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GCSE · Chemistry

C8 Chemical Analysis

AQA 8461/8462/8463 · Calculator allowed · about 95 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

Chemists distinguish between pure substances, mixtures and formulations.

(a)State what is meant by a 'pure substance' in chemistry.(1)

(b)A student says: 'Orange juice from a carton is a pure substance, because the label says it is 100% juice.' Explain why the student is incorrect.(2)

(c)State what is meant by a 'formulation'.(1)

(Total for Question 1 is 4 marks)

A technician wants to check whether a white solid is a pure sample of sulfur (melting point 115 degrees C).

(a)Describe how the technician could use melting point to test whether the solid is pure sulfur.(3)

(b)The solid started melting at 109 degrees C and finished melting at 118 degrees C. State, with a reason, whether the solid is pure sulfur.(2)

(Total for Question 2 is 5 marks)

A student carries out paper chromatography to investigate a food colouring, dye X, as part of a required practical.

(a)Give one reason why a pencil, rather than a pen, is used to draw the baseline (origin line) on the chromatography paper.(1)

(b)In the student's experiment, the solvent front travelled 8.4 cm from the baseline, and the single spot from dye X travelled 5.6 cm. Calculate the Rf value of dye X. Give your answer to 2 significant figures.(3)

(c)Dye X produced only one spot on the chromatogram. Explain what this shows about dye X, and how the chromatogram would look different if dye X were actually a mixture of two coloured compounds.(2)

(Total for Question 3 is 6 marks)

The student compares dye X to three reference food dyes, A, B and C, run on the same chromatogram using the same solvent. Figure 1 shows the Rf value of each reference dye.

(a)Use the Rf value you calculated for dye X in Question 3(b) and the data in Figure 1 to identify which reference dye, A, B or C, matches dye X.(1)

(b)Suggest one reason why matching Rf values in a single solvent is not, on its own, complete proof that dye X and the matching reference dye are the same compound.(2)

(Total for Question 4 is 3 marks)

Chemists use simple tests 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)

(Total for Question 5 is 4 marks)

Two further gases commonly tested for in the laboratory are carbon dioxide and chlorine.

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

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

(Total for Question 6 is 4 marks)

A technician uses flame tests as part of a required practical to identify unknown metal ions in four solid salts, labelled P, Q, R and S.

(a)Describe how the technician should safely carry out a flame test on a sample of one of the solid metal salts.(3)

(b)Table 1 shows the flame colour observed for each salt: P gave an orange-red flame, Q gave a lilac flame, R gave a yellow flame, and S gave a blue-green flame. Identify the metal ion present in each of salts P, Q, R and S.(4)

(Total for Question 7 is 7 marks)

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

(a)State the colour of the precipitate formed when sodium hydroxide solution is added to a solution containing copper(II) ions, Cu2+.(1)

(b)State the colour of the precipitate formed with iron(II) ions, Fe2+, and the colour formed with iron(III) ions, Fe3+.(2)

(c)Higher tier only. A solution contains either aluminium ions, Al3+, or calcium ions, Ca2+. Both ions form a white precipitate when a small volume of sodium hydroxide solution is first added. Describe a further test, using sodium hydroxide solution, that would distinguish between the two ions, including what would be observed for each.(3)

(Total for Question 8 is 6 marks)

A student wants to test whether a white solid is a metal carbonate.

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

(b)State what would be observed if the gas produced is bubbled through limewater and carbonate ions are present.(1)

(Total for Question 9 is 4 marks)

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

(a)Describe the method used to test for halide ions in solution, including the reagents that should be added.(3)

(b)State the colour of the precipitate formed for chloride ions, for bromide ions and for iodide ions.(3)

(Total for Question 10 is 6 marks)

A student wants to confirm whether a solution contains sulfate ions.

(Total for Question 11 is 4 marks)

Higher tier only. Flame emission spectroscopy is an instrumental method used to analyse metal ions in solution.

(a)Name the instrumental method used to identify metal ions in solution and to determine their concentration.(1)

(b)Give two advantages of using this instrumental method, rather than flame tests observed by eye, to identify metal ions.(2)

(Total for Question 12 is 3 marks)

A laboratory analyses a sample of impure copper sulfate crystals, obtained from an industrial process, with a total mass of 12.5 g. Chemical analysis shows the sample contains 10.75 g of pure hydrated copper sulfate, the rest being an insoluble impurity.

