Launch practical
About 60 minutes
- Set up the bench with the 12 items of apparatus.
- Take the readings yourself, into the results table you draw first.
- Process your results.
- 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 26 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. Watch the flame just above the loop.
What it shows
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
Each ion has its own test. Metal ions give flame colours and coloured hydroxide precipitates; carbonates fizz with acid, giving carbon dioxide. Halides give silver halide precipitates (white, cream, yellow); sulfates give white barium sulfate. Acid goes in first to remove carbonate, and it must not add the ion being tested for.
The graph to expect. This practical has no graph. The results are observations, used to identify the ions.
Key equation for the exam: Ba²⁺ + SO₄²⁻ → BaSO₄.
Equation to write. Learn each test with its positive result.
Apparatus
| Item | What it is for | Resolution |
|---|---|---|
| Safety goggles | Protect your eyes from the reagents. | — |
| Bunsen burner and heatproof mat | The hot blue flame for the flame tests. | — |
| Nichrome wire loop in a handle, and emery paper | Carries a little of each solution into the flame; the emery paper cleans it. | — |
| Lithium, sodium, potassium, calcium and copper(II) chloride solutions, 0.4 mol/dm³ | Known solutions for the flame tests. | — |
| Sodium carbonate, chloride, bromide, iodide and sulfate solutions, 0.4 mol/dm³ | Known solutions for the tests for negative ions. | — |
| Unknown solutions A, B and C | Each is one ionic compound for you to identify. | — |
| Test tubes in a rack | About 1 cm depth of each solution for each test. | — |
| Teat (dropping) pipettes | Add the reagents drop by drop and move the gas into the limewater. | — |
| Limewater (calcium hydroxide solution) | Turns milky with carbon dioxide. | — |
| Dilute hydrochloric and nitric acids (0.4 mol/dm³), sodium hydroxide (0.4 mol/dm³), silver nitrate (0.05 mol/dm³) and barium chloride (0.1 mol/dm³) solutions | The test reagents, in dropping bottles. | — |
| White tile | Stood behind the tubes so the colours of the precipitates are easy to see. | — |
| Residue bottle | Silver and barium mixtures go here, not down the sink. | — |
Method
In our own words, as the Lab runs it. Your school's method sheet may differ in detail.
Choose the Bunsen flame for the flame tests
Light the Bunsen with the air hole closed, then set the flame you will hold the wire in.
Do: Air hole open: a hot, blue (roaring) flame.
Why: A flame test needs a flame with no colour of its own. Then the only colour you see comes from the metal ions on the wire.
Choose the acid for the halide test
Before the silver nitrate, add a few drops of dilute acid to each sample.
Do: Dilute nitric acid, 0.4 mol/dm³.
Why: Carbonate ions also give a pale precipitate with silver nitrate. The acid removes them (as carbon dioxide) first. The acid must not bring in a halide of its own.
Choose the acid for the sulfate test
Before the barium chloride, add a few drops of dilute acid to each sample.
Do: Dilute hydrochloric acid, 0.4 mol/dm³.
Why: Carbonate ions also give a white precipitate (barium carbonate). The acid removes them first. It must not bring in sulfate ions of its own.
Make the flame station safeSafety
Stand the Bunsen on a heatproof mat, tie long hair back, and wear goggles for every test.
Why: The wire gets red hot, and silver nitrate, barium chloride and sodium hydroxide must be kept out of your eyes.
Label every tube before you pour
Write the name of the solution on a label for each test tube. Keep unknowns A, B and C in their own row of the rack.
Why: Thirteen solutions look the same. A tube with no label, or the wrong one, puts the right observation against the wrong solution.
Take the readings
Part A: flame tests on five known chlorides; the carbonate, halide and sulfate tests on known sodium salts. Part B: on each of unknowns A, B and C, a flame test, sodium hydroxide, and the carbonate, halide and sulfate tests.
Repeats: Each test is done once: the results are observations, not measurements, so there is no mean. Repeat any test that gave an unclear result, such as a mixed flame colour or a faint precipitate. Use a clean wire or a fresh sample, and compare it with the known solution beside it.
Make safe and clear upSafety
Turn off the Bunsen at the gas tap. Leave the hot wire to cool on the heatproof mat before you put it away. Pour the silver and barium mixtures into the residue bottle, not down the sink. Rinse the test tubes and stand them upside down in the rack to drain. Wipe the bench and wash your hands.
