost).
Record the total volume of CO₂ collected in the gas syringe when the reaction finishes (or record the volume after a fixed time).
Repeat with different masses of calcium carbonate (e.g. 0.50 g, 1.50 g, 2.00 g), keeping the acid volume/concentration and temperature the same.
Repeat trials for each mass and calculate a mean volume to improve reliability.

Indicative points matched: mass measured (balance), CaCO₃ into flask, volume of acid measured (measuring cylinder), add acid, stopper quickly, collect CO₂ in gas syringe, record volume (or time-based), repeat and change mass.

🧠 Exam technique: how to write this for full marks

Sequence matters: measure → add → seal → record. If you seal after gas starts, you lose CO₂ and lose validity.
State what you change: mass of CaCO₃ (independent variable).
State what you measure: volume of CO₂ in the gas syringe (dependent variable).
State at least two controls: e.g. volume of acid, concentration of acid, temperature, same apparatus.
Add repeats + mean to show good science (often what separates Level 2 from Level 3).

📐 Data handling (what you’d do after)

Make a results table: mass of CaCO₃ (g) vs volume of CO₂ (cm³).
Plot a graph: mass (x-axis) vs volume of CO₂ (y-axis) and describe the trend.

❌ Common reasons students lose marks (examiner-style)

Not using the given equipment properly (e.g. no mention of gas syringe to measure CO₂ volume).
Forgetting to seal the flask immediately → gas escapes → results not valid.
Not stating different masses (so there is no investigation, just one reaction).
No controlled variables (acid volume/concentration/temperature) → unfair test.
No repeats, so reliability is weak (often keeps answers at Level 2).

Quick mark checklist (Question 1 total = 12 marks)

Parts (01.1) to (01.5) = 6 marks

01.1: H⁺ (1)
01.2: indicator (1) + correct colour result (1)
01.3: solubility decreases (1)
01.4: pH increases (1)
01.5: (l) (1)

Part (01.6) = 6 marks

Clear, sequenced method using balance, measuring cylinder, conical flask, gas syringe
Change mass of CaCO₃, measure CO₂ volume
Seal quickly, control variables, repeat/mean

Topics

Chemistry · Required Practicals · C4: Chemical Changes · C8: Chemical Analysis

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 1 (Higher), 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.