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Specimen lab book: Water purification
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Study Hack Lab
RP 8 · Chemistry
Water purification
Lab book
Name
Specimen
Date
2 November 2026
Course
GCSE Chemistry
Board and specification
AQA GCSE Chemistry (8462), 4.10.1.2AQA GCSE Combined Science: Trilogy (8464), 5.10.1.2Apparatus and techniques: AT 2, AT 3, AT 4
Run
C29HO
Status
Finished: every stage done
Exam questions
30/30 marks
Aim
Analyse water from four sources (pH and dissolved solids), then purify sea water by distillation and test the sea water and the distilled water for sodium and chloride ions.
Contents
Plan and risk assessment
Apparatus
Method
Results
Graph
Analysis
Conclusion and evaluation
Make safe and clear up
Exam questions on your data
Simulated data produced by Study Hack Lab from the student's own run (seed 20261004). Every reading, value and mark here is from that run; nothing has been retyped. Layout v1, practical v2. Original material; not affiliated with AQA, which has not reviewed it.
1 Plan and risk assessment
Water from rivers, the sea and the ground carries dissolved salts. Evaporating a measured volume and weighing what is left gives the mass of dissolved solids; heating to constant mass makes sure every trace of water has gone. Distillation purifies water: it boils off, the vapour condenses in the cold condenser, and the salts stay behind. A flame test (sodium: yellow) and silver nitrate with nitric acid (chloride: white precipitate) check the water before and after.
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
Variables
Independent (you change)
The source of the water sample
Dependent (you measure)
pH (no unit); mass of dissolved solids in g/dm³; the ion tests before and after distilling (observations)
Control (you keep the same)
Volume of each sample evaporated; The balance; Heating to constant mass; The indicator
Prediction
Which water will leave the most solid behind when it is evaporated?
Sea waterRight first time
Your sea water had 35.5 g/dm³; the mineral water 0.32 g/dm³. Sea water has about 35 g of salts dissolved in every dm³, mostly sodium chloride. Fresh waters (tap, river, mineral) have well under 1 g per dm³: rain water picks up only a little from the rocks it flows through.
What will the pH of the sea water be?
A little above 7 (about 8): slightly alkalineRight first time
You measured pH 8. Sea water is slightly alkaline, about pH 8, because of the dissolved carbonate and hydrogencarbonate ions it contains.
You distil the sea water and test the distilled water. What will the tests show?
No flame colour and no precipitate: the salts stay behind in the flask.Right first time
Your distilled water gave no flame colour and no precipitate. Water boils at 100 °C but sodium chloride does not boil until over 1400 °C. Only the water evaporates, condenses and drips into the cylinder, so the distillate has no sodium ions and no chloride ions.
Risk assessment
Hazard
Harm it could cause
Control you chose
Solid spitting from an evaporating basin heated to dryness
Hot solid in the eyes or on the skin: burns and eye damage
Evaporate on a water bath and dry over a low flame; wear goggles.Right first time
Boiling water, steam, and hot glass and porcelain
Scalds and burns
Clamp the flask, boil gently, and let everything cool on a heatproof mat before you touch it.Right first time
Sea water boiling unevenly (bumping) in the flask
Boiling salt water spurts out: scalds
Add anti-bumping granules and fill the flask no more than half full.Right first time
Silver nitrate solution, 0.05 mol/dm³
Stains the skin; irritates the eyes
Wear goggles, wipe up spills at once, and pour residues into the residue bottle.Right first time
Dilute nitric acid, 0.4 mol/dm³ (irritant), and the dilute hydrochloric acid for the wire
Irritates the eyes and skin
Wear goggles and use only a few drops.Right first time
Untreated river water
Microbes in it can cause illness
Wash your hands afterwards and never taste any sample, the distilled water included.Right first time
2 Apparatus
✓Eye protection (goggles) Protects the eyes from spitting solid, hot water and splashes.
✓Water samples: tap, sea, bottled mineral and river water The water from different sources.
✓Test tubes in a rack, and a white tile Small samples for the pH and ion tests.
✓Universal indicator solution and colour chart Shows the pH of each sample as a colour.