While you take readings
Clean the wire between flame tests
Before each test, rub the wire loop with emery paper (or dip it in dilute hydrochloric acid). Then hold it in the flame until it gives no colour.
Why: Anything left on the loop from the last solution burns with its own colour in the next test. Sodium’s bright yellow hides the other colours.
Move the gas into the limewater
Squeeze the teat of the pipette, take up gas from above the fizzing liquid, and bubble it through the limewater. Repeat.
Why: A teat pipette holds only a few cm³ of gas. Limewater needs several pipettes of carbon dioxide before it turns milky.
Add the sodium hydroxide solution
Add sodium hydroxide solution to about 1 cm depth of each unknown in a test tube.
Do: A few drops at a time, then keep adding until it is in excess.
Why: Three metal ions give a white precipitate. Only aluminium hydroxide dissolves when more sodium hydroxide is added (in excess), so you must see what happens as you add more.
Hazards and control measures
| Hazard | What could happen | Control measure |
|---|---|---|
| Bunsen flame and the red-hot wire | Burns; hair or paper catching fire | Heatproof mat, hair tied back, hold the wire by its handle and let it cool on the mat. |
| Sodium hydroxide solution, 0.4 mol/dm³ (irritant) | Irritates the skin and can damage the eyes | Wear goggles; add it with a dropping pipette; wash splashes off with plenty of water. |
| Barium chloride solution, 0.1 mol/dm³ (harmful) | Harmful if swallowed | Never pipette by mouth; wash your hands after the practical; pour residues into the residue bottle. |
| Silver nitrate solution, 0.05 mol/dm³ (low hazard at this concentration; toxic to water life) | Stains the skin black; may irritate the eyes | Wear goggles, avoid skin contact, wash off splashes at once, and pour residues into the residue bottle. |
| Dilute nitric acid, 0.4 mol/dm³ (irritant) | Irritates the eyes and skin | Wear goggles; add only a few drops with a dropping pipette; wash splashes off with water. |
| Copper and iron salt solutions (harmful; toxic to water life) | Harmful if swallowed; harm life in rivers and streams | Wash your hands; pour the mixtures into the residue bottle. |
| Dilute hydrochloric acid, 0.4 mol/dm³ (low hazard at this concentration) | Can still sting the eyes and cuts in the skin | Wear goggles and add only a few drops. |
Variables
- Independent: you change
- None: this is identification, not an investigation
- Dependent: you measure
- The observation in each test (flame colour, precipitate colour, fizzing)
- Control: you keep the same
- Volume of each sample in cm depth. About 1 cm depth in each test tube.
- Concentration of the solutions in mol/dm³. The same concentrations every time (0.4 mol/dm³ for the salts).
- The order of reagents. Acid first, then the test reagent.
- The flame. The blue flame, with the wire at its edge.
Results from a specimen run
| Solution | Flame colour |
|---|---|
| Lithium chloride (known) | Crimson |
| Sodium chloride (known) | Yellow |
| Potassium chloride (known) | |
| Calcium chloride (known) | Orange-red |
| Copper(II) chloride (known) | Green |
| Unknown A | Yellow |
| Unknown B | Green |
| Unknown C | No colour: the flame stays blue |
| Test | Solution | Observation | Gas transferred in pipettes |
|---|---|---|---|
| Dilute hydrochloric acid; gas into limewater | Sodium carbonate | Fizzes; the gas turns limewater milky | 4 |
| Dilute nitric acid, silver nitrate | Sodium chloride | White precipitate | — |
| Dilute nitric acid, silver nitrate | Sodium bromide | Cream precipitate | — |
| Dilute nitric acid, silver nitrate | Sodium iodide | Yellow precipitate | — |
| Dilute hydrochloric acid, barium chloride | Sodium sulfate | White precipitate | — |
| Unknown | Sodium hydroxide | Dilute acid; limewater | Dilute nitric acid, silver nitrate | Dilute hydrochloric acid, barium chloride |
|---|---|---|---|---|
| A | No precipitate | No fizzing | Cream precipitate | No change |
| B | Blue precipitate | No fizzing | No change | White precipitate |
| C | White precipitate that dissolves in excess | No fizzing | White precipitate | No change |
Anomaly spotted: Flame test: potassium chloride, retaken.
Processing the data
- Identify A, B and C
- A NaBr, B CuSO₄, C AlCl₃
- The acid before silver nitrate
- Hydrochloric acid contains chloride ions, which give a white precipitate of silver chloride with any sample. Nitric acid removes carbonate ions and adds no halide.
