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GCSE · Chemistry

C8a Tests for Ions, Gases and Flame Tests

AQA 8464 · Calculator allowed · about 130 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

How Do Chemists Identify an Unknown Substance?

Original text written for Revision Library.

When a laboratory receives an unlabelled solid or solution, one of the first things a chemist does is run it through a set of standard qualitative tests, quick, simple procedures that turn an invisible identity into a visible clue. A wire dipped into a solid and held in a Bunsen flame might burn lilac, revealing potassium ions; a few drops of sodium hydroxide solution might turn cloudy white or vivid blue, exposing which metal ion is present, and whether it is the unusual case of aluminium, whose precipitate quietly dissolves again if enough alkali is added. Bubbles of gas released by an acid can be captured and tested: a squeaky pop for hydrogen, a relit splint for oxygen, a milky white cloud in limewater for carbon dioxide. Even the anions locked inside a compound leave their own fingerprints, a cream precipitate with silver nitrate for bromide, a white one with barium chloride for sulfate. None of these tests needs an expensive instrument; together they let a chemist build up, one observation at a time, a confident picture of exactly which ions an unknown sample contains. This topic covers flame tests for metal ions, metal hydroxide precipitate reactions, tests for carbonate, halide and sulfate ions, and tests for the gases hydrogen, oxygen, carbon dioxide and chlorine, including the required practical for identifying ions in unknown ionic compounds.

A flame test can be used to identify some metal ions from the colour they produce in a Bunsen flame.

(a)State the flame colour produced by lithium ions, Li+.(1)

(b)State the flame colour produced by sodium ions, Na+.(1)

(c)State the flame colour produced by potassium ions, K+.(1)

(d)State the flame colour produced by calcium ions, Ca2+.(1)

(Total for Question 1 is 4 marks)

Simple chemical tests can be used to identify common gases produced in reactions.

(a)Describe the test for hydrogen gas, and state the result of a positive test.(2)

(b)Describe the test for oxygen gas, and state the result of a positive test.(2)

(c)Describe the test for carbon dioxide gas, and state the result of a positive test.(2)

(d)Describe the test for chlorine gas, and state the result of a positive test.(2)

(Total for Question 2 is 8 marks)

Flame tests are a required practical technique used to identify metal ions from the colour they produce in a flame.

(a)Describe how a student should carry out a flame test on a solid ionic compound, using a nichrome wire loop.(3)

(b)Explain why the wire loop must be cleaned before it is used to test a new sample.(2)

(Total for Question 3 is 5 marks)

Adding a few drops of sodium hydroxide solution to a solution of a metal salt can produce a coloured precipitate, which can be used to identify the metal ion present.

State the colour of the precipitate formed when sodium hydroxide solution is added dropwise to separate solutions containing each of the following ions: calcium ions, Ca2+; copper(II) ions, Cu2+; iron(II) ions, Fe2+; iron(III) ions, Fe3+; and aluminium ions, Al3+.

(Total for Question 4 is 5 marks)

A sample contains a mixture of two different metal salts.

Suggest why a flame test on this mixture might not clearly show the flame colours of both metal ions present.

(Total for Question 5 is 2 marks)

A student reacts small samples of a solid with dilute acid and wants to identify the gases produced.

(a)The student adds dilute hydrochloric acid to a sample of magnesium ribbon and collects the gas produced in a test tube. Describe how the student could test this gas, and state the result that would show the gas is hydrogen.(2)

(b)The student also bubbles a separate sample of gas, produced when a different white solid reacts with dilute acid, through limewater, and the limewater turns cloudy. Identify the gas, and explain what this confirms about the white solid.(2)

(Total for Question 6 is 4 marks)

A student is given an unknown white solid and wants to test whether it is a metal carbonate.

