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GCSE · Chemistry

C8b Chromatography and Instrumental Analysis

AQA 8464 · Calculator allowed · about 80 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

Separating and Identifying Substances

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Chemists often need to check how pure a substance is, or work out what is in a mixture. Paper chromatography separates the different substances in a mixture, because they move through the paper at different rates, carried by a solvent. Comparing the distance a spot travels with the distance travelled by the solvent gives an Rf value, which can be compared with reference substances to identify the components of a mixture, such as a food colouring or an ink. Modern laboratories also use instrumental methods of analysis. These use machines, rather than test tubes, and give results that are faster, more sensitive and more accurate than traditional chemical tests, especially when only a very small sample is available. Flame emission spectroscopy is one such instrumental method: it identifies which metal ions are present in a sample, and how concentrated they are, by analysing the light given out when the sample is placed in a flame.

Chromatography and instrumental analysis rely on some key definitions.

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

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

(Total for Question 1 is 2 marks)

Paint and medicine tablets are both examples of formulations.

(a)Name one component you would expect to find in a paint formulation, and state its purpose.(2)

(b)Name one component you would expect to find in a medicine tablet, and state its purpose.(2)

(Total for Question 2 is 4 marks)

A student thinks a white solid might be a pure sample of compound X, which has a known melting point of 122 degC. She heats a sample of the solid slowly and finds that it starts to melt at 116 degC and is completely melted by 121 degC.

(a)Use the data to determine whether the solid is pure compound X. Explain your answer.(3)

(Total for Question 3 is 3 marks)

Rf values are used to identify substances separated by paper chromatography.

(a)State what is meant by the term 'Rf value'.(1)

(b)Give the equation used to calculate an Rf value.(1)

(Total for Question 4 is 2 marks)

In a paper chromatography experiment, the solvent front travelled 8.4 cm from the baseline. A spot of dye, spot A, travelled 5.6 cm from the baseline in the same time.

(a)Calculate the Rf value of spot A. Give your answer to 2 decimal places.(2)

(Total for Question 5 is 2 marks)

A packet of orange sweets contains a food colouring. A student separates the colouring using paper chromatography. The solvent front travels 9.0 cm from the baseline. Three spots are seen on the chromatogram, at distances of 2.7 cm, 6.3 cm and 8.1 cm from the baseline.
The table below shows the Rf value of four reference food dyes, run in the same solvent.

Dye	Rf value
Allura Red AC	0.30
Sunset Yellow	0.55
Tartrazine	0.70
Brilliant Blue	0.90

(a)Calculate the Rf value of each of the three spots.(3)

(b)Use the table to identify the three dyes present in the food colouring.(3)

(c)State whether the food colouring is a pure substance or a formulation. Give a reason for your answer.(2)

(Total for Question 6 is 8 marks)

Required practical. A technician wants to investigate the food dyes used in three different brands of orange squash, using paper chromatography, and to identify which dyes each brand contains by comparing them with reference samples.
Describe a method the technician could use to carry out this investigation and obtain valid, comparable results. In your answer, you should refer to how the chromatogram is set up, how it is run, and how the results would be analysed.

(Total for Question 7 is 6 marks)

Required practical. Answer the following questions about the paper chromatography practical.

(a)Explain why the baseline should be drawn in pencil rather than in ink.(1)

(b)Explain why the level of solvent in the beaker must be below the pencil baseline.(1)

(c)Explain why a lid is placed on the beaker during the experiment.(1)

(Total for Question 8 is 3 marks)

Chemists can identify substances using either simple chemical tests or instrumental methods of analysis.

(a)State three advantages of using instrumental methods of analysis, rather than chemical tests, to identify substances.(3)

(b)Give one disadvantage of using instrumental methods compared with simple chemical tests.(1)

(Total for Question 9 is 4 marks)

Higher tier only (Chemistry triple award). Flame emission spectroscopy is an instrumental method used to identify metal ions in solution.

(a)Describe how flame emission spectroscopy is used to analyse a sample.(4)

(Total for Question 10 is 4 marks)

Higher tier only (Chemistry triple award). The table shows the wavelength of the main emission line produced by four metal ions during flame emission spectroscopy.

Metal ion	Wavelength of main line (nm)
Lithium, Li+	671
Sodium, Na+	589
Potassium, K+	767
Calcium, Ca2+	422

A student analyses an unknown solution using flame emission spectroscopy. The spectrum produced has a single strong line at 589 nm.

(a)Identify the metal ion present in the unknown solution. Give a reason for your answer.(2)

(b)A second, unknown solution produces a spectrum with two lines: one at 589 nm and one at 671 nm. Suggest what this tells you about the composition of the second solution.(2)

(Total for Question 11 is 4 marks)

Higher tier only (Chemistry triple award). A calibration graph is produced for sodium ion concentration, by measuring the peak height (intensity) of the 589 nm emission line for five standard sodium solutions of known concentration.