✓pH meter and pH 7 buffer (resolution 0.01) Measures pH to 0.01 (if you choose it).
✓Top-pan balance (0.01 g) (resolution 0.01 g) Weighs the basins before and after evaporating.
✓Four evaporating basins (A–D) Hold each sample while the water evaporates.
✓Measuring cylinders: 25 cm³ and 250 cm³ (resolution 0.5 cm³ and 2 cm³) Measure the volume of each sample.
✓Water bath (beaker on a tripod and gauze), Bunsen burners and heatproof mats Evaporates the water gently.
✓Side-arm (distillation) flask, anti-bumping granules Holds the sea water while it boils.
✓Liebig condenser, retort stands and clamps, rubber tubing Cools the steam so it condenses into pure water.
✓Thermometer, −10 to 110 °C (resolution 1 °C divisions (read to 0.5 °C)) Measures the temperature of the vapour at the side arm.
✓25 cm³ measuring cylinder (receiver) (resolution 0.5 cm³) Collects and measures the distillate.
✓Nichrome wire loop in a handle For the flame test for sodium ions.
✓Dilute nitric acid (0.4 mol/dm³), silver nitrate solution (0.05 mol/dm³) and dilute hydrochloric acid Nitric acid and silver nitrate test for chloride ions; the hydrochloric acid cleans the flame-test wire.
3 Method: your choices
Which version are you doing?: AQA required practical 8: analysing and purifying waterpH and dissolved solids of four waters, then distil sea water and test it for ions.
Zero the balance: DoneA balance that does not read zero adds the same error to every mass you record.
Clean the flame-test wire: DoneSalt from earlier tests and from fingers sticks to the loop. Sodium gives a strong yellow flame even in tiny amounts.
Put on eye protectionSafety: DoneHot solid can spit out of an evaporating basin, boiling water can spurt from the flask, and silver nitrate and nitric acid irritate the eyes.
Choose how to measure the pH: Universal indicator and its colour chartYou match the colour to the chart by eye, so each pH is to the nearest whole number. Two waters with pH 7.2 and 7.4 look the same.
Choose how to evaporate the water: On a water bath, then dry over a low flameThe basin never gets hotter than about 100 °C while there is water left, so nothing spits while the water evaporates. It takes longer. Now and then a crust still spits a little while the last of it dries over the low flame.
Heat to constant mass: DoneA crust of salt can look dry but still hold water. When the mass stops changing, all the water has gone.
Choose how much mineral water to evaporate: 250 cm³, added a portion at a time as it evaporatesAbout 0.05–0.1 g of solid: the balance’s 0.01 g is about a tenth of it, so your result is within about ±15 %.
Add anti-bumping granules to the flaskSafety: DoneThey give the steam somewhere to form, so the water boils smoothly instead of in sudden violent bursts.
Connect the condenser’s cooling water: Water in at the bottom, out at the topThe jacket fills completely with cold water, so nearly all the steam condenses.
Position the thermometer: Bulb level with the side armIt reads the temperature of the steam going into the condenser: the boiling point of water, about 100 °C.
How hard to boil the sea water: Boil it gentlyOnly steam reaches the condenser, so the distillate is pure water. About 8–11 cm³ collects in 10 minutes.
4 Results
25 of 25 readings taken.
pH of each water sample · Universal indicator, matched to the colour chart.
Water sample
Colour of universal indicator
pH
Tap water
Blue-green: pH 8
8
Sea water
Blue-green: pH 8
8
Bottled mineral water
Blue-green: pH 8
8
River water
Green: pH 7
7
Evaporating to constant mass · Basins A–C: 25.0 cm³ of sea water each. Basin D: 250 cm³ of bottled mineral water. A dash means the mass was already constant.
Basin
Volume of water in cm³
Mass of empty basin in g
Mass after first heating in g
Mass after heating 2 in g
Mass after heating 3 in g
Mass of dissolved solids in g
Dissolved solids in g/dm³
A (sea)
25.0
87.58
88.51
88.47
88.46
0.88
35.2
B (sea)
25.0
74.88
75.81
75.78
75.77
0.89
35.6
C (sea)
25.0
75.76
76.71
76.66
76.65
0.89
35.6
D (mineral)
250
59.98
60.06
60.06
—
0.08
0.32
Mean (sea)
35.5
Distilling the sea water · 100 cm³ of sea water; distillate collected for 10 minutes in a 25 cm³ measuring cylinder.