(a)Describe a test the student could carry out, and state the result that would confirm the presence of carbonate ions.(3)

(b)The solid is calcium carbonate. Write a balanced symbol equation, including state symbols, for its reaction with dilute hydrochloric acid.(2)

(c)Write a balanced symbol equation, including state symbols, for the reaction that causes limewater to turn cloudy.(2)

(Total for Question 7 is 7 marks)

A technician wants to identify which halide ion, if any, is present in an unknown solution.

(a)Describe the test the technician should carry out to test for halide ions, including the reagents needed.(3)

(b)State the colour of the precipitate formed with silver nitrate solution for each of the following halide ions: chloride, Cl-; bromide, Br-; iodide, I-.(3)

(c)Write an ionic equation, including state symbols, for the reaction between silver ions and iodide ions.(2)

(Total for Question 8 is 8 marks)

A student is testing a solution to see whether it contains sulfate ions.

(a)Describe the test for sulfate ions, including the reagents used and a positive result.(3)

(b)Explain why dilute hydrochloric acid, rather than dilute nitric acid, is used to acidify the sample in this test.(2)

(Total for Question 9 is 5 marks)

Sodium hydroxide solution is added to solutions of two soluble salts.

(a)Write a balanced symbol equation, including state symbols, for the reaction between copper(II) sulfate solution and sodium hydroxide solution.(2)

(b)Write a balanced symbol equation, including state symbols, for the reaction between iron(III) chloride solution and sodium hydroxide solution.(3)

(Total for Question 10 is 5 marks)

Silver nitrate solution and barium chloride solution are used as test reagents for different anions.

(a)Write an ionic equation, including state symbols, for the reaction between silver ions and chloride ions.(2)

(b)Write an ionic equation, including state symbols, for the reaction between barium ions and sulfate ions.(2)

(Total for Question 11 is 4 marks)

Iron forms two common ions, Fe2+ and Fe3+, which can be distinguished using sodium hydroxide solution.

(a)State the colour of the precipitate formed when sodium hydroxide solution is added to a solution containing Fe2+ ions, and to a solution containing Fe3+ ions.(2)

(b)A student adds sodium hydroxide solution to a solution of iron(II) sulfate and leaves the test tube open to the air. After several minutes, the outer edges of the green precipitate have turned brown. Suggest an explanation for this observation.(2)

(Total for Question 12 is 4 marks)

Sodium hydroxide solution is added dropwise to two separate solutions, one containing calcium ions, Ca2+, and one containing aluminium ions, Al3+.

(a)State what would be observed if excess sodium hydroxide solution is added to the solution containing calcium ions.(1)

(b)Write a balanced ionic equation, including state symbols, for the reaction between aluminium ions and hydroxide ions to form the white precipitate.(2)

(Total for Question 13 is 3 marks)

A technician suspects that a 25.0 cm³ sample of solution contains sulfate ions. Dilute hydrochloric acid is added, followed by excess barium chloride solution. A white precipitate of barium sulfate forms; it is filtered, washed, dried and weighed, giving a mass of 0.466 g. (Relative formula mass, Mr, of BaSO4 = 233)

(a)Explain why dilute hydrochloric acid is added to the sample before the barium chloride solution.(2)

(b)Calculate the number of moles of barium sulfate precipitate formed.(2)

(c)State the number of moles of sulfate ions, SO4²-, that were present in the original 25.0 cm³ sample, and use this to calculate the concentration of sulfate ions in the original solution, in mol/dm^3.(3)

(Total for Question 14 is 7 marks)

A student reacts 50.0 cm³ of 0.100 mol/dm³ sodium chloride solution with excess silver nitrate solution. A white precipitate of silver chloride forms. (Relative formula mass, Mr, of AgCl = 143.5)

(a)Calculate the number of moles of sodium chloride used.(2)

(b)State the number of moles of silver chloride precipitate that would form, and use this to calculate its mass, to 3 significant figures.(3)

(Total for Question 15 is 5 marks)

A technician is given an unknown solid and told that it is a single, soluble ionic compound. It could contain one of the cations calcium, copper(II), iron(II), iron(III) or aluminium, and one of the anions carbonate, chloride, bromide, iodide or sulfate.