(a)Calculate the percentage purity of the sample of copper sulfate crystals. Give your answer to 3 significant figures.(3)

(b)Suggest one reason why a formulation, such as a fertiliser or a medicine, might deliberately be made using less than 100% of the pure active substance.(1)

(Total for Question 13 is 4 marks)

A technician has 25.0 cm3 of a 2.00 mol/dm3 stock solution of sodium chloride, which is diluted with water to a total volume of 250 cm3 to make a reference standard for ion analysis.

(Total for Question 14 is 3 marks)

A white solid could be either sodium carbonate or sodium sulfate.

(Total for Question 15 is 3 marks)

A student is given an unknown white solid, compound Y, and told to dissolve a sample in distilled water for testing.

(Total for Question 16 is 6 marks)

A quality-control chemist tests a sports drink formulation by paper chromatography, comparing it against three reference food dyes, L, M and N, run on the same solvent. The solvent front travels 9.60 cm. The single dye spot from the drink travels 6.72 cm. Figure 2 shows the Rf value of each reference dye.

(a)Calculate the Rf value of the dye spot from the sports drink.(2)

(b)Based on this result, the chemist concludes that the sports drink contains dye L, and only dye L. Evaluate this conclusion, referring to the limitations of using a single solvent in paper chromatography.(3)

(Total for Question 17 is 5 marks)

Higher tier only. To calibrate a flame emission spectrometer, a chemist must prepare a series of standard solutions of known concentration from a 1.00 mol/dm3 stock solution of potassium chloride.

(a)Describe how the chemist could accurately prepare 100 cm3 of a 0.100 mol/dm3 standard solution of potassium chloride from the stock solution, naming the apparatus used to measure the two volumes involved.(3)

(b)Calculate the number of moles of potassium chloride present in the 100 cm3 of 0.100 mol/dm3 standard solution.(2)

(Total for Question 18 is 5 marks)

Iron(III) chloride solution reacts with sodium hydroxide solution to form the brown precipitate used to identify Fe3+ ions.

(Total for Question 19 is 3 marks)

Two students separately test an unlabelled solution, Z. Student 1 carries out a flame test on a solid sample of Z and observes a lilac flame. Student 2 adds sodium hydroxide solution, drop by drop and then in excess, to a separate sample of Z's solution and sees no precipitate form at any point. Student 1 then adds dilute nitric acid followed by silver nitrate solution to a third, fresh sample of Z's solution, and a yellow precipitate forms.

(a)Using the evidence given, identify the metal ion and the halide ion present in solution Z.(2)

(b)Explain why Student 2's sodium hydroxide test alone was not able to confirm the identity of the metal ion in Z, even though it gave a valid result.(2)

(Total for Question 20 is 4 marks)

Mark scheme · C8 Chemical Analysis

Question 1

(a) B1 a single element or compound, not mixed with any other substance oe
(a) Answer: A pure substance is a single element or compound that is not mixed with any other substance.
(b) M1 orange juice contains several different substances (e.g. water, sugars, citric acid, vitamins) mixed together oe
(b) A1 so it is a mixture, not a single element or compound, even though every component came from the orange (100% juice describes its origin, not its chemical purity) oe
(b) Answer: Orange juice contains many different substances, such as water, sugars and citric acid, mixed together, so it is a mixture rather than a single pure compound; '100% juice' describes where it came from, not its chemical purity.
(c) B1 a mixture that has been designed as a useful product, in which each component is present in a carefully measured (controlled) quantity oe
(c) Answer: A formulation is a mixture that has been designed as a useful product, with each component present in a carefully controlled amount.

Question 2

(a) B1 heat a small sample of the solid slowly (e.g. in a capillary tube using a melting point apparatus) oe
(a) B1 record the temperature at which the solid starts to melt and the temperature at which it has completely melted oe
(a) B1 compare the melting range measured to the known melting point of pure sulfur (115 degrees C); a pure substance melts sharply at a single, exact temperature oe
(a) Answer: Heat a small sample slowly using a melting point apparatus, recording the temperature at which melting starts and finishes, then compare this to the known melting point of pure sulfur (115 degrees C). A pure substance melts sharply at one exact temperature.
(b) M1 the solid is not pure sulfur oe
(b) A1 because it melts over a range of temperatures (109-118 degrees C) rather than sharply at 115 degrees C; impurities lower and widen the melting point range oe
(b) Answer: It is not pure sulfur, because it melts gradually over a range (109 to 118 degrees C) instead of sharply at exactly 115 degrees C; the presence of an impurity lowers and widens the melting point range.

