AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Foundation), November 2020: Question 3
11 marks · Low Demand difficulty · Short Answer
Investigate the mass of dissolved solids in water samples using evaporation, addressing apparatus, experimental errors, means, and proportion calculations.
Practise this questionQuestion
Question text
03 A student investigated the mass of dissolved solids in water samples.
Figure 4 shows the apparatus used.
Figure 4
This is the method used.
1. Record the mass of a dry evaporating basin.
2. Pour 25 cm3 of the water sample into the evaporating basin.
3. Place the evaporating basin on the beaker for 10 minutes.
4. Record the mass of the evaporating basin and contents.
03.1 What is used to find the mass of the evaporating basin?
[1 mark]
Tick ( ) one box.
Balance
Beaker
Measuring cylinder
Thermometer 11
One error is that droplets of water collect on the bottom of the evaporating basin.
03.2 Suggest how this error affects the mass of the evaporating basin and contents.
[1 mark]
03.3 How can this error be corrected?
[1 mark]
03.4 Another error in the method is that not all the water was removed from the
water sample.
How can this error be corrected?
[1 mark]
Tick ( ) one box.
Add more boiling water to the beaker.
Heat until the mass of the evaporating basin and contents is constant.
Stir the water sample in the evaporating12 basin with a glass rod.
03.5 The water in the water sample turns into steam.
What is the name of this process?
[1 mark]
Another student did the experiment correctly with three water samples A, B and C.
Table 2 shows the results.
Table 2
Mass of dissolved solids in g
Water sample
Test 1 Test 2 Test 3 Mean
A 0.23 0.23 0.20 X
B 0.03 0.07 0.02 0.04
C 1.45 1.60 1.45 1.50
03.6 The range is the difference between the largest value and the smallest value.
Which water sample has the greatest range of results?
[1 mark]
Tick ( ) one box.
A
B
C 13
03.7 Calculate the mean mass X for water sample A.
Use Table 2.
[2 marks]
X = g
03.8 What is the dependent variable in this experiment?
[1 mark]
Tick ( ) one box.
Mass of dissolved solids
Time taken for water to heat
Type of water sample
Volume of boiling water
03.9 A different water sample contains 3.6 g of dissolved solids in 150 cm3
Calculate the mass of dissolved solids in 25 cm3 of this sample.
[2 marks]
Mass = g
Mark scheme
Show the mark scheme
AO /
Question Answers Extra information Mark
Spec. Ref.
03.1 balance 1 AO1
5.10.1.2
RPA13
03.2 mass was greater / more than 1 AO3
expected 5.10.1.2
RPA13
03.3 dry the bottom of the 1 AO3
evaporating basin 5.10.1.2
RPA13
or
use an electric heater
03.4 heat until the mass of the evaporating basin and contents is 1 AO3
constant. 5.10.1.2
RPA13
03.5 evaporation ignore boiling 1 AO1
5.10.1.2
RPA13
03.6 C 1 AO2
5.3.1.4
5.10.1.2
RPA13
03.7 0.23 + 0.23 + 0.20 0.66 1 AO2
or
33 5.10.1.2
RPA13
= 0.22 (g) 1
03.8 mass of dissolved solids 1 AO1
5.10.1.2
RPA13
03.9 25 1 AO2
× 3.6 or × 3.6 1
150 6 5.3.2.5
5.10.1.2
RPA13
= 0.6 (g) 1
Total 11
How to answer it
AQA GCSE Chemistry: Water Analysis Practical
What this question tests
- Required Practical 13: Analysis and purification of water samples by evaporation to dryness.
- Apparatus recall: Identifying the correct scientific instrument to measure mass.
- Evaluating experimental errors: Sources of systematic errors (condensation, incomplete evaporation) and how to fix them.
- Scientific concepts: Understanding the term "heating to constant mass" and state changes (evaporation).
- Working scientifically: Identifying dependent variables, calculating data range, finding a mean value, and scaling concentrations via ratios.