Plan a series of tests the technician could carry out on separate samples of the compound to identify both the cation and the anion present. In your answer you should refer to: how you would test for the cation; how you would test for the anion; and any safety precautions needed.

(Total for Question 16 is 6 marks)

An unknown white solid is tested by a student. Test 1: a small sample is used in a flame test, and a lilac flame is produced. Test 2: a separate sample is dissolved in water and sodium hydroxide solution is added; no precipitate forms. Test 3: dilute hydrochloric acid is added to a separate sample of the solid; effervescence occurs, and the gas produced turns limewater cloudy.

(a)State what Test 1 shows about the identity of the solid.(1)

(b)Explain what Test 2 shows about the identity of the solid.(2)

(c)Identify the gas produced in Test 3, and state what this confirms about the solid.(2)

(d)Using the results of all three tests, identify the unknown solid, giving both its name and chemical formula.(2)

(Total for Question 17 is 7 marks)

Higher tier only. Aluminium hydroxide is unusual among the metal hydroxides tested in this topic because it is soluble in excess sodium hydroxide solution.

(a)State what would be observed if excess sodium hydroxide solution is added to a solution containing aluminium ions, Al3+, and compare this with what would be observed for calcium ions, Ca2+.(3)

(b)Write a balanced ionic equation, including state symbols, for aluminium hydroxide dissolving in excess hydroxide ions to form the soluble ion Al(OH)4-.(2)

(c)Explain why both the dropwise addition and the excess addition of sodium hydroxide solution must be observed in order to distinguish a solution of calcium ions from a solution of aluminium ions.(2)

(Total for Question 18 is 7 marks)

A student carries out a flame test on an unknown solid and observes an orange-red flame. The student concludes the solid must contain calcium ions, Ca2+. However, the wire loop used had not been cleaned after a previous flame test on a sample of sodium chloride.

(a)Evaluate the student's conclusion.(2)

(b)Suggest how the student should repeat the test to obtain a more reliable result.(2)

(Total for Question 19 is 4 marks)

An unknown blue solution, X, is tested as follows. A flame test on a sample produces a green flame. Sodium hydroxide solution is added dropwise to a separate sample of X; a blue precipitate forms, which remains insoluble even when excess sodium hydroxide solution is added. A third sample of X is acidified with dilute hydrochloric acid, and barium chloride solution is then added; a white precipitate forms.

(a)Identify the cation present in solution X, giving a reason from the relevant tests.(2)

(b)Identify the anion present in solution X, giving a reason.(2)

(c)State the full name and formula of compound X.(2)

(d)Write ionic equations, including state symbols, for the two precipitation reactions described.(4)

(Total for Question 20 is 10 marks)

Mark scheme · C8a Tests for Ions, Gases and Flame Tests

Question 1

(a) B1 red (crimson) (oe, cao)
(a) Answer: Red (crimson).
(b) B1 yellow (oe, cao)
(b) Answer: Yellow.
(c) B1 lilac (oe, cao)
(c) Answer: Lilac.
(d) B1 orange-red (brick-red) (oe, cao)
(d) Answer: Orange-red (brick-red).

Question 2

(a) B1 hold a lit (burning) splint at the mouth of the test tube containing the gas (oe)
(a) B1 a positive test gives a squeaky pop, as the hydrogen ignites/burns rapidly (oe)
(a) Answer: Hold a lit splint at the mouth of the test tube; hydrogen gives a squeaky pop.
(b) B1 insert a glowing splint into the test tube containing the gas (oe)
(b) B1 a positive test relights (re-ignites) the glowing splint (oe, cao)
(b) Answer: Insert a glowing splint into the test tube; oxygen relights the glowing splint.
(c) B1 bubble the gas through (or shake with) limewater (calcium hydroxide solution) (oe)
(c) B1 a positive test turns the limewater cloudy/milky (oe, cao)
(c) Answer: Bubble the gas through limewater; carbon dioxide turns limewater cloudy.
(d) B1 hold a piece of damp litmus paper in the gas (oe)
(d) B1 a positive test bleaches the litmus paper, turning it white (oe, cao)
(d) Answer: Hold damp litmus paper in the gas; chlorine bleaches it white.