Distilled
Temperature of the vapour in °C
Volume of distillate in cm³
Sea water
100.5
9.5
Testing for ions · Flame test with a nichrome wire loop; a few drops of dilute nitric acid, then silver nitrate solution.
Water
Flame colour
With silver nitrate
Sea water
Yellow (orange-yellow)
White precipitate
Distilled water
No colour: the flame stays blue
Stays clear: no precipitate
Anomaly: No anomalous reading.
5 Graph
pH of each water
6 Analysis
Mass of solid in basin A
mass of solid = mass of basin and solid − mass of empty basin
Use basin A’s constant mass (the last one you recorded) to work out the mass of solid.
0.880 gRight first time
Dissolved solids in basin A
dissolved solids (g/dm³) = mass of solid (g) ÷ volume of water (dm³)
Work out the mass of dissolved solids in every dm³ of sea water. 25.0 cm³ = 0.025 dm³.
35.2 g/dm³Right first time
Mean dissolved solids in sea water
mean = sum of the results kept ÷ number of results kept
Work out the mean of your three sea-water results.
35.5 g/dm³Right first time
Dissolved solids in the mineral water
dissolved solids (g/dm³) = mass of solid (g) ÷ volume of water (dm³)
Work out the dissolved solids in the mineral water (basin D, 250 cm³ = 0.25 dm³).
0.32 g/dm³Right first time
The balance and the mineral water
The balance reads to 0.01 g. The mineral water left 0.08 g of solid. What does this mean for your mineral water result?
The 0.01 g is a fairly small part of the mass (a quarter or less), so the result is roughly right.Right first time
Why a bar chart
Why do you show these results on a bar chart, not a line graph?
The source of the water is a categoric variable: there is nothing in between tap water and sea water.Right first time
Which ions are there?
Distilled water: no sodium or chloride ionsRight first time
7 Conclusion and evaluation
Quantity
Your value
Accepted value
Sea water (class value)
35.5 g/dm³
35.2 g/dm³
Mineral water (its label)
0.32 g/dm³
0.33 g/dm³
Vapour temperature
100.5 °C
100.4 °C
Conclusion
What do your results show?
Sea water has about 35 g of dissolved solids in every dm³, far more than the mineral water (about 0.32 g/dm³). Distilling it leaves the salts behind: the distilled water gave no flame colour and no precipitate.Right first time
Distillation separates water from the salts dissolved in it: water boils at 100 °C, sodium chloride not until over 1400 °C. Only water vapour reached the condenser.
Sources of error
Not heating to constant mass: systematic or random?
SystematicRight first time
Water is still trapped in the solid, so every mass of solid, and the dissolved solids per dm³, is too high.
Solid spitting out of the basin as the last water boils off: systematic or random?
RandomRight first time
A trial loses a different amount of solid each time, so its result is too low and does not agree with the others.
The resolution of the balance (0.01 g) compared with the mass of solid from the mineral water: systematic or random?
RandomRight first time
The mass of solid is only a few times the balance’s resolution, so the result could be out by 15 % or more.
Judging the colour of universal indicator by eye: systematic or random?
RandomRight first time
The pH is only to the nearest whole number, and two people may match the same colour to different squares.
Boiling the flask too hard during distillation: systematic or random?
SystematicRight first time
Droplets of salt water are thrown into the condenser, so the distillate always contains some salt and tests positive.
The thermometer bulb down in the boiling liquid instead of level with the side arm: systematic or random?
SystematicRight first time
It reads the boiling salt solution, which is hotter than the steam that distils, so the boiling point recorded is too high.
Cooling water in at the top of the condenser: systematic or random?
SystematicRight first time
The jacket is only partly full, so it cools less well: some steam escapes without condensing and a little less distillate is collected.
A dirty flame-test wire: systematic or random?
SystematicRight first time
Sodium on the loop gives a yellow flame whatever you test, so the distilled water seems to contain sodium ions.
