Tests for gases & ions, flame emission spectroscopy (RP7) – GCSE Chemistry exam questions with mark scheme

1Grade 1–3Foundation4 marks

The table shows four gases and five test results.

Gas	Letter	Test result
hydrogen		A turns limewater milky
oxygen		B turns damp red litmus paper blue
carbon dioxide		C bleaches damp litmus paper white
chlorine		D burns with a squeaky pop with a lighted splint
		E relights a glowing splint

Match each gas to the test result that identifies it. Write the correct letter, A, B, C, D or E, next to each gas. Use each letter once or not at all. [4]

hydrogen → D (burns with a squeaky pop with a lighted splint) for each gas: if more than one letter is given, no mark for that gas(1)
oxygen → E (relights a glowing splint)(1)
carbon dioxide → A (turns limewater milky)(1)
chlorine → C (bleaches damp litmus paper white)(1)
Answer: hydrogen D (squeaky pop); oxygen E (relights glowing splint); carbon dioxide A (limewater milky); chlorine C (bleaches damp litmus)
Examiner tip: Lighted splint is for hydrogen; glowing splint is for oxygen — learn which is which.

2Grade 6–7HigherTriple only5 marks

A student tests a solution of an unknown compound, Y.

Test 1: sodium hydroxide solution is added. A green precipitate forms.
Test 2: dilute nitric acid and then silver nitrate solution are added. A cream precipitate forms.

(a) Identify the metal ion and the halide ion in Y and give the name of Y. [3]

(b) Write a balanced ionic equation, with state symbols, for the formation of the green precipitate. [2]

(a) iron(II) / Fe²⁺(1)
(a) bromide / Br⁻(1)
(a) iron(II) bromide; allow ecf from their ions(1)
(b) Fe²⁺ + 2OH⁻ → Fe(OH)₂ (correct species and balanced) allow a correct balanced full equation (FeBr₂ + 2NaOH → Fe(OH)₂ + 2NaBr) for this mark only(1)
(b) state symbols (aq) + (aq) → (s) dependent on correct species in the first (b) mark(1)
Answer: (a) Fe²⁺, Br⁻, iron(II) bromide (b) Fe²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(s)
Examiner tip: Green = iron(II), brown = iron(III), blue = copper(II); cream = bromide, white = chloride, yellow = iodide.

3Grade 4–5Triple only4 marks

A student tests four solutions. Complete the table. [4]

Ion tested for	Reagent(s) added	Positive result
carbonate	dilute acid, gas bubbled into limewater	(i) ......
sulfate	(ii) ......	white precipitate
lithium	flame test	(iii) ......
calcium	flame test	(iv) ......

(i) limewater turns milky / cloudy ignore fizzing / effervescence; fizzing alone does not score(1)
(ii) dilute hydrochloric acid then barium chloride solution (both needed) allow acidified barium chloride; do not accept sulfuric acid(1)
(iii) crimson flame; allow red(1)
(iv) orange-red flame; allow brick red; do not accept 'red' alone(1)
Answer: (i) limewater turns milky / cloudy (ii) dilute HCl then BaCl₂(aq) (iii) crimson (iv) orange-red
Examiner tip: Lithium is crimson and calcium is orange-red — 'red' alone for calcium is not accepted.

4Grade 6–7Triple only4 marks

A sample contains a mixture of potassium and sodium ions. A flame test is not useful for identifying both ions.

(a) Explain why. [1]

(b) Describe how flame emission spectroscopy could identify both ions and find the concentration of sodium ions. [3]

(a) one flame colour masks / hides the other (the yellow of sodium hides the lilac of potassium) allow the colours mix / you only see the yellow (sodium) flame(1)
(b) sample put into a flame and the light given out is passed through a spectroscope allow spectrometer(1)
(b) the line spectrum produced is compared with reference spectra / each ion has a unique (line) spectrum so both ions identified(1)
(b) the intensity / brightness of the lines (for sodium) is measured to find the concentration allow compared with (a calibration from) standard solutions of known concentration(1)
Answer: (a) the colours mask each other (b) spectroscope produces a line spectrum compared with known spectra; intensity measured against standards gives concentration
Examiner tip: Flame tests fail for mixtures because colours mask each other — spectroscopy is quantitative as well as qualitative.

5Grade 4–5Triple only6 marks

A student tests solutions of two ionic compounds, X and Y. The table shows the results.