Question 3

(a) B1 pencil marks are insoluble in the solvent, so they will not move up the paper and interfere with (or be mistaken for) the dye spots oe
(a) Answer: Pencil is insoluble in the solvent, so the baseline will not move or dissolve and interfere with the results, unlike ink.
(b) M1 Rf = distance travelled by spot / distance travelled by solvent
(b) M1 5.6 / 8.4
(b) A1 0.67 (2 sf) cao
(b) Answer: Rf = 0.67
(c) M1 one spot shows that dye X is a single pure compound (or that its components are chemically identical/travel at the same rate) oe
(c) A1 if dye X were a mixture of two compounds, two separate spots (at two different Rf values/heights) would be seen instead, because the two compounds would have different solubilities/attractions to the paper and solvent oe
(c) Answer: One spot indicates dye X is a single pure compound. If it were a mixture of two coloured compounds, two separate spots at different heights (different Rf values) would appear, because each compound has a different solubility and would travel a different distance up the paper.

Question 4

(a) B1 dye C (ft: award if consistent with their Rf value from 3(b))
(a) Answer: Dye C, since it also has an Rf value of 0.67.
(b) M1 two different compounds could coincidentally have the same, or a very similar, Rf value in this one solvent oe
(b) A1 so the test should be repeated using one or more different solvents; only if the Rf values match in every solvent tested can the identification be considered reliable oe
(b) Answer: Two different compounds could happen to share the same Rf value in this particular solvent, so the identification is not certain; repeating the test with a different solvent, and only accepting the match if the Rf values agree every time, would make the conclusion much more reliable.

Question 5

(a) B1 hold a lit splint at the open end of the test tube containing the gas oe
(a) B1 a positive test gives a squeaky pop (as the hydrogen ignites/burns rapidly)
(a) Answer: Hold a lit splint at the mouth of the test tube; hydrogen gives a squeaky pop as it ignites.
(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
(b) Answer: Insert a glowing splint into the test tube; oxygen relights the glowing splint.

Question 6

(a) B1 bubble the gas through limewater (calcium hydroxide solution) oe
(a) B1 a positive test turns the limewater cloudy/milky (white precipitate of calcium carbonate forms)
(a) Answer: Bubble the gas through limewater; carbon dioxide turns limewater cloudy.
(b) B1 hold a piece of damp litmus paper (or damp universal indicator paper) at the mouth of the test tube oe
(b) B1 a positive test bleaches the litmus paper, turning it white
(b) Answer: Hold damp litmus paper at the mouth of the test tube; chlorine bleaches the paper white.

Question 7

(a) B1 dip a clean (nichrome or platinum) 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, non-luminous part of a blue Bunsen flame oe
(a) B1 observe and record the colour of the flame produced; wear eye protection throughout oe
(a) Answer: Dip a clean wire loop into the solid (or a paste of the solid with a little dilute hydrochloric acid), hold it in the clear blue part of a roaring Bunsen flame, and observe the colour produced, wearing eye protection.
(b) B1 P: calcium ion (Ca2+)
(b) B1 Q: potassium ion (K+)
(b) B1 R: sodium ion (Na+)
(b) B1 S: copper ion (Cu2+)
(b) Answer: P contains calcium ions (Ca2+); Q contains potassium ions (K+); R contains sodium ions (Na+); S contains copper ions (Cu2+).

Question 8

(a) B1 blue (precipitate)
(a) Answer: Blue precipitate.
(b) B1 Fe2+: green precipitate
(b) B1 Fe3+: brown (orange-brown) precipitate
(b) Answer: Fe2+ gives a green precipitate; Fe3+ gives a brown precipitate.
(c) M1 add an excess (large excess) of sodium hydroxide solution to each sample oe
(c) A1 if the ion is Al3+, the white precipitate dissolves (redissolves) in the excess sodium hydroxide, forming a colourless solution oe
(c) A1 if the ion is Ca2+, the white precipitate remains (does not dissolve) even in excess sodium hydroxide
(c) Answer: Add excess sodium hydroxide solution: if the ion is Al3+, the white precipitate dissolves in the excess to give a colourless solution; if the ion is Ca2+, the white precipitate stays undissolved even in excess sodium hydroxide.

