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Study Hack Lab

RP 7Chemistry

Identifying ions

Lab book

Name: Specimen
Date: 2 November 2026
Course: GCSE Chemistry
Board and specification: AQA GCSE Chemistry (8462), 4.8.3Apparatus and techniques: AT 2, AT 8
Run: C29HO
Status: Finished: every stage done
Exam questions: 30/30 marks

Aim

See the result of each test on known solutions. Then use flame tests, sodium hydroxide, and the carbonate, halide and sulfate tests to identify the ions in three unknown single ionic compounds.

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

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.

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

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; Concentration of the solutions in mol/dm³; The order of reagents; The flame

Prediction

What colour will potassium chloride give in a flame test?

Lilac Right first time

Potassium ions give a lilac flame. It is faint, and a little sodium (yellow) easily hides it, so the wire must be clean. Lithium is crimson, sodium yellow, calcium orange-red and copper green.

Silver nitrate is added (after nitric acid) to chloride, bromide and iodide solutions. What colours are the precipitates, in that order?

White, cream, yellow Right first time

Silver chloride is white, silver bromide cream and silver iodide yellow. The colours are close: compare the tubes side by side against a white tile.

Aluminium, calcium and magnesium ions all give a white precipitate with a little sodium hydroxide. What happens when more sodium hydroxide is added?

Only the aluminium hydroxide precipitate dissolves. Right first time

Aluminium hydroxide reacts with excess sodium hydroxide and dissolves; calcium and magnesium hydroxides stay. That is the only way to tell aluminium apart with this test.

Risk assessment

Hazard	Harm it could cause	Control you chose
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. Right first time
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. Right first time
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. Right first time
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. Right first time
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. Right first time
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. Right first time
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. Right first time

2 Apparatus

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.

3 Method: your choices

Choose the Bunsen flame for the flame tests: Air hole open: a hot, blue (roaring) flameThe blue flame has almost no colour of its own, so each metal ion’s colour shows clearly at the edge of the flame.
Choose the acid for the halide test: Dilute nitric acid, 0.4 mol/dm³Removes any carbonate and adds no halide ions: only a halide gives a precipitate.
Choose the acid for the sulfate test: Dilute hydrochloric acid, 0.4 mol/dm³Removes any carbonate and adds no sulfate: only a sulfate gives the white precipitate.
Make the flame station safeSafety: DoneThe 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: DoneThirteen solutions look the same. A tube with no label, or the wrong one, puts the right observation against the wrong solution.
Clean the wire between flame tests: DoneAnything 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: DoneA 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: A few drops at a time, then keep adding until it is in excessYou see each precipitate form, then whether it dissolves in excess: aluminium can be told from calcium and magnesium.

4 Results

26 of 26 readings taken.

Flame tests · Wire held at the edge of the blue Bunsen flame.

Solution	Flame colour
Lithium chloride (known)	Crimson
Sodium chloride (known)	Yellow
Potassium chloride (known)	~~Yellow, crossed out,~~ Lilac
Calcium chloride (known)	Orange-red
Copper(II) chloride (known)	Green
Unknown A	Yellow
Unknown B	Green
Unknown C	No colour: the flame stays blue

Tests for negative ions: known sodium salts

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	—

Unknowns A, B and C · Sodium hydroxide added a few drops at a time, then in excess.

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

~~0.0~~ Struck through: the anomaly, retaken; the new reading is beside it.

Anomaly: Flame test: potassium chloride, retaken. Likely cause: sodium on the loop (from fingers or the previous solution): its bright yellow hides potassium’s faint lilac.

5 Graph

This practical has no graph: the results are in the table above.

