AQA GCSE Chemistry Chemistry Paper 2 (Higher), June 2025: Question 1
11 marks · Standard Demand difficulty · Short Answer
Describe and analyze an experimental method to determine the mass and mean concentration of dissolved solids in different sea water samples.
Practise this questionQuestion
Question text
01 A student investigated the mass of dissolved solids in sea water.
This is the method used.
1. Weigh an empty evaporating basin.
2. Measure 10.0 cm3 of sea water sample A into the evaporating basin
using a burette.
3. Heat the evaporating basin until all the water seems to have evaporated.
4. Weigh the evaporating basin and contents.
5. Calculate the mass of dissolved solids in the sample.
6. Repeat steps 1 to 5 with sea water samples B, C and D.
01.1 Why did the student use a burette rather than a measuring cylinder to measure
10.0 cm3 of each sample?
[1 mark]
01.2 What was the dependent variable in this investigation?
[1 mark]
01.3 Describe how the student could make sure all of the water had evaporated in step 3.
[2 marks]
01.4 How did the student calculate the mass of dissolved solids in each sea water sample?
[1 mark]
Table 1 shows the results.
Table 1
Sea water sample A B C D
Mass of dissolved solids in grams 0.35 0.37 0.33 0.34
01.5 Calculate the mean concentration of dissolved solids in 10.0 cm3 samples of
sea water A, B, C and D.
Use Table 1 and the equation:
3 mean mass of dissolved solids (g)
mean concentration of dissolved solids (g/dm ) = 3
volume of sample (dm )
Give your answer in g/dm3.
[5 marks]
Mean concentration = g/dm3
01.6 Suggest why sea water samples A, B, C and D contain different masses
of dissolved solids.
[1 mark]
Mark scheme
Show the mark scheme
Question 1
AO /
Question Answers Extra information Mark
Spec. Ref.
01.1 (a burette is) more accurate allow (a burette has) a higher 1 AO3
(than a measuring cylinder) resolution 4.10.1.2
RPA8
ignore precise
AO /
Spec. Ref.
01.2 the mass of (dissolved) solids 1 AO1
4.10.1.2
RPA8
AO /
Spec. Ref.
01.3 reheat (the evaporating basin 1 AO3
and contents) 4.10.1.2
RPA8
(and reweigh) until constant 1
mass
allow for 2 marks
heat to constant mass
AO /
Spec. Ref.
01.4 subtract the mass of the (empty) allow subtract initial mass from 1 AO1
evaporating basin from the the final mass 4.10.1.2
mass of the evaporating basin RPA8
and contents (at the end of the
experiment)
AO /
Spec. Ref.
01.5 (allow mean mass =) (allow mean mass =) 1 AO2
0.35 + 0.37 + 0.33 + 0.34 1.39 4.3.2.5
44 4.10.1.2
RPA8
= 0.3475 (g) 1
(conversion 10 cm3 =) 0.01 dm3 1
0.3475 allow correct use of incorrectly 1
(mean concentration =)
0.01 determined mean mass
allow correct use of incorrect /
no conversion of volume
= 34.75 (g/dm3) allow 34.75 (g/dm3) correctly 1
rounded to at least 2 significant
figures
alternative approach 1
(conversion 10 cm3 =)
0.01 dm3 (1)
(concentrations =)
0.35 0.37 0.33 0.34 allow correct use of incorrect /
(1) no conversion of volume
0.01 0.01 0.01 0.01
= 35 37 33 34 (1)
(mean concentration =) (allow mean concentration =)
35 + 37 + 33 + 34 139
(1)
allow correct use of incorrectly
determined concentrations
= 34.75 (g/dm3) (1) allow 34.75 (g/dm3) correctly
rounded to at least 2 significant
figures
8 alternative approach 2 using
total mass and total volume
(total mass =
0.35 + 0.37 + 0.33 + 0.34 =)
1.39 (g) (1)
(total volume =
10 + 10 + 10 + 10 =)
40 cm3 (1)
(conversion 40 cm3 =)
0.04 dm3 (1)
1.39 allow correct use of incorrectly
(mean concentration =) (1)
0.04 determined total mass
allow correct use of incorrect /
no conversion of total volume
= 34.75 (g/dm3) (1) allow 34.75 (g/dm3) correctly
rounded to at least 2 significant
figures
AO /
Spec. Ref.