Question 9

(a) B1 add dilute acid (e.g. dilute hydrochloric acid) to a sample of the solid oe
(a) B1 collect any gas produced and bubble it through limewater (or test with a lit/glowing splint is not suitable; must use limewater to confirm CO2 specifically) oe
(a) B1 if carbonate ions are present, effervescence (bubbling/fizzing) occurs and the gas turns limewater cloudy, confirming carbon dioxide oe
(a) Answer: Add dilute acid to the solid; if carbonate ions are present, the solid will fizz and produce a gas. Bubble this gas through limewater: if it turns cloudy, this confirms the gas is carbon dioxide and so the solid contains carbonate ions.
(b) B1 the limewater turns cloudy/milky
(b) Answer: The limewater turns cloudy.

Question 10

(a) B1 add a few drops of dilute nitric acid to the solution oe
(a) B1 then add a few drops of silver nitrate solution oe
(a) B1 a coloured precipitate forming confirms the presence of a halide ion (the colour identifies which one)
(a) Answer: Add dilute nitric acid to the solution, then add silver nitrate solution; a coloured precipitate forming confirms a halide ion is present, and its colour identifies which one.
(b) B1 chloride: white precipitate
(b) B1 bromide: cream precipitate
(b) B1 iodide: yellow precipitate
(b) Answer: Chloride ions give a white precipitate; bromide ions give a cream precipitate; iodide ions give a yellow precipitate.

Question 11

B1 add a few drops of dilute hydrochloric acid to the solution oe
B1 then add a few drops of barium chloride solution oe
B1 if sulfate ions are present, a white precipitate forms (of barium sulfate)
B1 the dilute hydrochloric acid is added first to remove/prevent a false positive from any carbonate ions present, which would otherwise also give a precipitate with barium ions oe
Answer: Add dilute hydrochloric acid to the solution (to remove any carbonate that could give a false positive), then add barium chloride solution; a white precipitate of barium sulfate confirms sulfate ions are present.

Question 12

(a) B1 flame emission spectroscopy
(a) Answer: Flame emission spectroscopy.
(b) B1 it is more sensitive/accurate than judging a flame colour by eye, so it can detect and identify very small amounts of an ion, and can distinguish ions that give similar flame colours oe
(b) B1 it can identify the concentration of the ion present, and can analyse mixtures of metal ions in a single sample, which is not reliable using a flame test observed by eye oe
(b) Answer: It is far more sensitive and accurate than judging colour by eye, so it can identify ions present in very small amounts or distinguish ions with similar flame colours; it can also measure the concentration of an ion and analyse mixtures of metal ions in one sample, which a flame test viewed by eye cannot reliably do.

Question 13

(a) M1 percentage purity = (mass of pure substance / total mass of sample) x 100
(a) M1 (10.75 / 12.5) x 100
(a) A1 86.0% cao
(a) Answer: 86.0%
(b) B1 the pure active substance may be too concentrated/strong to use safely or effectively on its own, so it is diluted with a filler/carrier substance to give the correct, safer dose or strength oe
(b) Answer: The pure active substance may be too concentrated to use safely, so it is mixed with a filler or carrier to give a controlled, safer dose or strength.

Question 14

M1 concentration1 x volume1 = concentration2 x volume2 stated or used oe
M1 2.00 x 25.0 = C2 x 250, so C2 = (2.00 x 25.0) / 250
A1 0.200 mol/dm3 cao
Answer: 0.200 mol/dm3

Question 15

M1 add dilute hydrochloric acid to a sample of the solid oe
A1 if the solid is sodium carbonate, effervescence (fizzing) occurs and a gas is produced that turns limewater cloudy (carbon dioxide) oe
A1 if the solid is sodium sulfate, no fizzing/gas is produced (sulfate does not react with dilute acid in this way) oe
Answer: Add dilute hydrochloric acid to a sample of the solid: sodium carbonate fizzes, producing carbon dioxide gas that turns limewater cloudy, whereas sodium sulfate produces no fizzing or gas.