- The acid before barium chloride
- The acid removes carbonate ions, which would give a white precipitate of barium carbonate. Sulfuric acid contains sulfate ions, so it would give barium sulfate with every sample.
- Aluminium, calcium or magnesium?
- The white precipitate dissolves when more sodium hydroxide is added (in excess).
- The sulfate test as an equation (Higher)
- Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
- Going further: a mixture
- Sodium and lithium: sodium’s yellow hid the crimson in the flame test.
| Quantity | Specimen run | Accepted |
|---|---|---|
| Ions identified correctly (3 metal ions, 3 non-metal ions) | 6 | 6 |
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: Air hole closed: the yellow safety flame
The safety flame is already bright yellow (glowing soot). Every flame test looks yellow, so you cannot tell the metals apart. This is a common slip at this practical.
The fix: Choose the Bunsen flame for the flame tests. A flame test needs a flame with no colour of its own. Then the only colour you see comes from the metal ions on the wire.
Method: Dilute hydrochloric acid, 0.4 mol/dm³
Hydrochloric acid contains chloride ions, so every sample gives a white silver chloride precipitate: chloride is found where there is none.
The fix: Choose the acid for the halide test. Carbonate ions also give a pale precipitate with silver nitrate. The acid removes them (as carbon dioxide) first. The acid must not bring in a halide of its own.
Method: No acid: the silver nitrate alone
A carbonate sample gives a pale precipitate of silver carbonate, which looks like silver chloride.
The fix: Choose the acid for the halide test. Carbonate ions also give a pale precipitate with silver nitrate. The acid removes them (as carbon dioxide) first. The acid must not bring in a halide of its own.
Flame test: lithium chloride: Recorded “Yellow”
The fix: Look again: a deep, bright red.
Flame test: lithium chloride: Recorded “Orange-red”
The fix: Look again: it is a deep red with no orange in it. Orange-red is calcium.
Exam answer: Leaving out: Did not clean the wire between tests
The fix: Say what went wrong: the wire was not cleaned between tests.
Exam answer: Leaving out: Ions from the previous solution (sodium) stayed on the wire, so the colours mixed / the yellow hid the lilac
The fix: Explain the colour: ions from the previous solution (sodium) were still on the wire, and sodium’s bright yellow hid the faint lilac.
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 1Flame tests: contamination · Analysing and evaluating[2 marks]
Table 1 shows the flame colours another student recorded for the known solutions, in the order they tested them.
Table 1 Solution Flame colour Lithium chloride Crimson Sodium chloride Yellow Potassium chloride Yellow Calcium chloride Orange-red Copper(II) chloride Green The result for potassium chloride is wrong. Suggest what the student did wrong, and explain why it gave the wrong colour.
Worked answer and how the marks are given
A full-mark answer
The wire was not cleaned between tests. Sodium ions from the previous solution were left on the wire, and the bright yellow sodium colour masked the lilac.
Mark scheme
- Did not clean the wire between tests
- Ions from the previous solution (sodium) stayed on the wire, so the colours mixed / the yellow hid the lilac
Where marks are usually lost
- Missing: Did not clean the wire between tests. Say what went wrong: the wire was not cleaned between tests.
- Missing: Ions from the previous solution (sodium) stayed on the wire, so the colours mixed / the yellow hid the lilac. Explain the colour: ions from the previous solution (sodium) were still on the wire, and sodium’s bright yellow hid the faint lilac.
- Question 2Concentration from a precipitate · Applying knowledge[3 marks]
A student added excess barium chloride solution to 25.0 cm³ of a sodium sulfate solution. All the sulfate ions formed a precipitate of barium sulfate:
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
The student filtered, washed and dried the precipitate. Its mass was 0.515 g.
Relative formula mass (Mr) of BaSO₄ = 233
Calculate the concentration of sulfate ions in the solution, in mol/dm³.
Concentration =
Worked answer and how the marks are given
A full-mark answer
amount of BaSO₄ = 0.515 ÷ 233 = 0.00221 mol
25.0 cm³ = 0.0250 dm³
concentration = 0.00221 ÷ 0.0250 = 0.088 mol/dm³Mark scheme
- amount of BaSO₄ = 0.515 ÷ 233 = 0.00221 mol (method)
- 25.0 cm³ = 0.0250 dm³ (or × 1000 ÷ 25.0) (method)
- 0.088 mol/dm³ (allow error carried forward from their amount in moles)
- Allow answers from 0.08664 to 0.09018 (the marker's tolerance; correct rounding of the student's own working is also accepted).