6 Analysis

Identify A, B and C
A NaBr, B CuSO₄, C AlCl₃ Right first time
The acid before silver nitrate
You added dilute nitric acid before the silver nitrate. Why must it be nitric acid, and not hydrochloric acid?
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. Right first time
The acid before barium chloride
Why is dilute hydrochloric acid added before barium chloride, and why not sulfuric acid?
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. Right first time
Aluminium, calcium or magnesium?
Aluminium, calcium and magnesium ions all give a white precipitate with a few drops of sodium hydroxide. Which observation identifies aluminium?
The white precipitate dissolves when more sodium hydroxide is added (in excess). Right first time
The sulfate test as an equation (Higher)
Unknown B (CuSO₄) gave a white precipitate with barium chloride. Which is the ionic equation for the formation of the white precipitate?
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) Right first time
Going further: a mixture
A flame test on a solid gave only a yellow flame. A flame emission spectroscope showed bright lines at 589, 610, 671 nm. Use the reference lines to say which metal ions are in the solid.
Sodium and lithium: sodium’s yellow hid the crimson in the flame test. Right first time

7 Conclusion and evaluation

Quantity	Your value	Accepted value
Ions identified correctly (3 metal ions, 3 non-metal ions)	6	6

Conclusion

What does this practical show about identifying ions?

Each ion gives its own result in its own test. So a single compound can be identified by testing for the metal ion and the non-metal ion separately. Right first time

The metal ion came from the flame test and sodium hydroxide. The non-metal ion came from the acid, silver nitrate and barium chloride tests. The bottles were A sodium, B copper(II), C aluminium.

Sources of error

A wire not cleaned between flame tests: systematic or random?

Systematic Right first time

Ions from the previous solution, and sodium, stay on the loop: yellow or mixed colours where there should be one clear colour.

Sodium from fingers, the bench or the tap water on the loop: systematic or random?

Random Right first time

A strong yellow appears in one test and hides a faint colour such as lilac.

The yellow (safety) flame: systematic or random?

Systematic Right first time

The flame is already yellow, so every flame test looks yellow.

Hydrochloric acid added before the silver nitrate: systematic or random?

Systematic Right first time

The acid adds chloride ions, so every sample gives a white precipitate.

Sulfuric acid, or no acid, before the barium chloride: systematic or random?

Systematic Right first time

Sulfuric acid adds sulfate ions (a white precipitate every time); with no acid a carbonate gives white barium carbonate.

Sodium hydroxide not added drop by drop and then in excess: systematic or random?

Systematic Right first time

A few drops cannot tell aluminium from calcium or magnesium; poured in at once, aluminium hydroxide dissolves before it is seen.

Only one pipette of gas bubbled through the limewater: systematic or random?

Systematic Right first time

Not enough carbon dioxide reaches the limewater, so it stays clear and a carbonate is missed.

Judging similar colours by eye (cream and white; crimson and orange-red): systematic or random?

Random Right first time

Close colours can be mistaken, especially in poor light.

Iron(II) hydroxide turning brown at the surface: systematic or random?

Random Right first time

Left to stand, the green precipitate browns as air oxidises it, and can be mistaken for iron(III).

Improvement

Which change would most improve your results?

Do not touch the loop; use distilled water; look through blue cobalt glass, or use a flame emission spectroscope. Right first time

Repeatable or reproducible

You test unknown A with silver nitrate a second time, with the same method, and see the same cream precipitate. This shows your result is…

Repeatable Right first time

Repeatable: the same person, method and equipment give the same result.

A technician tests unknown A with flame emission spectroscopy and identifies the same metal ion that you did. This shows the result is…

Reproducible Right first time

Reproducible: a different person, with a different method or equipment, gets the same result.