Improvement
Which change would most improve your results?
Use a pH meter calibrated with a buffer solution.Right first time
Repeatable or reproducible
Your two kept sea-water trials agree to within 1 g/dm³. This shows your results are…
RepeatableRight first time
Repeatable: the same person, method and equipment give similar results.
Another group, with different basins and a different balance, finds almost the same dissolved solids for the class’s sea water. This shows the result is…
ReproducibleRight first time
Reproducible: a different person or different equipment gives similar results.
8 Make safe and clear up
Turn off the Bunsen burners, then the condenser water; leave the flask, basins and tripods to cool on the heatproof mats
Pour the silver nitrate mixtures into the residue bottle, and the other solutions down the sink with plenty of water
Soak the salt crust out of the basins in warm water, and rinse the flask, test tubes and cylinders
Wash your hands (the river water may carry microbes), then take off your goggles last
9 Exam questions on your data
On your own readings, self-marked against the mark scheme, with error carried forward where it applies. Total 30/30.
01.1[1 mark]
A student evaporated water samples from different sources to compare the mass of solid dissolved in each.
Give one control variable in this investigation.
Your answer
The volume of each water sample
1/1 self-marked
Volume of (each) water sample; allow same balance / same (type of) basin / heating to constant mass / same heating time or temperature; ignore amount of water
Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.
Model answer: The volume of each water sample evaporated.
01.2[1 mark]
A student heated an evaporating basin of sea water over a Bunsen burner until it was dry.
Give one reason why the student should wear eye protection.
Your answer
Hot salt could spit out of the basin into the eyes and damage them.
1/1 self-marked
Hot solid / salt / liquid could spit (spurt) into the eyes; allow eye damage or burns to the eyes; ignore "it is hot" or "dangerous" on its own
Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.
Model answer: Hot solid could spit out of the basin into the eyes and burn or damage them.
02.1[1 mark]
Another student evaporated 25.0 cm³ of the same sea water in each of three trials. Table 1 shows their results.
Which trial is anomalous?
Select one row.
Table 1
Trial
Mass of empty basin in g
Mass of basin and dry solid in g
Mass of dissolved solids in g
1
88.88
89.67
0.79
2
87.83
88.70
0.87
3
47.31
48.20
0.89
Your answer
trial 1
1/1
Trial 1: much less solid than the other two
Full marks. Clear and precise. Compare the masses of solid: trial 1 (0.79 g) is much less than the others.
Model answer: Trial 1: it left much less solid than the other two trials.
02.2[1 mark]
Another student evaporated 25.0 cm³ of the same sea water in each of three trials. Table 1 shows their results.
Table 1
Trial
Mass of empty basin in g
Mass of basin and dry solid in g
Mass of dissolved solids in g
1
88.88
89.67
0.79
2
87.83
88.70
0.87
3
47.31
48.20
0.89
Suggest one reason why the mass of solid in trial 1 is lower than in the other trials.
Your answer
Some solid spat out of the basin while it was being heated.
1/1 self-marked
Some solid spat (jumped) out of the basin; allow some (solution / solid) was spilt or lost; allow less sea water was measured; do not accept not heated to constant mass
Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.
Model answer: Some of the solid spat out of the basin as the last of the water boiled off.
02.3[2 marks]
Four groups each evaporated the same sea water to constant mass. Table 2 shows the mass of dissolved solids they found in 1 dm³.
Table 2
Group
Dissolved solids in g/dm³
A
34.0
B
35.4
C
34.8
D
36.6
Calculate the mean mass of dissolved solids in g/dm³.
Model answer: (34.0 + 35.4 + 34.8 + 36.6) ÷ 4 = 140.8 ÷ 4
mean = 35.2 g/dm³
02.4[2 marks]
Assume all of the dissolved solid in the sea water is sodium chloride. Sodium chloride contains 39.3 % sodium by mass.
Use your answer to Question 02.3 to calculate the mass of sodium in 1 dm³ of the sea water.