Question 3

(a) B1 dip the (nichrome) wire loop into dilute hydrochloric acid, then rinse it in distilled water, to clean it (oe)
(a) B1 dip the clean wire loop into the solid sample, or into a paste made with the solid and a few drops of dilute hydrochloric acid, so a small amount sticks to the loop (oe)
(a) B1 hold the wire loop in the clear, roaring (blue) part of a Bunsen flame and observe/record the colour produced; wear eye protection throughout (oe)
(a) Answer: Clean the wire loop with dilute hydrochloric acid and water, dip it into the sample, then hold it in a blue Bunsen flame and observe the colour, wearing eye protection.
(b) B1 to remove any of the previous sample remaining on the wire (oe)
(b) B1 otherwise contamination could produce or mask a flame colour that does not belong to the new sample, causing a misleading result (oe)
(b) Answer: Cleaning removes traces of the previous sample, which could otherwise contaminate the result and produce or mask a misleading flame colour.

Question 4

B1 calcium ions, Ca2+: white precipitate
B1 copper(II) ions, Cu2+: blue precipitate
B1 iron(II) ions, Fe2+: green precipitate
B1 iron(III) ions, Fe3+: brown (orange-brown) precipitate
B1 aluminium ions, Al3+: white precipitate
Answer: Ca2+: white; Cu2+: blue; Fe2+: green; Fe3+: brown; Al3+: white.

Question 5

B1 one flame colour may be much stronger/more intense than the other (oe)
B1 so it can mask/hide the weaker colour, making it difficult to see or identify both ions present (oe)
Answer: A stronger flame colour from one ion can mask the weaker colour of the other, making both ions difficult to identify.

Question 6

(a) B1 hold a lit (burning) splint at the mouth of the test tube (oe)
(a) B1 a squeaky pop is heard, confirming the gas is hydrogen (oe, cao)
(a) Answer: Hold a lit splint at the mouth of the test tube; a squeaky pop confirms hydrogen.
(b) B1 carbon dioxide (cao)
(b) B1 this confirms the white solid contains carbonate ions, since carbon dioxide is the characteristic gas produced when a carbonate reacts with an acid (oe)
(b) Answer: Carbon dioxide; this confirms the solid contains carbonate ions.

Question 7

(a) B1 add some dilute acid (e.g. dilute hydrochloric acid) to a sample of the solid (oe)
(a) B1 if carbonate ions are present, effervescence/bubbling occurs and a gas is produced; pass this gas through limewater (oe)
(a) B1 the limewater turns cloudy/milky, confirming the gas is carbon dioxide and the solid contains carbonate ions (oe)
(a) Answer: Add dilute acid to the solid; effervescence produces a gas that turns limewater cloudy, confirming carbonate ions.
(b) M1 correct formulae and balancing: CaCO3 + 2HCl -> CaCl2 + H2O + CO2
(b) A1 correct state symbols throughout: CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + H2O(l) + CO2(g) (cao)
(b) Answer: CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + H2O(l) + CO2(g)
(c) M1 correct formulae and balancing: CO2 + Ca(OH)2 -> CaCO3 + H2O
(c) A1 correct state symbols throughout: CO2(g) + Ca(OH)2(aq) -> CaCO3(s) + H2O(l) (cao)
(c) Answer: CO2(g) + Ca(OH)2(aq) -> CaCO3(s) + H2O(l)