8 Make safe and clear up

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

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 did a flame test on potassium chloride solution with a clean wire.
What colour flame shows that potassium ions are present?
Tick (✓) one box.
- Crimson
- Lilac (your choice)
- Orange-red
- Yellow
Your answer
Lilac
1/1
- Lilac
Full marks. That is exactly what the mark scheme is looking for.
Model answer: Lilac.
01.2[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.
Your answer
The student did not clean the wire between the tests. Sodium from the previous solution was left on the wire, so the yellow colour masked the lilac flame.
2/2 self-marked
- 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
Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.
Model 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.
01.3[1 mark]
Metal ions can also be identified with an instrument that analyses the light given out when a sample is put into a flame.
Name this instrumental method.
Your answer
Flame emission spectroscopy
1/1 self-marked
- Flame emission spectroscopy
Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.
Model answer: Flame emission spectroscopy (do not accept F.E.S.).
01.4[1 mark]
Which is an advantage of flame emission spectroscopy compared with a flame test?
Tick (✓) one box.
- It only tells you whether an ion is present or not.
- It is more sensitive: it can detect very small amounts of an ion. (your choice)
- It does not need a flame.
- It is cheaper.
Your answer
It is more sensitive: it can detect very small amounts of an ion.
1/1
- It is more sensitive: it can detect very small amounts of an ion.
Full marks. Clear and precise.
Model answer: It is more sensitive: it can detect very small amounts (it is also accurate and rapid).
02.1[3 marks]
A sample of water may contain aluminium ions, Al³⁺.
Describe a test to show that the water contains aluminium ions. Give the results.
Your answer
Add sodium hydroxide solution a few drops at a time. A white precipitate forms. Add excess sodium hydroxide and the precipitate dissolves.
3/3 self-marked
- Add sodium hydroxide solution
- White precipitate (forms)
- The precipitate dissolves in excess sodium hydroxide
Self-marked: 3 of 3 marks. This is your own judgement against the mark scheme.
Model answer: Add sodium hydroxide solution. A white precipitate forms. The precipitate dissolves when excess sodium hydroxide is added.
02.2[2 marks]
A solution may contain copper(II) ions, Cu²⁺.
Describe a test for copper(II) ions. Give the result.
Your answer
Add sodium hydroxide solution. A blue precipitate forms.
2/2 self-marked
- Add sodium hydroxide solution
- Blue precipitate (two colours such as green/blue are not credited)
- or: do a flame test
- or: a green (allow blue-green) flame
Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.
Model answer: Add sodium hydroxide solution: a blue precipitate forms. (Or: flame test, green flame; allow blue-green flame. The result mark depends on the test mark; two colours for the precipitate are not credited.)
02.3[1 mark]
Your unknown C contained aluminium ions, Al³⁺.
When sodium hydroxide solution is added, aluminium ions react with hydroxide ions, OH⁻, to form a white precipitate of aluminium hydroxide.
Which is the balanced ionic equation for the formation of the precipitate?
Tick (✓) one box.
- Al³⁺(aq) + 2OH⁻(aq) → Al(OH)₂(s)
- Al³⁺(aq) + 3OH⁻(aq) → Al(OH)₃(s) (your choice)
- Al³⁺(aq) + 3OH⁻(aq) → Al(OH)₃(aq)
- Al(s) + 3OH⁻(aq) → Al(OH)₃(s)
Your answer
Al³⁺(aq) + 3OH⁻(aq) → Al(OH)₃(s)
1/1
- Al³⁺(aq) + 3OH⁻(aq) → Al(OH)₃(s)
Full marks. That is exactly what the mark scheme is looking for.
Model answer: Al³⁺(aq) + 3OH⁻(aq) → Al(OH)₃(s) (3 OH⁻ ions balance the 3+ charge; the precipitate is a solid, (s).)
02.4[1 mark]
Table 2 shows your results for unknowns A, B and C.
Which unknown contains the ion of a Group 1 metal?
Select one row.
Table 2
Unknown Flame test Sodium hydroxide solution
A Yellow No precipitate
B Green Blue precipitate
C No colour: the flame stays blue White precipitate that dissolves in excess
