Launch practical
About 100 minutes
- Set up the bench with the 15 items of apparatus.
- Take the readings yourself, into the results table you draw first.
- Process your results and plot the graph.
- Answer exam questions on your own results, which you mark against the mark scheme.
Runs in your browser: a 3D bench with 2D close-ups of each instrument for exact readings. Nothing to install and no account.
What you measure, and how
One run takes 25 readings. You take every one yourself, the way you would in class.
- On the 3D bench (Fig. 1) you set out the apparatus and read each instrument where it stands.
- In the 2D close-up (Fig. 2) the same instrument is drawn large, from the same run, for an exact reading. It is also how the practical runs on a device that cannot show 3D.
- In your table you write what you read, to the instrument’s resolution, and the notebook checks it against the instrument.
How to read it. Hold the tube against white, next to the chart, and pick the square it matches best.
What it shows
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
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.
The graph to expect. This practical has no line graph. The pH of each source goes on a bar chart; the result is the mean dissolved solids in sea water.
Key equation for the exam: No equation: learn the tests for pure water..
No equation to learn. Pure water boils at 100 °C and leaves no solid when evaporated.
Apparatus
| Item | What it is for | Resolution |
|---|---|---|
| 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 | Measures pH to 0.01 (if you choose it). | 0.01 |
| Top-pan balance (0.01 g) | Weighs the basins before and after evaporating. | 0.01 g |
| Four evaporating basins (A–D) | Hold each sample while the water evaporates. | — |
| Measuring cylinders: 25 cm³ and 250 cm³ | Measure the volume of each sample. | 0.5 cm³ and 2 cm³ |
| 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 | Measures the temperature of the vapour at the side arm. | 1 °C divisions (read to 0.5 °C) |
| 25 cm³ measuring cylinder (receiver) | Collects and measures the distillate. | 0.5 cm³ |
| 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. | — |
Method
In our own words, as the Lab runs it. Your school's method sheet may differ in detail.
Which version are you doing?
Boards set this practical differently. Your course setting picks one; you can change it before you start.
Why: AQA analyses and purifies water samples; other boards distil a mixture to separate it. The exam questions follow your board.
Zero the balance
With nothing on the pan, press TARE (or ZERO) so the display reads 0.00 g.
Why: A balance that does not read zero adds the same error to every mass you record.
Clean the flame-test wire
Dip the nichrome wire loop in dilute hydrochloric acid, then hold it in the hottest part of a blue Bunsen flame. Repeat until it gives the flame no colour.
Why: Salt from earlier tests and from fingers sticks to the loop. Sodium gives a strong yellow flame even in tiny amounts.
Put on eye protectionSafety
Wear goggles from the moment the Bunsen burners are lit until everything is cool and cleared away.
Why: Hot 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
You will test a small sample of each water.
Do: Universal indicator and its colour chart.
Why: pH tells you whether each sample is acidic, neutral or alkaline.
Choose how to evaporate the water
Each basin of water must be heated until all the water has gone and only the dissolved solid is left.
Do: On a water bath, then dry over a low flame.
Why: As the last of the water boils off, a crust forms and steam bursts through it, throwing solid out of the basin.
Heat to constant mass
After the first heating and weighing, heat the basin again for 5 minutes, let it cool and reweigh it. Repeat until two masses in a row agree (within 0.01 g).
Why: A 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
Mineral water has far less dissolved solid than sea water: a few tenths of a gram in every dm³.
Do: 250 cm³, added a portion at a time as it evaporates.
Why: The balance reads to 0.01 g. The mass of solid needs to be many times bigger than that to measure it well.
Add anti-bumping granules to the flaskSafety
Drop two anti-bumping granules into the flask with the sea water.
Why: They give the steam somewhere to form, so the water boils smoothly instead of in sudden violent bursts.
Connect the condenser’s cooling water
Fit the rubber tubing from the cold tap to one connection on the condenser’s outer jacket, and from the other connection to the sink.
Do: Water in at the bottom, out at the top.
Why: The cold water in the jacket cools the steam inside the inner tube, so it condenses into liquid water.
Position the thermometer
The thermometer goes through the bung at the top of the flask.
Do: Bulb level with the side arm.
Why: The thermometer should tell you the temperature of the vapour that goes into the condenser: the boiling point of what you collect.
Take the readings
PH of each sample; then for each evaporating basin the empty mass and the mass of basin and solid after each heating, until constant (sea water three times, mineral water once); the vapour temperature and the volume of distillate; four ion tests.