Question 16

Level 1 (1-2): Simple, isolated tests are named with little or no detail of the reagents used or the results expected. The answer does not clearly link tests to the identification of both a cation and an anion.
Level 2 (3-4): A logical sequence of at least two suitable tests is described, with the reagents used and the expected positive results given for most tests. Some linkage is shown between the tests and identifying either the cation or the anion, but coverage of both ions is incomplete.
Level 3 (5-6): A complete, logically ordered sequence of tests is described, covering both the metal cation and the negative ion, with correct reagents and the specific results (including relevant colours) that would confirm the identity of each ion.
Indicative content:
- Carry out a flame test on a separate solid sample: dip a clean wire loop in the solid (or a paste with dilute hydrochloric acid) and hold it in a blue Bunsen flame; the flame colour identifies the metal ion present (e.g. lilac for potassium, yellow for sodium, orange-red for calcium, blue-green for copper).
- Alternatively, or in addition, add a few drops of sodium hydroxide solution to the solution of compound Y; a coloured precipitate identifies certain metal ions (e.g. blue for Cu2+, green for Fe2+, brown for Fe3+, white for Ca2+/Mg2+/Al3+).
- To test for carbonate ions, add dilute acid to a fresh sample: effervescence and a gas that turns limewater cloudy confirms carbonate ions are present.
- To test for sulfate ions, add dilute hydrochloric acid followed by barium chloride solution to a fresh sample of the solution; a white precipitate confirms sulfate ions.
- To test for halide ions, add dilute nitric acid followed by silver nitrate solution to a fresh sample; a white, cream or yellow precipitate identifies chloride, bromide or iodide ions respectively.
- The tests should be carried out on separate small samples of the solution so that reagents from one test do not interfere with (contaminate) another; a systematic sequence (cation test, then testing for each possible anion) avoids missing an ion or getting a false result.

Question 17

(a) M1 6.72 / 9.60
(a) A1 0.70 cao
(a) Answer: 0.70
(b) M1 the calculated Rf value (0.70) does match dye L exactly, so this is reasonable initial evidence that dye L is present oe
(b) M1 however, the conclusion is not fully certain from one solvent alone, because a different dye (not among L, M, N) could coincidentally share the same Rf value of 0.70 in this solvent oe
(b) A1 also, only one spot was seen, which shows the drink's dye component is a single (pure) substance, but repeating the chromatography with a second, different solvent would be needed to confirm the match with dye L reliably oe
(b) Answer: The Rf value matching dye L exactly (0.70) is good initial evidence, but it is not fully conclusive on its own, since a different dye not in the reference table could coincidentally give the same Rf value in this one solvent; the single spot does show the drink contains only one dye, but the identification would only become reliable if the match were confirmed by repeating the chromatography using a second, different solvent.

Question 18

(a) B1 use a (volumetric) pipette to accurately measure 10.0 cm3 of the 1.00 mol/dm3 stock solution into a volumetric flask oe
(a) B1 add distilled water to make the total volume up to the 100 cm3 graduation mark on the volumetric flask oe
(a) B1 mix (invert/swirl) the flask thoroughly to give an evenly diluted solution
(a) Answer: Use a volumetric pipette to measure exactly 10.0 cm3 of the stock solution into a 100 cm3 volumetric flask, then add distilled water carefully up to the graduation mark, and mix thoroughly.
(b) M1 moles = concentration x volume (in dm3); 0.100 x (100/1000)
(b) A1 0.0100 mol (or 1.00 x 10^-2 mol) cao
(b) Answer: 0.0100 mol

Question 19

M1 correct formulae for all four species: FeCl3, NaOH, Fe(OH)3, NaCl
M1 correctly balanced: FeCl3 + 3NaOH -> Fe(OH)3 + 3NaCl
A1 correct state symbols throughout: FeCl3(aq) + 3NaOH(aq) -> Fe(OH)3(s) + 3NaCl(aq)
Answer: FeCl3(aq) + 3NaOH(aq) -> Fe(OH)3(s) + 3NaCl(aq)

Question 20

(a) B1 potassium ion (K+), shown by the lilac flame
(a) B1 iodide ion (I-), shown by the yellow precipitate with silver nitrate
(a) Answer: The metal ion is potassium (K+), and the halide ion is iodide (I-).
(b) M1 group 1 metal ions, such as potassium (and sodium and lithium), do not form a precipitate with sodium hydroxide solution oe
(b) A1 so seeing no precipitate only rules out the metal ions that do form one (e.g. Cu2+, Fe2+, Fe3+, Ca2+, Mg2+, Al3+); it cannot, by itself, distinguish between the different group 1 metal ions, which is why the flame test was still needed oe
(b) Answer: Group 1 metal ions, including potassium, do not form a precipitate with sodium hydroxide, so getting no precipitate only rules out ions such as Cu2+, Fe2+, Fe3+, Ca2+, Mg2+ and Al3+, which do form one; it cannot distinguish between different group 1 ions, so the flame test was still needed to identify potassium specifically.

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