Your answer
unknown A
1/1
- Unknown A: no precipitate with sodium hydroxide
Full marks. Well done. Unknown A: Group 1 hydroxides are soluble, so it gives no precipitate with sodium hydroxide.
Model answer: Unknown A: it gives no precipitate with sodium hydroxide, because Group 1 hydroxides are soluble.
03.1[2 marks]
A student wants to show that a solution contains sulfate ions, SO₄²⁻.
Describe the test for sulfate ions. Give the result.
Your answer
Add dilute hydrochloric acid and then barium chloride solution. A white precipitate forms.
2/2 self-marked
- (Add dilute hydrochloric acid, then) barium chloride solution (allow barium nitrate; allow dilute nitric acid in place of the hydrochloric acid; do not accept sulfuric acid)
- White precipitate
Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.
Model answer: Add dilute hydrochloric acid, then barium chloride solution. A white precipitate forms. (The result mark depends on the test mark; do not accept sulfuric acid.)
03.2[2 marks]
A solution contains bromide ions, Br⁻.
Describe a test for bromide ions. Give the result.
Your answer
Add dilute nitric acid then silver nitrate solution. A cream precipitate forms.
2/2 self-marked
- Add (nitric acid and) silver nitrate solution
- Cream precipitate
Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.
Model answer: Add dilute nitric acid, then silver nitrate solution. A cream precipitate forms. (Ignore “bromine”; do not accept “cream solution”.)
03.3[2 marks]
In the halide test, dilute nitric acid is added before the silver nitrate solution.
Explain why dilute hydrochloric acid must not be used in the halide test.
Your answer
Hydrochloric acid contains chloride ions. They would react with the silver nitrate to give a white precipitate of silver chloride, even if the sample has no chloride ions.
2/2 self-marked
- Hydrochloric acid contains chloride ions
- So a white precipitate (of silver chloride) forms whatever the sample: a false positive for chloride
Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.
Model answer: Hydrochloric acid contains chloride ions. These react with silver nitrate to give a white precipitate of silver chloride, even if the sample contains no chloride. So the result would be wrong.
03.4[2 marks]
A white solid may be a carbonate.
Describe a test for carbonate ions. Give the results.
Your answer
Add dilute hydrochloric acid and it fizzes. Bubble the gas through limewater and the limewater turns milky.
2/2 self-marked
- Add dilute acid: fizzing (effervescence)
- Gas bubbled through limewater turns it milky (cloudy)
Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.
Model answer: Add dilute hydrochloric acid: it fizzes. Bubble the gas through limewater: the limewater turns milky (cloudy).
03.5[1 mark]
Barium chloride solution has this hazard label: “Harmful if swallowed”.
Give one precaution the student should take when using barium chloride solution.
Your answer
Wash your hands after using barium chloride solution.
1/1 self-marked
- Wash hands after use / do not eat or drink / no mouth pipetting / wear goggles or gloves / dispose of residues safely
Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.
Model answer: Wash your hands after using it (or: do not eat or drink in the lab; never pipette by mouth; wear gloves).
04[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.
Your answer
0.088
Working: 0.515 / 233 = 0.00221 25.0 cm3 = 0.0250 dm3 0.00221 / 0.0250
3/3
- amount of BaSO₄ = 0.515 ÷ 233 = 0.00221 mol
- 25.0 cm³ = 0.0250 dm³ (or × 1000 ÷ 25.0)
- 0.088 mol/dm³ (allow error carried forward from their amount in moles)
Full marks. That is exactly what the mark scheme is looking for.
Model answer: amount of BaSO₄ = 0.515 ÷ 233 = 0.00221 mol 25.0 cm³ = 0.0250 dm³ concentration = 0.00221 ÷ 0.0250 = 0.088 mol/dm³
05[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.
Your 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.
6/6 self-marked
- 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
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: 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.