Your answer
13.8
Working: 35.2 × 39.3 ÷ 100
= 13.8 g/dm³
2/2
35.2 × 39.3 ÷ 100 (allow their mean × 0.393)
= 13.8 (g/dm³) (allow correct use of an incorrect mean)
Full marks. Well done.
Model answer: 35.2 × 39.3 ÷ 100
= 13.8 g/dm³
02.5[2 marks]
Another student heated each basin only once before weighing it. They did not heat to constant mass.
Explain how this would affect their value for the mass of dissolved solids.
Your answer
Not all the water would have evaporated, so the mass of dissolved solids would be too high.
2/2 self-marked
Not all the water had evaporated (some water was left in the solid)
So the mass (of solid) would be too high (greater than it should be)
Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.
Model answer: Not all the water would have evaporated, so the mass of dissolved solids would be too high.
02.6[2 marks]
You evaporated 250 cm³ of mineral water. It left 0.08 g of solid. 25.0 cm³ of sea water left 0.88 g. You used the same balance for both.
Explain why the value for the dissolved solids in the mineral water is much less certain than the value for the sea water.
Your answer
The mass of solid from the mineral water is very small, so the resolution of the balance, 0.01 g, is a large percentage of it.
2/2 self-marked
The mass of solid (from the mineral water) is very small
So the resolution of the balance (0.01 g) is a large fraction (percentage) of it; allow a large percentage uncertainty / error
Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.
Model answer: The mass of solid from the mineral water is very small, so the resolution of the balance (0.01 g) is a large percentage of it.
02.7[1 mark]
A student measured the pH of each water sample with universal indicator.
Give one advantage of using a pH probe instead of universal indicator.
Your answer
It gives a numerical value, so it is more precise.
1/1 self-marked
More precise / gives a numerical value / reads to more decimal places / no colour to judge (removes human error); ignore more accurate on its own
Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.
Model answer: It gives a numerical value to 0.01, so it is more precise (no colour has to be judged by eye).
03.1[2 marks]
Sea water contains sodium ions.
Describe a test to show that sodium ions are present. Give the result.
Your answer
Do a flame test: dip a clean nichrome wire in the sea water and hold it in a blue Bunsen flame. The flame turns yellow.
2/2 self-marked
Flame test: dip a (clean) wire loop in the sample and hold it in a (blue Bunsen) flame; allow flame emission spectroscopy
Yellow (flame) (MP2 dependent on MP1); allow orange-yellow; do not accept yellow precipitate
Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.
Model answer: Flame test: dip a clean nichrome wire loop in the sea water and hold it in a blue Bunsen flame. The flame turns yellow.
03.2[2 marks]
Sea water also contains chloride ions.
Describe a test to show that chloride ions are present. Give the result.
Your answer
Add dilute nitric acid and then silver nitrate solution. A white precipitate forms.
2/2 self-marked
Add (dilute nitric acid and) silver nitrate (solution); allow acidified silver nitrate; do not accept hydrochloric acid
White precipitate (MP2 dependent on MP1)
Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.
Model answer: Add a few drops of dilute nitric acid, then silver nitrate solution. A white precipitate forms.
03.3[1 mark]
You tested your distilled water. In the flame test you saw: no colour: the flame stays blue. With nitric acid and silver nitrate you saw: stays clear: no precipitate.
What do these results show about the distilled water?
Your answer
The distilled water contains no sodium ions and no chloride ions, so distillation removed the salt.
1/1 self-marked
It contains no sodium ions and no chloride ions that these tests can detect (the salt was removed); allow the salt / ions were removed by distillation. Not: it is pure (these two tests only look for sodium and chloride ions)
Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.
Model answer: The distilled water contains no sodium ions and no chloride ions: distillation removed the salt.
03.4[1 mark]
Your thermometer bulb was level with the side arm. It read 100.5 °C while the distillate collected.
Give the reason why the bulb of the thermometer should be level with the side arm of the flask.
Your answer
So it measures the temperature of the vapour going into the condenser, which is the boiling point of the substance being collected.
1/1 self-marked
To measure the temperature of the vapour (steam / gas) entering the condenser; allow the boiling point of the substance being collected (distilled)
Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.
Model answer: So it measures the temperature of the vapour going into the condenser: the boiling point of the substance being collected.