Question 8

(a) B1 acidify the sample with a few drops of dilute nitric acid (oe)
(a) B1 then add silver nitrate solution (oe)
(a) B1 observe whether a precipitate forms, and note its colour (oe)
(a) Answer: Acidify with dilute nitric acid, then add silver nitrate solution and observe the colour of any precipitate.
(b) B1 chloride, Cl-: white precipitate
(b) B1 bromide, Br-: cream precipitate
(b) B1 iodide, I-: yellow precipitate
(b) Answer: Cl-: white; Br-: cream; I-: yellow.
(c) M1 correct formulae and balance: Ag+ + I- -> AgI
(c) A1 correct state symbols: Ag+(aq) + I-(aq) -> AgI(s) (cao)
(c) Answer: Ag+(aq) + I-(aq) -> AgI(s)

Question 9

(a) B1 acidify the sample with dilute hydrochloric acid (oe)
(a) B1 then add barium chloride solution (or barium nitrate solution) (oe)
(a) B1 a white precipitate forms if sulfate ions are present (oe, cao)
(a) Answer: Acidify with dilute hydrochloric acid, then add barium chloride solution; a white precipitate confirms sulfate ions.
(b) B1 hydrochloric acid does not introduce any ions that would interfere with (give a false positive for) the sulfate test (oe)
(b) B1 because chloride ions do not form a precipitate with barium ions, so the acid used to remove any carbonate present does not affect the reliability of the test (oe)
(b) Answer: Hydrochloric acid does not introduce ions that would give a false positive with barium ions, since chloride does not precipitate with barium.

Question 10

(a) M1 correct formulae and balance: CuSO4 + 2NaOH -> Cu(OH)2 + Na2SO4
(a) A1 correct state symbols: CuSO4(aq) + 2NaOH(aq) -> Cu(OH)2(s) + Na2SO4(aq) (cao)
(a) Answer: CuSO4(aq) + 2NaOH(aq) -> Cu(OH)2(s) + Na2SO4(aq)
(b) M1 correct formulae: FeCl3 + NaOH -> Fe(OH)3 + NaCl
(b) M1 correctly balanced: FeCl3 + 3NaOH -> Fe(OH)3 + 3NaCl
(b) A1 correct state symbols throughout: FeCl3(aq) + 3NaOH(aq) -> Fe(OH)3(s) + 3NaCl(aq) (cao)
(b) Answer: FeCl3(aq) + 3NaOH(aq) -> Fe(OH)3(s) + 3NaCl(aq)

Question 11

(a) M1 correct formulae and balance: Ag+ + Cl- -> AgCl
(a) A1 correct state symbols: Ag+(aq) + Cl-(aq) -> AgCl(s) (cao)
(a) Answer: Ag+(aq) + Cl-(aq) -> AgCl(s)
(b) M1 correct formulae and balance: Ba2+ + SO4²- -> BaSO4
(b) A1 correct state symbols: Ba2+(aq) + SO4²-(aq) -> BaSO4(s) (cao)
(b) Answer: Ba2+(aq) + SO4²-(aq) -> BaSO4(s)

Question 12

(a) B1 Fe2+: green precipitate
(a) B1 Fe3+: brown precipitate
(a) Answer: Fe2+: green precipitate. Fe3+: brown precipitate.
(b) B1 the iron(II) hydroxide precipitate has been oxidised by oxygen in the air (oe)
(b) B1 forming iron(III) hydroxide, which is brown, at the surface/edges where it is in contact with the air (oe)
(b) Answer: The iron(II) hydroxide has been oxidised by oxygen in the air to iron(III) hydroxide, which is brown.

Question 13

(a) B1 the white precipitate remains (does not dissolve/redissolve) (oe, cao)
(a) Answer: The white precipitate remains and does not dissolve.
(b) M1 correct formulae and balance: Al3+ + 3OH- -> Al(OH)3
(b) A1 correct state symbols: Al3+(aq) + 3OH-(aq) -> Al(OH)3(s) (cao)
(b) Answer: Al3+(aq) + 3OH-(aq) -> Al(OH)3(s)