Solution	Flame colour
Lithium chloride	Crimson
Sodium chloride	Yellow
Potassium chloride	Yellow
Calcium chloride	Orange-red
Copper(II) chloride	Green

Unknown	Flame test	Sodium hydroxide solution
A	Yellow	No precipitate
B	Green	Blue precipitate
C	No colour: the flame stays blue	White precipitate that dissolves in excess

Unseen data: the same skills, new apparatus

01[2 marks]

A student dissolves a sample of a garden fertiliser sold as potash in distilled water. The label says it is a single ionic compound. The student tests the solution.

Results

Test	Observation
Flame test (clean wire, blue flame)	Lilac
Sodium hydroxide solution, a few drops then in excess	No precipitate
Dilute hydrochloric acid; gas into limewater	Fizzes; the gas turns limewater milky
Dilute nitric acid, then silver nitrate	No change
Dilute hydrochloric acid, then barium chloride	No change

Which TWO statements are correct? Tick (✓) two boxes.

The metal ion is lithium ions, Li⁺.
The metal ion is potassium ions, K⁺. (your choice)
The flame test alone identifies both ions in the compound.
The negative ion is carbonate ions, CO₃²⁻. (your choice)
The negative ion is sulfate ions, SO₄²⁻.

Your answer

The metal ion is potassium ions, K⁺.; The negative ion is carbonate ions, CO₃²⁻.

2/2

The metal ion is potassium ions, K⁺.
The negative ion is carbonate ions, CO₃²⁻.

Full marks. Your answer meets all the points for this question.

Model answer: A lilac flame and no precipitate with sodium hydroxide: potassium ions, K⁺. The one positive test for negative ions shows carbonate ions, CO₃²⁻.

02[2 marks]
A student tests a sample of pond water for halide ions. The student adds dilute hydrochloric acid, then silver nitrate solution. A white precipitate forms. The student concludes that the sample contains chloride ions.
Which TWO statements are correct? Tick (✓) two boxes.
- The conclusion is valid: a white precipitate with silver nitrate shows chloride ions.
- The conclusion is not valid: hydrochloric acid contains chloride ions, so a white precipitate of silver chloride forms with any sample. (your choice)
- The acid only makes the precipitate form faster, so it can be left out.
- The test should be repeated with dilute nitric acid instead of hydrochloric acid. (your choice)
- The test should be repeated with dilute sulfuric acid instead of hydrochloric acid.
Your answer
The conclusion is not valid: hydrochloric acid contains chloride ions, so a white precipitate of silver chloride forms with any sample.; The test should be repeated with dilute nitric acid instead of hydrochloric acid.
2/2
- The conclusion is not valid: hydrochloric acid contains chloride ions, so a white precipitate of silver chloride forms with any sample.
- The test should be repeated with dilute nitric acid instead of hydrochloric acid.
Full marks. Your answer meets all the points for this question.
Model answer: Hydrochloric acid adds chloride ions, so silver chloride forms whatever the sample: the result says nothing about the sample. Dilute nitric acid removes carbonate ions without adding a halide, so repeat with nitric acid.
03[2 marks]
A water company tests a sample of mineral water with flame emission spectroscopy. The spectrum has bright lines at 404, 423, 616, 646, 767 nm.
Reference spectra
Metal ion Bright lines in nm
lithium (Li⁺) 610, 671
sodium (Na⁺) 589
potassium (K⁺) 404, 767
calcium (Ca²⁺) 423, 616, 646
copper(II) (Cu²⁺) 510, 515, 522
Which TWO statements are correct? Tick (✓) two boxes.
- The sample contains potassium ions and calcium ions. (your choice)
- The sample contains potassium ions and copper(II) ions.
- Flame emission spectroscopy can identify only one metal ion in a sample.
- Flame emission spectroscopy can identify the metal ions in a mixture, where in a flame test one colour can hide another. (your choice)
- A flame test would show both metal ions just as clearly.
Your answer
The sample contains potassium ions and calcium ions.; Flame emission spectroscopy can identify the metal ions in a mixture, where in a flame test one colour can hide another.
2/2
- The sample contains potassium ions and calcium ions.
- Flame emission spectroscopy can identify the metal ions in a mixture, where in a flame test one colour can hide another.
Full marks. Your answer meets all the points for this question.
Model answer: Every line matches potassium (404, 767 nm) or calcium (423, 616, 646 nm). Each metal ion gives its own lines, so the instrument can identify ions in a mixture (and in small amounts) where a flame test cannot.

Metal ion	Bright lines in nm
lithium (Li⁺)	610, 671
sodium (Na⁺)	589
potassium (K⁺)	404, 767
calcium (Ca²⁺)	423, 616, 646
copper(II) (Cu²⁺)	510, 515, 522

Identifying ions

Aim

Contents

1 Plan and risk assessment

Variables

Prediction

Risk assessment

2 Apparatus

3 Method: your choices

4 Results

5 Graph

6 Analysis

7 Conclusion and evaluation

Conclusion

Sources of error

Improvement

Repeatable or reproducible

8 Make safe and clear up

9 Exam questions on your data

Unseen data: the same skills, new apparatus