Test	Compound X	Compound Y
Flame test	crimson flame	green flame
Add sodium hydroxide solution	no change	blue precipitate
Add dilute nitric acid, then silver nitrate solution	yellow precipitate	no change
Add dilute hydrochloric acid, then barium chloride solution	no change	white precipitate

(a) Name compound X. [2]

(b) Name compound Y. [2]

(i) Explain why dilute hydrochloric acid must not be used instead. [1]

(ii) Suggest why an acid is added at all. [1]

(a) lithium (ion) identified allow Li⁺(1)
(a) iodide (ion) identified, giving lithium iodide allow LiI; do not accept iodine / lithium iodine(1)
(b) copper(II) / copper (ion) identified allow Cu²⁺(1)
(b) sulfate (ion) identified, giving copper(II) sulfate allow copper sulfate / CuSO₄; do not accept sulfide / sulfur(1)
(c)(i) hydrochloric acid contains chloride ions, which would form a (white) precipitate with silver nitrate (so a false positive / the result would be hidden) allow it would add chloride ions(1)
(c)(ii) to react with / remove carbonate ions (which would also form a precipitate) allow to remove other ions that would give a precipitate / to prevent a false positive(1)
Answer: (a) lithium iodide (b) copper(II) sulfate (c)(i) HCl adds chloride ions, which give a white precipitate with silver nitrate (ii) to remove carbonate ions that would also form a precipitate
Examiner tip: Name the ion, not the element: ‘iodide’ and ‘sulfate’, never ‘iodine’ or ‘sulfur’.

6Grade 6–7Triple only6 marks

A technician finds three bottles of white solid with the labels missing. She knows the solids are:

lithium bromide
potassium sulfate
sodium carbonate.

Plan an investigation, using chemical tests, to identify both the positive ion and the negative ion in each solid.

Your plan should include the results you would expect. [6]

Dissolve a small sample of each solid in (distilled) water; use a fresh sample of each solution for every test(1)
Flame test: clean a nichrome / platinum wire loop (e.g. dip in hydrochloric acid and heat until there is no colour), dip it in the solid / solution, hold it in a blue (roaring) Bunsen flame(1)
Lithium bromide gives a crimson flame(1)
Potassium sulfate gives a lilac flame(1)
Sodium carbonate gives a yellow flame(1)
Clean the wire between samples so one colour does not contaminate / mask another(1)
Carbonate: add dilute acid (e.g. hydrochloric acid); fizzing / bubbles; bubble the gas through limewater which turns milky / cloudy (carbon dioxide)(1)
Bromide: add dilute nitric acid then silver nitrate solution; a cream precipitate forms(1)
Sulfate: add dilute hydrochloric acid then barium chloride solution; a white precipitate forms(1)
Acid is added first in the halide and sulfate tests to remove carbonate ions, which would otherwise give a precipitate(1)
Each negative-ion test gives a positive result for only one of the three solids, so each solid is identified by two independent tests (its flame colour and its negative-ion test)(1)
Safety: wear eye protection; barium chloride is toxic — handle with care(1)

Level	Marks	What it looks like
Level 3	5–6	A logically ordered plan that would identify all six ions: flame tests with correct colours for all three metal ions AND correct tests with reagents and results for carbonate, bromide and sulfate. Includes some detail of method (e.g. cleaning the wire, acidifying first, fresh samples).
Level 2	3–4	A plan that would identify most ions: correct flame test colours for at least two metal ions and a correct reagent and result for at least two negative ions, OR all ion tests named with some results missing or incorrect. Some method detail may be missing.
Level 1	1–2	Some relevant tests are described, e.g. a flame test or one negative-ion test, with at least one correct result. Plan lacks structure and would not identify all three solids.
No relevant content	0	Nothing worthy of credit.

Answer: Flame tests: crimson = lithium bromide, lilac = potassium sulfate, yellow = sodium carbonate. Confirm negative ions: acid + limewater turns milky (carbonate); HNO₃ then AgNO₃ gives cream precipitate (bromide); HCl then BaCl₂ gives white precipitate (sulfate).
Examiner tip: Six-mark plans lose marks for missing results — every test needs a reagent AND the expected observation.

7Grade 6–7HigherTriple only6 marks

A student has three solutions, A, B and C. One contains aluminium ions, one contains calcium ions and one contains magnesium ions.

When a few drops of sodium hydroxide solution are added to each solution, a white precipitate forms in all three.

(a) Describe how the student could use more sodium hydroxide solution to identify the solution containing aluminium ions. Give the result. [2]

(b) Describe how the student could then tell which of the other two solutions contains calcium ions. Give the result. [2]

Write a balanced equation for the reaction between aluminium sulfate and sodium hydroxide to form the white precipitate. [2]

(a) add (sodium hydroxide solution) in excess / keep adding sodium hydroxide(1)
(a) the precipitate (with aluminium ions) dissolves / disappears / forms a colourless solution; the other two precipitates remain allow ‘the aluminium one dissolves’; ignore ‘reacts’(1)
(b) (carry out a) flame test on each of the two (original) solutions do not accept a test with more sodium hydroxide(1)
(b) calcium (ions) give an orange-red flame (magnesium does not give this colour) allow orange / red-orange / brick-red; do not accept red on its own or crimson(1)
(c) correct formulae: Al₂(SO₄)₃ + NaOH → Al(OH)₃ + Na₂SO₄(1)
(c) balanced: Al₂(SO₄)₃ + 6NaOH → 2Al(OH)₃ + 3Na₂SO₄ allow multiples; ignore state symbols; allow Al³⁺ + 3OH⁻ → Al(OH)₃ for 1 mark only(1)
Answer: (a) add excess NaOH: the aluminium hydroxide precipitate dissolves (b) flame test: calcium gives orange-red (c) Al₂(SO₄)₃ + 6NaOH → 2Al(OH)₃ + 3Na₂SO₄
Examiner tip: Only aluminium hydroxide dissolves in excess NaOH — calcium and magnesium need a second test (a flame test) to tell them apart.