- Error carried forward: method and answer marks follow correctly from the student's own earlier value.
- A correct answer with no working scores full marks.
Where marks are usually lost
- Answer 0.0000884 (gains 1 mark). Change 25.0 cm³ to dm³ first (÷ 1000): concentration = 0.00221 ÷ 0.0250.
- Answer 0.0000553 (gains 1 mark). Divide by the volume, don’t multiply: concentration = 0.00221 ÷ 0.0250.
- Answer 4800 (gains 1 mark). Divide the mass by the Mr, don’t multiply: amount = 0.515 ÷ 233.
- Question 3Planning tests for ions · Knowledge[6 marks]
A medicine contains potassium bromide, KBr. The medicine is a white solid that dissolves in water.
Plan tests to show that the medicine contains potassium ions and bromide ions. Give the results of the tests.
Worked answer and how the marks are given
A full-mark answer
Potassium ions:
1. Dissolve the medicine in distilled water in a test tube.
2. Clean a metal wire loop by dipping it in acid and holding it in the flame until it gives no colour.
3. Dip the wire in the sample and hold it in a blue (roaring) Bunsen flame.
4. A lilac flame shows potassium ions.
Bromide ions:
5. Add dilute nitric acid to the sample in a test tube.
6. Add silver nitrate solution with a dropping pipette: a cream precipitate shows bromide ions.Mark scheme
Indicative content:
- Dissolve (crushed) sample in (distilled) water in a test tube
- Clean the metal wire (e.g. dip in acid / hold in the flame until no colour)
- Put the sample on the wire and hold it in the flame
- A blue (non-luminous / roaring) Bunsen flame
- Lilac flame shows potassium ions
- Add (dilute) nitric acid
- Add silver nitrate solution (with a dropping pipette)
- Cream precipitate shows bromide ions Do not accept “cream… solution / liquid”.
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 |
|---|---|---|---|
| A wire not cleaned between flame tests | Systematic | Ions from the previous solution, and sodium, stay on the loop: yellow or mixed colours where there should be one clear colour. | Clean the loop with emery paper (or dilute hydrochloric acid) and hold it in the flame until it gives no colour, before every test. |
| Sodium from fingers, the bench or the tap water on the loop | Random | A strong yellow appears in one test and hides a faint colour such as lilac. | Do not touch the loop; use distilled water; look through blue cobalt glass, or use a flame emission spectroscope. |
| The yellow (safety) flame | Systematic | The flame is already yellow, so every flame test looks yellow. | Open the air hole and use the hot blue flame. |
| Hydrochloric acid added before the silver nitrate | Systematic | The acid adds chloride ions, so every sample gives a white precipitate. | Use dilute nitric acid before silver nitrate. |
| Sulfuric acid, or no acid, before the barium chloride | Systematic | Sulfuric acid adds sulfate ions (a white precipitate every time); with no acid a carbonate gives white barium carbonate. | Add dilute hydrochloric acid first. |
| Sodium hydroxide not added drop by drop and then in excess | Systematic | A few drops cannot tell aluminium from calcium or magnesium; poured in at once, aluminium hydroxide dissolves before it is seen. | Add it a few drops at a time, then keep adding until it is in excess, watching all the time. |
| Only one pipette of gas bubbled through the limewater | Systematic | Not enough carbon dioxide reaches the limewater, so it stays clear and a carbonate is missed. | Keep transferring the gas until the limewater changes (or bubble it through with a delivery tube). |
| Judging similar colours by eye (cream and white; crimson and orange-red) | Random | Close colours can be mistaken, especially in poor light. | Compare each tube with the known results side by side against a white tile, or use flame emission spectroscopy. |
| Iron(II) hydroxide turning brown at the surface | Random | Left to stand, the green precipitate browns as air oxidises it, and can be mistaken for iron(III). | Record the colour as soon as the precipitate forms. |
Repeatable. When you repeat a test on the same solution with the same method and equipment, you see the same colour or precipitate.
Reproducible. Another student, or a different method such as flame emission spectroscopy, identifies the same ions in the same unknowns.
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 7 | This page | — |
| Edexcel | 9.6C | Same activity | The ions to know are listed in spec points 9.2C to 9.5C; check them against this lab. |
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.