AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Foundation), 2023: Question 3
12 marks · Standard Demand difficulty · Short Answer
Analyze water samples for dissolved solids, identify methods for producing potable water, and evaluate the impact of fluoridation using graphical data.
Practise this questionQuestion
Question text
03 Some types of water contain dissolved substances.
A student investigated the mass of dissolved solids in distilled water and in sea water.
Figure 5 shows the apparatus.
Figure 5
This is the method used.
1. Weigh an evaporating basin.
2. Add a sample of distilled water to the evaporating basin.
3. Weigh the evaporating basin and the water sample.
4. Heat the water sample.
5. Weigh the evaporating basin and contents.
6. Repeat steps 1 to 5 two more times.
7. Repeat steps 1 to 6 with sea water.
03.1 Give one safety precaution the student should take.
[1 mark]
03.2 The method used did not give valid results.
Which two improvements could the student make to give valid results?
[2 marks]
Tick ( ) two boxes.
*14* Heat until the mass of the evaporating basin and contents does not change.
Leave some water in the evaporating basin after heating.
Record the results in a table and plot a bar graph.
Use 25 cm3 of water for each experiment.
Use a beaker instead of an evaporating basin.
03.3 A different student used a method which gave valid results.
Table 2 shows the results.
Table 2
Mass of dissolved solids in grams
Type of water Test 1 Test 2 Test 3
Distilled water 0.00 0.00 0.00
Sea water 0.31 0.32 0.27
The student concludes that distilled water is pure.
Suggest one reason why.
[1 mark]
Tap water is potable.
One step in producing potable water is sterilisation.
*0153.*4 Why is potable water sterilised?
[1 mark]
Tick ( ) one box.
To improve the taste
To kill bacteria
To remove dissolved solids
03.5 Chlorine is used to sterilise water.
The reaction between chlorine and water is reversible.
Complete the sentence.
[1 mark]
Equilibrium is reached when the forward and reverse reactions
occur at the same .
03.6 The test for chlorine uses damp litmus paper.
Complete the sentence.
Choose the answer from the box.
[1 mark]
blue green white
When damp litmus paper is put into chlorine gas
the litmus paper turns .
There is evidence that adding fluoride ions to drinking water reduces tooth decay.
*16* Figure 6 shows the percentage of people showing tooth decay.
Figure 6
03.7 Compare the effects of adding fluoride ions to drinking water on tooth decay in people
between the ages of 25 and 74.
Use data from Figure 6.
[4 marks]
03.8 Student A says that water companies should add fluoride ions to all drinking water.
Student B disagrees.
Suggest one reason why Student B may think that fluoride ions should not be added
to all drinking water.
[1 mark]
Mark scheme
Show the mark scheme
Question 3
AO /
Question Answers Extra information Mark
Spec. Ref.
03.1 wear safety glasses / goggles allow tie hair back 1 AO3
5.10.1.2
RPA13
AO /
Spec. Ref.
03.2 heat until the mass of the 1 AO3
evaporating basin and contents 5.10.1.2
does not change RPA13
use 25 cm3 of water for each 1
experiment
AO /
Spec. Ref.
03.3 no dissolved solids (in distilled 1 AO3
water) 5.10.1.2
RPA13
AO /
Spec. Ref.
03.4 to kill bacteria 1 AO1
5.10.1.2
AO /
Spec. Ref.
03.5 rate 1 AO1
5.6.2.3
AO /
Spec. Ref.
03.6 white 1 AO1
– OMBINED SCIENCE: TRILOGY – 2F – 5.8.2.4
AO /
Question Answers Mark
Spec. Ref.
Level 2: Scientifically relevant features are identified; the way(s) in 3–4 AO3
12 03.7
which they are similar / different is made clear and (where
appropriate) the magnitude of the similarity / difference is noted.