8Grade 6–7Triple only5 marks

A water company uses flame emission spectroscopy to analyse a sample, S, from a borehole.

The diagram shows simplified line spectra for five metal ions and for sample S.

(a) Identify the two metal ions present in sample S. [2]

(b) Explain how the spectra show that sample S does not contain sodium ions. [1]

(a) lithium (ions) / Li⁺(1)
(a) potassium (ions) / K⁺; max 1 mark if three or more ions are given(1)
(b) there is no line in the spectrum of S at the same wavelength as the sodium line (589 nm / about 590 nm) allow the sodium line is missing from S(1)
(c) any two from: more accurate; more sensitive / detects very small amounts / low concentrations; more rapid / faster; can identify ions in a mixture (where flame colours would mask each other); can measure the concentration of the ions(1)
(c) second advantage from the list above ignore ‘easier’ / ‘cheaper’ / ‘more reliable’ without qualification(1)
Answer: (a) lithium and potassium (b) S has no line at the wavelength of the sodium line (about 590 nm) (c) e.g. more sensitive; can measure concentration (also accurate, rapid, works for mixtures)
Examiner tip: Every line of a reference spectrum must appear in the sample — check both lithium lines and both potassium lines, not just one.

9Grade 8–9HigherTriple only5 marks

A student uses flame emission spectroscopy to find the concentration of sodium ions in a bottle of mineral water.

She first measures the emission intensity for standard solutions of sodium ions.

Concentration of sodium ions in mg/dm³	0	2	4	6	8	10
Emission intensity (arbitrary units)	0.0	14.8	30.2	44.9	60.1	75.0

She then dilutes 10.0 cm³ of the mineral water to a total volume of 100.0 cm³ with distilled water. The diluted sample has an emission intensity of 33.0

(a) Determine the concentration of sodium ions in the diluted sample. [2]

(b) Calculate the concentration of sodium ions in the undiluted mineral water. [1]

(d) Suggest why the student diluted the mineral water before measuring it. [1]

(a) uses the linear / proportional relationship, e.g. 75.0 ÷ 10 = 7.5 per mg/dm³ or plots a calibration graph and reads off at intensity 33.0(1)
(a) 4.4 (mg/dm³) allow 4.3–4.5(1)
(b) 44 (mg/dm³) allow ecf: their (a) × 10; allow 43–45(1)
(c) 22 (mg) allow ecf: their (b) × 0.5 (correct conversion of 500 cm³ to 0.5 dm³)(1)
(d) the undiluted water would give an intensity / concentration above the range of the standard solutions (above 75 / 10 mg/dm³) allow so the reading is within the calibration range; ignore ‘to make it more accurate’ unqualified(1)
Answer: (a) 4.4 mg/dm³ (b) 44 mg/dm³ (c) 22 mg (d) undiluted reading (≈330) would be outside the range of the standards
Examiner tip: After using the calibration you must multiply back by the dilution factor (here ×10) — many students stop at the diluted value.

10Grade 1–3Foundation6 marks

A student adds magnesium ribbon to dilute hydrochloric acid in a test tube. Bubbles of a gas are given off.

(a) Name the gas. Describe the test for this gas and give the result. [3]

(b) In a different experiment, another student collects a colourless gas. She puts a glowing splint into the gas and the splint goes out. She says: “This proves the gas is carbon dioxide.”

(i) Explain why the student’s conclusion is not valid. [1]

(ii) Describe the test the student should use to show the gas is carbon dioxide. Give the result for carbon dioxide. [2]

(a) hydrogen allow H₂; do not accept H(1)
(a) (hold a) lighted / burning splint (at the mouth of the test tube) do not accept glowing splint(1)
(a) (burns with a) squeaky pop allow pop / squeaky pop sound; ignore ‘explodes’ on its own; result mark is dependent on a lighted / burning splint(1)
(b)(i) other gases would also put out the (glowing) splint / do not relight a glowing splint allow a named unreactive gas such as nitrogen / argon that would also do this; allow the test only shows the gas is not oxygen; ignore hydrogen as the named gas (a glowing splint may ignite it)(1)
(b)(ii) bubble / pass the gas through limewater allow calcium hydroxide solution; allow shake the gas with limewater / add limewater to the gas(1)
(b)(ii) limewater turns milky / cloudy allow white precipitate forms; do not accept ‘turns white’ on its own; result mark is dependent on limewater(1)
Answer: (a) hydrogen; lighted splint gives a squeaky pop (b)(i) other gases (e.g. nitrogen) also put out a glowing splint (ii) bubble through limewater, which turns milky / cloudy
Examiner tip: Lighted splint for hydrogen, glowing splint for oxygen — mixing them up loses both the method and the result mark.

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