Question 14

(a) B1 to remove/react with any carbonate ions present (oe)
(a) B1 which would otherwise also form a white precipitate (barium carbonate) with barium chloride, giving a false positive result for sulfate (oe)
(a) Answer: It removes any carbonate ions present, which would otherwise also give a white precipitate with barium chloride, causing a false positive.
(b) M1 moles = mass / Mr = 0.466 / 233
(b) A1 0.00200 mol (2.00 x 10^-3 mol) (cao)
(b) Answer: 0.00200 mol
(c) M1 moles of SO4²- = moles of BaSO4 = 0.00200 mol (1:1 ratio) (ecf from b)
(c) M1 concentration = moles / volume in dm³ = 0.00200 / 0.0250
(c) A1 0.0800 mol/dm³ (awrt 0.08, ecf)
(c) Answer: 0.00200 mol of sulfate ions; concentration = 0.0800 mol/dm³

Question 15

(a) M1 moles = concentration x volume (dm³) = 0.100 x (50.0/1000)
(a) A1 0.00500 mol (cao)
(a) Answer: 0.00500 mol
(b) M1 moles of AgCl = moles of NaCl = 0.00500 mol (1:1 ratio) (ecf from a)
(b) M1 mass = moles x Mr = 0.00500 x 143.5
(b) A1 0.718 g (awrt 0.718, ecf)
(b) Answer: 0.00500 mol of AgCl; mass = 0.718 g

Question 16

Level 3 (5-6): A logically sequenced plan is described that would identify both the cation and the anion. The answer includes correctly separated samples, the correct reagent for each test, the observations that distinguish each possible ion, and reference to safety precautions.
Level 2 (3-4): A plan is described with some steps in a logical order, covering tests for either the cation or the anion (or both, with gaps). Some correct reagents and observations are given, but the plan is not fully complete or has minor omissions.
Level 1 (1-2): Simple statements are made about one or two tests, with little or no logical structure and little reference to how the results would be interpreted.
Level 0 (0): No relevant content.
Indicative content:
- Use separate samples of the compound for each test, so the results do not interfere with each other.
- Flame test: use a clean nichrome wire loop (cleaned in dilute hydrochloric acid), dip it in a sample of the solid, hold it in a blue Bunsen flame and observe the colour (e.g. green indicates copper).
- Metal hydroxide test: dissolve a sample in water, add sodium hydroxide solution dropwise and note the colour of any precipitate (white for calcium or aluminium, blue for copper, green for iron(II), brown for iron(III)).
- Add sodium hydroxide solution in excess to the same sample: only the aluminium hydroxide precipitate dissolves, which distinguishes aluminium from calcium (both give a white precipitate).
- Test for carbonate: add dilute acid to a separate sample and pass any gas produced through limewater; the limewater turns cloudy if carbon dioxide (and therefore carbonate) is present.
- Test for halide: acidify a separate sample with dilute nitric acid, then add silver nitrate solution, and note the precipitate colour (white for chloride, cream for bromide, yellow for iodide).
- Test for sulfate: acidify a separate sample with dilute hydrochloric acid, then add barium chloride solution; a white precipitate confirms sulfate.
- Combine the results of all tests to identify both the cation and the anion, and hence the full compound.
- Safety: wear eye protection throughout, since the reagents include acids and irritant/corrosive solutions.

Question 17

(a) B1 it contains potassium ions, K+ (cao)
(a) Answer: It contains potassium ions, K+.
(b) B1 no precipitate forms with sodium hydroxide solution because potassium hydroxide is soluble, so this is consistent with the metal ion being potassium (as shown by Test 1) (oe)
(b) B1 it rules out calcium, copper(II), iron(II), iron(III) and aluminium ions, since all of these would have given a coloured or white precipitate with sodium hydroxide solution (oe)
(b) Answer: No precipitate is consistent with a soluble Group 1 hydroxide (potassium), and rules out calcium, copper, iron(II), iron(III) and aluminium.
(c) B1 carbon dioxide (cao)
(c) B1 this confirms the solid contains carbonate ions, CO3²- (oe)
(c) Answer: Carbon dioxide; this confirms the solid contains carbonate ions.
(d) B1 potassium carbonate (ecf from a and c)
(d) B1 K2CO3 (cao, ecf)
(d) Answer: Potassium carbonate, K2CO3.