Level 1: Relevant features are identified and differences noted. 1–2 AO2
No relevant content 0
5.10.1.2
Indicative content
relevant features
• fluoride reduces tooth decay (in all age groups)
• the reduction in tooth decay using fluoride generally increases
with age
• tooth decay increases in 25-54 age groups
• tooth decay decreases in 55-64 age group
• tooth decay increases in 65-74 age group
magnitude
• fluoride has the least effect in the 25-34 age group
• fluoride has the greatest effect in the 55-64 age group
• using fluoride decreases tooth decay by 2% in 25-34 age group
• using fluoride decreases tooth decay by 3% in 35-44 age group
• using fluoride decreases tooth decay by 9% in 45-54 age group
• using fluoride decreases tooth decay by 10% in 55-64 age group
• using fluoride decreases tooth decay by 7% in 65-74 age group
AO /
Spec. Ref.
03.8 any one from: 1 AO2
• could be a risk to health 5.10.1.2
• could have side effects
• could discolour teeth
• people should be able to
decide for themselves
Total Question 3 12
How to answer it
Water Purity and Potable Water Analysis
What this question tests
This question assesses your knowledge of Required Practical 13 (Analysis and Purification of Water), the difference between pure and potable water, chemical tests for gases, dynamic equilibrium, and the ability to interpret complex data from bar charts regarding public health (fluoridation).
Practical Skills: Evaporation
Key Knowledge
- Safety: When heating liquids to dryness, spitting can occur. Safety goggles are essential.
- Validity: To ensure all water has evaporated, you must "heat to constant mass."
- Control Variables: To compare samples fairly, the starting volume of water must be the same (e.g., 25 cm³).
Correct Answers
- 03.1: Wear safety glasses / goggles.
- 03.2:
1. Heat until the mass of the evaporating basin and contents does not change.
2. Use 25 cm³ of water for each experiment.
Pure vs. Potable Water
Exam Technique
In chemistry, "Pure" means a substance containing only one type of element or compound. If distilled water leaves 0.00g of solid after evaporation, it contains no dissolved salts, proving it is pure.
Common Errors
Don't confuse Potable with Pure. Potable water is safe to drink but often contains dissolved minerals. Sterilisation (using chlorine, UV, or ozone) is done specifically to kill microbes/bacteria, not to remove solids.
03.4 Answer: To kill bacteria. (1 mark)
Equilibrium and Gas Tests
Dynamic Equilibrium
In a reversible reaction, equilibrium is reached when the rate of the forward reaction equals the rate of the reverse reaction.
Chlorine Test Mechanism
Chlorine gas is an oxidising agent. When it meets damp litmus paper:
- It first turns blue litmus red (because it forms an acidic solution).
- It then bleaches the paper, turning it white.
Correct Answers
03.5: ...at the same rate.
03.6: ...the litmus paper turns white.
Data Analysis: Fluoride and Tooth Decay
Compare the effects of adding fluoride ions. (4 marks)
How to get 4/4 Marks
This is a "Level 2" response question. You must identify patterns and use specific data from Figure 6.
- The General Trend: Fluoride reduces tooth decay in every age group shown.
- The Magnitude: The reduction is most significant in older age groups. For example, in the 55-64 group, decay drops from 12% to 2% (a 10% reduction).
- Specific Comparison: In the youngest group (25-34), the effect is smallest (only a 2% reduction).
- Age Trend: Without fluoride, tooth decay generally increases as people get older (until 64).
Data Extraction (Step-by-Step)
- Identify the "Without Fluoride" bar (light grey) vs "With Fluoride" bar (dark grey).
- Calculate the difference for at least two groups to show "magnitude."
- Example: 45-54 age group: 14% (without) - 5% (with) = 9% reduction.
Ethics and Health
Arguments against Fluoridation
While fluoride helps teeth, some people object to "mass medication" where they have no choice in what is added to their water supply.
Accepted Answers (Choose one)
- It could be a risk to health / have side effects.
- It could discolour teeth (fluorosis).
- People should be able to decide for themselves (freedom of choice).
Topics
Chemistry · C10: Using Resources · C6: The Rate and Extent of Chemical Change · C8: Chemical Analysis
Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 2 (Foundation), 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.