Repeats: AQA’s method: repeat the evaporation and calculate a mean, leaving out anomalous results. You do three trials with sea water. A mean reduces the effect of random errors (measuring the volume, the last digit of the balance), but not systematic ones (water left in the solid if you do not heat to constant mass). The mineral water is done once, with a large volume: a bigger mass of solid does more for its precision than repeats would. The distillation is done once, as in AQA’s handbook.
Make safe and clear upSafety
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.
While you take readings
How hard to boil the sea water
Heat the flask until the water boils, then turn the flame down.
Do: Boil it gently.
Why: Bubbles bursting at the surface throw out tiny droplets of salt water. Boiling hard throws them up into the side arm.
Hazards and control measures
| Hazard | What could happen | Control measure |
|---|---|---|
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| Untreated river water | Microbes in it can cause illness | Wash your hands afterwards and never taste any sample, the distilled water included. |
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. 25.0 cm³ of sea water each time, measured with the same measuring cylinder.
- The balance. The same 0.01 g balance for every weighing.
- Heating to constant mass. Reheat each basin until two masses agree within 0.01 g.
- The indicator. The same universal indicator and colour chart for every sample.
Results from a specimen run
| 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 |
| 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) | blank | blank | blank | blank | blank | blank | 35.5 |
| Distilled | Temperature of the vapour in °C | Volume of distillate in cm³ |
|---|---|---|
| Sea water | 100.5 | 9.5 |
| 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 |
Processing the data
- Mass of solid in basin A
- 0.880 g
- Dissolved solids in basin A
- 35.2 g/dm³
- Mean dissolved solids in sea water
- 35.5 g/dm³
- Dissolved solids in the mineral water
- 0.32 g/dm³
- The balance and the mineral water
- The 0.01 g is a fairly small part of the mass (a quarter or less), so the result is roughly right.
- Why a bar chart
- The source of the water is a categoric variable: there is nothing in between tap water and sea water.
- Which ions are there?
- Distilled water: no sodium or chloride ions
| Quantity | Specimen run | Accepted |
|---|---|---|
| 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 |
Across 120 simulated careful groups, mean dissolved solids in sea water came out between 33.8 and 37.0 g/dm³ (median 35.2). That is the simulation's model, not a promise of what a real class gets.
Common mistakes and the fix
Mistakes to check in this practical, and what to do instead. In a write-up or an exam answer, the mark goes to naming the step and saying what it does to the readings.
Method: Use it as it is
If the empty pan does not read 0.00 g, every mass in your table is out by that amount (a zero error). Your mass of solid is still right: the same error is in both masses, so it cancels when you subtract.
The fix: Zero the balance. A balance that does not read zero adds the same error to every mass you record.
Method: Use it straight from the box
Sodium on the dirty loop turns the flame yellow in every test, so even pure distilled water seems to contain sodium ions (a false positive).
The fix: Clean the flame-test wire. Salt from earlier tests and from fingers sticks to the loop. Sodium gives a strong yellow flame even in tiny amounts.
Method: Skip it
Your eyes are unprotected all lesson. Spitting solid from a basin, a spurt of boiling water or a splash of silver nitrate could injure them. Your readings would be the same, so this is not a data error: it is flagged in the safety review.
The fix: Put on eye protection. Hot solid can spit out of an evaporating basin, boiling water can spurt from the flask, and silver nitrate and nitric acid irritate the eyes.
Universal indicator in the tap water: Recorded “Orange: pH 4”
The fix: Look again: hold the tube against the white tile next to the colour chart and find the square it matches best.
Universal indicator in the tap water: Recorded “Green: pH 7”
The fix: Close: compare the tube with the chart again, side by side against a white background. It matches the square above that one.
Exam answer: Leaving out: 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
The fix: Name something kept the same: the volume of each water sample.
Exam answer: Leaving out: 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
The fix: Say what could happen to the eyes: hot solid could spit out of the basin into them.
Worked exam-style questions
Written for Study Hack in the style of exam questions, on the specimen run above. They are not taken from any exam board's papers. Try each one before you open the worked answer.
- Question 1Control variables · Knowledge[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.
Worked answer and how the marks are given
A full-mark answer
The volume of each water sample evaporated.
Mark scheme
- 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 Do not accept “source”, “type / kind of water”.
Where marks are usually lost
- Missing: 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. Name something kept the same: the volume of each water sample.
- Question 2Mean of repeats · Applying knowledge[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.