03.5[1 mark]
Suggest why most drinking water in the UK is not produced by distilling sea water.
Your answer
Boiling the water needs a lot of energy, so distillation is expensive.
1/1 self-marked
It needs a lot of energy (fuel) to boil the water, so it is expensive; allow the UK has enough fresh water (rain, rivers, lakes)
Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.
Model answer: Boiling the water needs a lot of energy, so it is expensive; the UK has plenty of fresh water to treat instead.
03.6[2 marks]
In the UK, most potable water is made from fresh water, such as river water.
Describe two stages in producing potable water from river water.
Your answer
The water is passed through filter beds to remove solids, then it is sterilised with chlorine to kill harmful microbes.
2/2 self-marked
Passed through filter beds (filtered) to remove (insoluble) solids
Sterilised with chlorine, ozone or ultraviolet (UV) light (to kill harmful microbes)
Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.
Model answer: The water is passed through filter beds to remove solids. Then it is sterilised with chlorine (or ozone or UV light) to kill harmful microbes.
03.7[2 marks]
Waste water (sewage) from homes must be treated before it can go back into a river.
Give two stages in the treatment of waste water.
Your answer
First it is screened to remove large objects and grit. Then the sludge is broken down by anaerobic digestion.
2/2 self-marked
Screening / grit removal (removes large objects and grit)
Sedimentation (to produce sewage sludge and effluent)
Anaerobic digestion of (sewage) sludge
Aerobic biological treatment of (the liquid) effluent
Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.
Model answer: Any two of: screening and grit removal; sedimentation to give sludge and effluent; anaerobic digestion of the sludge; aerobic biological treatment of the effluent.
04[6 marks]
A student wants to find the mass of solid dissolved in 1 dm³ of a sample of river water.
Plan a method the student could use.
Your answer
1. Weigh a clean, dry evaporating basin on a balance.
2. Measure 250 cm³ of the river water with a measuring cylinder. Pour it into the basin a portion at a time.
3. Heat the basin on a water bath to evaporate the water, adding more as it evaporates.
4. Let the basin cool, then reweigh it.
5. Heat it again, cool and reweigh, and repeat until the mass is constant.
6. Subtract the mass of the empty basin from the final mass to find the mass of dissolved solids.
7. Repeat the whole experiment and calculate a mean, leaving out anomalous results.
8. Multiply by 1000 ÷ 250 (by 4) to give the mass of dissolved solids in 1 dm³.
6/6 self-marked
Weigh the (empty) evaporating basin
Add a measured volume of the water (measuring cylinder)
Heat to evaporate the water (water bath / Bunsen)
Reweigh (the basin and solid) once cool
Repeat heating until constant mass
Subtract the mass of the basin from the final mass
Repeat and calculate a mean, discarding anomalous results
Calculate the mass in 1 dm³ (scale up from the volume used)
Self-marked: Level 3, 6 of 6 marks. You ticked 8 of 8 indicative points. This is your own judgement against the mark scheme.
Model answer: 1. Weigh a clean, dry evaporating basin on a balance.
2. Measure 250 cm³ of the river water with a measuring cylinder. Pour it into the basin a portion at a time.
3. Heat the basin on a water bath to evaporate the water, adding more as it evaporates.
4. Let the basin cool, then reweigh it.
5. Heat it again, cool and reweigh, and repeat until the mass is constant.
6. Subtract the mass of the empty basin from the final mass to find the mass of dissolved solids.
7. Repeat the whole experiment and calculate a mean, leaving out anomalous results.
8. Multiply by 1000 ÷ 250 (by 4) to give the mass of dissolved solids in 1 dm³.
Unseen data: the same skills, new apparatus
01[3 marks]
A student evaporates 25.0 cm³ of water from a river estuary in a weighed evaporating basin. They heat it, let it cool and weigh it, three times.
Mass of the evaporating basin
Mass in g
Empty basin
70.15
After heating 1
70.64
After heating 2
70.61
After heating 3
70.61
Calculate the mass of dissolved solids in 1 dm³ of the water. Give your answer in g/dm³.
Mass of solid = 70.61 − 70.15 = 0.46 g (the constant mass).