Question 18

(a) B1 the initial white precipitate of aluminium hydroxide dissolves in excess sodium hydroxide solution (oe)
(a) B1 forming a colourless solution (oe, cao)
(a) B1 this differs from calcium ions, where the white precipitate of calcium hydroxide remains/does not dissolve, even in excess sodium hydroxide solution (oe)
(a) Answer: The aluminium hydroxide precipitate dissolves in excess NaOH to give a colourless solution; the calcium hydroxide precipitate does not dissolve.
(b) M1 correct formulae and balance: Al(OH)3 + OH- -> Al(OH)4-
(b) A1 correct state symbols: Al(OH)3(s) + OH-(aq) -> Al(OH)4-(aq) (cao)
(b) Answer: Al(OH)3(s) + OH-(aq) -> Al(OH)4-(aq)
(c) B1 both ions form a white precipitate when sodium hydroxide is first added dropwise, so this step alone cannot distinguish between them (oe)
(c) B1 only when excess sodium hydroxide is added does a difference appear, since only the aluminium hydroxide precipitate dissolves, so both stages must be observed to reliably distinguish the two ions (oe)
(c) Answer: Both give a white precipitate dropwise, so only observing the excess stage (where only aluminium hydroxide dissolves) allows the two ions to be told apart.

Question 19

(a) B1 the conclusion may not be valid/reliable, because the wire was not cleaned properly and so may have been contaminated with residual sodium ions from the previous sample (oe)
(a) B1 sodium ions produce a strong yellow flame that could mask or combine with a weaker flame colour, so the orange-red colour observed might not reliably indicate calcium ions alone (oe)
(a) Answer: The conclusion may be unreliable, since contamination from sodium ions could mask or combine with the true flame colour.
(b) B1 clean the wire loop thoroughly (e.g. by dipping it in dilute hydrochloric acid then rinsing in distilled water) before testing the sample again (oe)
(b) B1 repeat the test with the freshly cleaned wire loop to check the result is reproducible/consistent (oe)
(b) Answer: Clean the wire loop thoroughly before repeating the test, and check the result is reproducible.

Question 20

(a) B1 copper(II) ions, Cu2+ (cao)
(a) B1 because the flame test is green and the precipitate with sodium hydroxide is blue and insoluble in excess, both characteristic of copper(II) ions (oe)
(a) Answer: Copper(II) ions, Cu2+, shown by the green flame and the blue, insoluble precipitate with NaOH.
(b) B1 sulfate ions, SO4²- (cao)
(b) B1 because a white precipitate formed when the solution was acidified with dilute hydrochloric acid and barium chloride solution was added, the characteristic positive test for sulfate ions (oe)
(b) Answer: Sulfate ions, SO4²-, shown by the white precipitate with acidified barium chloride solution.
(c) B1 copper(II) sulfate (ecf from a and b)
(c) B1 CuSO4 (cao, ecf)
(c) Answer: Copper(II) sulfate, CuSO4.
(d) M1 correct formulae and balance for the hydroxide precipitate: Cu2+ + 2OH- -> Cu(OH)2
(d) A1 correct state symbols: Cu2+(aq) + 2OH-(aq) -> Cu(OH)2(s) (cao)
(d) M1 correct formulae and balance for the sulfate precipitate: Ba2+ + SO4²- -> BaSO4
(d) A1 correct state symbols: Ba2+(aq) + SO4²-(aq) -> BaSO4(s) (cao)
(d) Answer: Cu2+(aq) + 2OH-(aq) -> Cu(OH)2(s), and Ba2+(aq) + SO4²-(aq) -> BaSO4(s)

Tests for Ions, Gases and Flame Tests - Worksheets, Questions and Revision