Mean = g/dm³
Worked answer and how the marks are given
A full-mark answer
(34.0 + 35.4 + 34.8 + 36.6) ÷ 4 = 140.8 ÷ 4
mean = 35.2 g/dm³Mark scheme
- (34.0 + 35.4 + 34.8 + 36.6) ÷ 4 (method)
- = 35.2 (g/dm³)
- Allow answers from 35.15 to 35.25 g/dm³ (the marker's tolerance; correct rounding of the student's own working is also accepted).
- Allow the unit written as g/dm3, g dm-3, gdm-3, g per dm3, g/l, g per litre and 2 other spellings.
- A correct answer with no working scores full marks.
Where marks are usually lost
- Answer 46.9 g/dm³ (no marks). There are four values, so divide by 4: 140.8 ÷ 4 = 35.2 g/dm³.
- Answer 141 g/dm³ (gains 1 mark). 140.8 is the total. Divide it by the number of values (4) to get the mean: 35.2 g/dm³.
- Missing: (34.0 + 35.4 + 34.8 + 36.6) ÷ 4. Add the four values and divide by 4.
- Question 3Planning: mass of dissolved solids · Knowledge[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.
Worked answer and how the marks are given
A full-mark 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.Mark scheme
Indicative content:
- 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)
Levels of response Level Marks What the answer does 3 5–6 A coherent method that would give valid results: all the important steps are there, in a sensible order. 2 3–4 A method that might not give valid results: most of the important steps are there, but some are missing or out of order. 1 1–2 A method that would not give valid results: some relevant steps are given, but they are not linked into a workable method. In Study Hack a six-mark answer is not given an automatic level: you tick the points your answer makes, then choose the level that fits it.
- No relevant content: 0 marks.
Evaluate
| Source of error | Type | Effect | Improvement |
|---|---|---|---|
| Not heating to constant mass | Systematic | Water is still trapped in the solid, so every mass of solid, and the dissolved solids per dm³, is too high. | Heat, cool and reweigh again and again until two masses in a row agree within 0.01 g. |
| Solid spitting out of the basin as the last water boils off | Random | A trial loses a different amount of solid each time, so its result is too low and does not agree with the others. | Evaporate on a water bath and dry the solid over a low flame; leave out any trial that spat. |
| The resolution of the balance (0.01 g) compared with the mass of solid from the mineral water | Random | The mass of solid is only a few times the balance’s resolution, so the result could be out by 15 % or more. | Evaporate a larger volume (250 cm³ or more), or use a balance that reads to 0.001 g. |
| Judging the colour of universal indicator by eye | Random | The pH is only to the nearest whole number, and two people may match the same colour to different squares. | Use a pH meter calibrated with a buffer solution. |
| Boiling the flask too hard during distillation | Systematic | Droplets of salt water are thrown into the condenser, so the distillate always contains some salt and tests positive. | Boil gently, with anti-bumping granules, and never fill the flask more than half full. |
| The thermometer bulb down in the boiling liquid instead of level with the side arm | Systematic | It reads the boiling salt solution, which is hotter than the steam that distils, so the boiling point recorded is too high. | Put the bulb level with the side arm, where the vapour goes into the condenser. |
| Cooling water in at the top of the condenser | Systematic | The jacket is only partly full, so it cools less well: some steam escapes without condensing and a little less distillate is collected. | Connect the cold water to the bottom connection and the outflow to the top one. |
| A dirty flame-test wire | Systematic | Sodium on the loop gives a yellow flame whatever you test, so the distilled water seems to contain sodium ions. | Clean the wire in acid and the flame until it gives no colour, before every test. |
Repeatable. Your three trials with the same sea water, the same method and the same balance agree closely (within about 1 g/dm³), apart from any trial that spat.
Reproducible. Another group, with different basins and a different balance, finds about the same dissolved solids per dm³ for the class’s sea water.
Other exam boards
How this practical lines up with other GCSE boards' required or core practicals. Codes only; check your own specification for the exact activity.
| Board | Practical | Match | What is different |
|---|---|---|---|
| AQA | Required practical 8 | This page | — |
| Edexcel | 2.11 | Similar, with differences | Only the simple distillation part matches; this lab distils sea water. |
Print it
- Blank write-up: the nine-step lab book with the correct table headings, ready to print for a real lesson.
- Specimen lab book: this page's run written up in full, with its exam answers ticked against the mark scheme.
- Printable method card: the practical on one index card. Practise with the cards in Study Hack.
- Practical workbook: a full simulated data set to process on paper, with exam questions and answers.
- Teacher and technician pack: apparatus lists, technician notes and marking points.