01.6 the samples were taken from 1 AO3
different places 4.10.1.2
Total Question 1 11
How to answer it
Determining Dissolved Solids in Sea Water (RPA 8)
This question assesses practical skills from Required Practical Activity 8: Analysis and Purification of Water Samples. You must demonstrate an understanding of apparatus accuracy (burette vs measuring cylinder), identifying scientific variables, the classic technique of heating to constant mass, mass-by-difference calculations, unit conversions (cm³ to dm³), concentration calculations, and evaluating experimental differences between real-world samples.
Question 01.1
Apparatus Selection: Burette vs Measuring Cylinder
1 Mark✅ Correct Answer
A burette is more accurate (or has a higher resolution) than a measuring cylinder.
❌ Common Errors
Writing that a burette is "more precise" or "more reliable". The mark scheme explicitly states: ignore precise. Accuracy or resolution are the required scientific terms.
Question 01.2
Identifying the Dependent Variable
1 Mark✅ Correct Answer
The mass of (dissolved) solids.
💡 Key Knowledge
- Independent variable: What the experimenter changes (the sea water sample A, B, C, D).
- Dependent variable: What is measured (the mass of dissolved solids remaining).
- Control variable: What is kept the same (the volume of water used: 10.0 cm³).
Question 01.3
Ensuring Complete Evaporation
2 Marks✅ Correct Answer
- Reheat the evaporating basin and contents [1 mark]
- Reweigh until the mass is constant [1 mark]
Note: Stating "heat to constant mass" earns both marks directly.
🧠 Exam Technique: Key Laboratory Phrase
Whenever an exam asks how to ensure all water has evaporated or a thermal decomposition is complete, the standard response is always: "Heat, reweigh, and repeat until the mass remains constant."
❌ Common Errors
Saying "look closely to see if there is no liquid left" or "heat it for a longer time". Visual inspection does not prove dryness because trapped moisture cannot be seen.
Question 01.4
Determining Mass of Dissolved Solids
1 Mark✅ Correct Answer
Subtract the mass of the empty evaporating basin from the mass of the evaporating basin and contents at the end.
(Or: "Subtract initial mass from final mass")
💡 Key Knowledge
This method is known as weighing by difference:
Mass of dissolved solid = (Basin + solid) − (Empty basin)
Question 01.5
Calculation: Mean Concentration in g/dm³
5 Marks📐 Step-by-Step Calculation
Given Data:
- Sample masses: A = 0.35 g, B = 0.37 g, C = 0.33 g, D = 0.34 g
- Volume of sample = 10.0 cm³
- Formula: mean concentration (g/dm³) = mean mass of dissolved solids (g) ÷ volume of sample (dm³)
Step 1: Calculate the mean mass of dissolved solids
Total mass = 0.35 + 0.37 + 0.33 + 0.34 = 1.39 g [1 mark]
Mean mass = 1.39 ÷ 4 = 0.3475 g [1 mark]
Step 2: Convert volume from cm³ to dm³
To convert cm³ to dm³, divide by 1000:
Volume = 10.0 ÷ 1000 = 0.01 dm³ [1 mark]
Step 3: Calculate the mean concentration
Mean concentration = 0.3475 g ÷ 0.01 dm³ [1 mark]
Mean concentration = 34.75 g/dm³ [1 mark]
(Allow rounding to at least 2 significant figures, e.g. 34.8 or 35 g/dm³)
🧠 Alternative Method (Equal Marks)
Calculate total mass and total volume:
- Total mass = 1.39 g
- Total volume = 4 × 10.0 cm³ = 40.0 cm³ = 0.04 dm³
- Concentration = 1.39 ÷ 0.04 = 34.75 g/dm³
❌ Calculation Traps
- Forgetting the volume conversion: Dividing by 10 instead of 0.01 gives an answer 1000 times too small (0.03475 g/dm³).
- Arithmetic slips: Forgetting to divide by 4 when calculating the mean mass.
Question 01.6
Explaining Differences in Results
1 Mark✅ Correct Answer
The sea water samples were taken from different places / locations (or different depths / different sources).
❌ Common Errors
Blaming "human error" or "student did not heat properly". The question asks why the sea water samples themselves contain different masses of solids, not why experimental error occurred.
Topics
Chemistry · Required Practicals · Required Practicals · C3: Quantitative Chemistry · C10: Using Resources
Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 2 (Higher), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.