Measuring Mass
What is used to find the mass of the evaporating basin? [1 mark]
✅ Correct Answer
Balance [1 mark]
❌ Common Errors
- Writing "scales" — while often understood, GCSE specifications specifically credit balance (or digital balance).
- Selecting measuring cylinder (which measures volume, not mass).
Condensation Error on Apparatus
Suggest how droplets on the bottom affect mass and how to correct this error. [2 marks]
✅ Correct Answers
03.2: The recorded mass was greater / more than expected. [1 mark]
03.3: Dry the bottom of the evaporating basin (or use an electric heater). [1 mark]
💡 Key Knowledge
Steam from the water bath condenses into liquid water on the cooler underside of the ceramic basin. If weighed with these droplets attached, you weigh the basin + dissolved solid + condensed water, leading to an artificially high mass.
🧠 Exam Technique
When asked how an error affects a measurement, clearly state the direction of change: say "greater than" or "too high" rather than just "it changes the results".
Ensuring Complete Evaporation
How to correct the error that not all water was removed? [1 mark]
✅ Correct Answer
Tick: Heat until the mass of the evaporating basin and contents is constant [1 mark]
💡 Key Knowledge: Heating to Constant Mass
This is a classic standard laboratory procedure in chemistry:
- Heat the evaporating basin and contents.
- Allow to cool and weigh on a balance.
- Reheat, cool, and reweigh.
- Repeat until two successive weighings give the exact same mass.
State Change
The water in the water sample turns into steam. What is the name of this process? [1 mark]
✅ Correct Answer
Evaporation [1 mark]
❌ Common Errors
The mark scheme notes: ignore boiling. Students often write "boiling" or "steam". The transition of liquid water to vapour when heated below or at its boiling point is scientifically termed evaporation.
Range of Experimental Data
Which water sample has the greatest range of results? [1 mark]
✅ Correct Answer
Tick box: C [1 mark]
📐 Calculation Check
Range = Highest value − Lowest value
- Sample A: 0.23 − 0.20 = 0.03 g
- Sample B: 0.07 − 0.02 = 0.05 g
- Sample C: 1.60 − 1.45 = 0.15 g (Greatest!)
Calculating the Mean
Calculate the mean mass X for water sample A. [2 marks]
📐 Step-by-Step Calculation
0.23 + 0.23 + 0.20 = 0.66 [1 mark]
0.66 ÷ 3 = 0.22 [1 mark]
X = 0.22 g
🧠 Exam Technique
- Always check for anomalies before calculating a mean. Here, 0.23, 0.23, and 0.20 are close together, so none are discarded.
- Maintain consistent decimal places matching the raw data (2 decimal places).
Scientific Variables
What is the dependent variable in this experiment? [1 mark]
✅ Correct Answer
Tick: Mass of dissolved solids [1 mark]
💡 Variable Quick Reference
- Independent variable: What you change → Type of water sample (A, B, C).
- Dependent variable: What you measure → Mass of dissolved solids.
- Control variables: What you keep constant → Volume of water sample (25 cm³).
Proportional Scaling Calculation
A sample contains 3.6 g of dissolved solids in 150 cm³. Calculate the mass in 25 cm³. [2 marks]
📐 Step-by-Step Calculation
Fraction of volume taken = 25 ÷ 150 = 1/6
Working: (25 ÷ 150) × 3.6 [1 mark]
Mass per cm³ = 3.6 g ÷ 150 cm³ = 0.024 g/cm³
Mass in 25 cm³ = 0.024 × 25 [1 mark]
Mass = 0.6 (g) [1 mark]
❌ Common Errors
- Multiplying 150 × 3.6 instead of dividing to find the concentration.
- Inverting the ratio (doing 150 ÷ 25 = 6, and then multiplying 3.6 × 6 = 21.6 g instead of dividing by 6). A smaller volume must contain a smaller mass!
Topics
Chemistry · C10: Using Resources · C3: Quantitative Chemistry
Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 2 (Foundation), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.