Volume = 25.0 ÷ 1000 = 0.025 dm³.
Mass ÷ volume in dm³, in g/dm³.
Full marks. Well done.
Model answer: Heatings 2 and 3 agree, so the mass is constant. Mass of solid = 70.61 − 70.15 = 0.46 g. Volume = 0.025 dm³. Dissolved solids = 0.46 ÷ 0.025 = 18.4 g/dm³.
02[2 marks]
A student tests four water samples. They measure the pH with a pH meter, evaporate 100 cm³ of each to find the dissolved solids, and measure the boiling point at normal air pressure.
Results
Sample
pH
Dissolved solids in g/dm³
Boiling point in °C
A
7.0
0.37
100.0
B
5.7
0.01
100.0
C
7.0
0.00
100.0
D
8.1
34.5
100.5
Which TWO statements are correct? Tick (✓) two boxes.
✓ Sample C’s results are what pure water gives: no residue to 0.01 g when it is evaporated, and it boils at 100.0 °C. (your choice)
Sample A is pure water, because its pH is 7.0.
✓ Sample A could be potable but is not pure: it can be safe to drink, but it has substances dissolved in it. (your choice)
Sample D could be made potable by filtering it through sand.
Sample D boils above 100 °C because it has been sterilised.
Your answer
Sample C’s results are what pure water gives: no residue to 0.01 g when it is evaporated, and it boils at 100.0 °C.; Sample A could be potable but is not pure: it can be safe to drink, but it has substances dissolved in it.
2/2
Sample C’s results are what pure water gives: no residue to 0.01 g when it is evaporated, and it boils at 100.0 °C.
Sample A could be potable but is not pure: it can be safe to drink, but it has substances dissolved in it.
Full marks. Well done.
Model answer: Pure water is a single substance: no dissolved solids, and it boils at exactly 100 °C. Sample C matches that to the precision measured (0.00 g left, 100.0 °C), which is consistent with pure water; a tiny amount of impurity below that precision could not be seen. Sample A has a pH of 7.0 but leaves dissolved solids: a pH of 7 does not show water is pure, and drinking (potable) water is not pure. Filtering does not remove dissolved salts from sample D: that needs distillation or reverse osmosis.
03[2 marks]
Two students each distil 100 cm³ of sea water for 10 minutes, then add dilute nitric acid and silver nitrate solution to the distillate.
How each student set up and what they found
Student 1
Student 2
Thermometer bulb
Level with the side arm
In the boiling liquid
Cooling water
In at the bottom
In at the top
Heating
Gently
Strongly
Temperature in °C
100.2
101.4
Volume of distillate in cm³
9.5
14.5
With silver nitrate
Stays clear
Faint white cloudiness
Which TWO statements explain the differences in the results? Tick (✓) two boxes.
✓ Student 2’s thermometer read higher because its bulb was in the boiling salty water, which boils above 100 °C. The bulb should be level with the side arm. (your choice)
✓ Student 2 boiled the flask hard, so droplets of salty water were carried into the condenser: that is why their distillate went cloudy. (your choice)
Student 2 collected more water because cold water in at the top of the condenser cools the vapour better.
Boiling harder gives purer water, because the water spends less time in the flask.
Some of the salt boiled off at 100 °C in both flasks, so neither distillate was pure.
Your answer
Student 2’s thermometer read higher because its bulb was in the boiling salty water, which boils above 100 °C. The bulb should be level with the side arm.; Student 2 boiled the flask hard, so droplets of salty water were carried into the condenser: that is why their distillate went cloudy.
2/2
Student 2’s thermometer read higher because its bulb was in the boiling salty water, which boils above 100 °C. The bulb should be level with the side arm.
Student 2 boiled the flask hard, so droplets of salty water were carried into the condenser: that is why their distillate went cloudy.
Full marks. That is exactly what the mark scheme is looking for.
Model answer: Student 2’s thermometer measured the boiling salt solution, which boils above 100 °C; at the side arm it measures the vapour that distils. Boiling hard collected more distillate, but spray carried salty water over, so it gave a cloudiness with silver nitrate. Water in at the top leaves the condenser part-empty: it cools worse, not better.