Concentration (mg/dm³)	Peak height (arbitrary units)
2	10
4	20
6	30
8	40
10	50

The calibration graph is a straight line that passes through the origin. An unknown sample gives a peak height of 35 units.

(a)Show that the peak height is directly proportional to the sodium ion concentration.(2)

(b)Use the calibration data to calculate the concentration of sodium ions in the unknown sample.(2)

(Total for Question 12 is 4 marks)

Higher tier only (Chemistry triple award). Flame tests and flame emission spectroscopy can both be used to identify metal ions in a solution.

(a)Give two advantages of using flame emission spectroscopy, rather than a flame test, to identify the metal ions present in a solution.(2)

(Total for Question 13 is 2 marks)

A tin of white gloss paint has a mass of 5.0 kg. The table shows the formulation of the paint by percentage mass.

Component	Percentage of mass
Titanium dioxide pigment	18%
Resin (binder)	45%
Solvent	30%
Additives	7%

(a)Calculate the mass of titanium dioxide pigment in the tin.(2)

(b)Calculate the mass of solvent in the tin.(2)

(Total for Question 14 is 4 marks)

In a chromatography experiment, a dye has an Rf value of 0.75. The solvent front travelled 12.0 cm from the baseline.

(a)Calculate the distance travelled by the dye spot.(2)

(Total for Question 15 is 2 marks)

A forensic scientist analyses ink from a ransom note using paper chromatography, to compare it with ink from four suspects' pens. The solvent front travels 10.0 cm from the baseline. The table shows the distance travelled by the single spot produced by each ink sample.

Ink sample	Distance travelled (cm)
Ransom note	6.5
Suspect 1	4.0
Suspect 2	6.5
Suspect 3	8.2
Suspect 4	3.0

(a)Calculate the Rf value of the ransom note ink.(2)

(b)Identify which suspect's pen matches the ransom note ink. Justify your answer using Rf values.(2)

(c)The forensic scientist repeats the experiment using a different solvent and obtains a different set of Rf values. Explain why it is important to run all the ink samples on the same chromatogram, using the same solvent, when comparing Rf values.(2)

(Total for Question 16 is 6 marks)

A student analyses a sample of white powder using paper chromatography and obtains only one spot, in every solvent they try.

(a)Explain what this suggests about the powder, and describe one other test the student could carry out to provide further evidence.(3)

(Total for Question 17 is 3 marks)

A pharmaceutical company manufactures headache tablets. Each tablet is a formulation containing an active drug plus other components. Quality-control chemists use several techniques to check the tablets meet specification.

(a)One test measures the melting point of the active drug extracted from a tablet. State how the results of this test could show that the active drug has been contaminated.(2)

(b)A second test uses paper chromatography to check that the tablet coating dye is the single permitted dye, dye X, and is not contaminated with a banned dye, dye Y. Describe how the chemists could use chromatography, including Rf values, to check for contamination with dye Y.(3)

(c)Higher tier only (Chemistry triple award). The company also uses flame emission spectroscopy to check for trace metal contamination in the tablets, rather than simple chemical tests. Evaluate why flame emission spectroscopy is more suitable than chemical tests for this quality-control purpose.(6)

(Total for Question 18 is 11 marks)

Mark scheme · C8b Chromatography and Instrumental Analysis

Question 1

(a) B1 a substance that contains only one element or one compound, with nothing else mixed in, oe
(a) Answer: A pure substance contains only one element or one compound throughout, with no other substance mixed in.
(b) B1 a mixture that has been designed as a useful product, made by mixing components in carefully measured quantities, oe
(b) Answer: A formulation is a mixture that has been designed as a useful product, with each component present in a carefully measured quantity.

Question 2

(a) B1 component named, e.g. pigment (accept solvent, binder/resin, or additive)
(a) B1 purpose correctly linked to the named component, e.g. pigment gives the paint its colour, oe
(a) Answer: E.g. pigment - gives the paint its colour.
(b) B1 component named, e.g. the active drug (accept binder, filler, or coating)
(b) B1 purpose correctly linked to the named component, e.g. the active drug produces the medical effect, oe
(b) Answer: E.g. the active drug - produces the medical effect of the tablet.

Question 3

(a) B1 the sample melts over a range of temperatures (116 degC to 121 degC) rather than sharply at one exact temperature, oe
(a) B1 a pure substance melts sharply at one fixed temperature (its melting point); here that would be 122 degC, oe
(a) B1 so the solid is not pure compound X / it is impure, since it melts over a range and below the known melting point of the pure compound (ecf from the first two marking points)
(a) Answer: The solid is impure. It melts over a range of temperatures (116 to 121 degC), and below the known melting point of pure compound X (122 degC), instead of sharply at 122 degC.

Question 4

(a) B1 the distance travelled by a spot (solute) divided by the distance travelled by the solvent, measured from the same baseline, oe
(a) Answer: The Rf value is the distance travelled by a spot divided by the distance travelled by the solvent, both measured from the same baseline.
(b) B1 Rf = distance moved by spot / distance moved by solvent, oe
(b) Answer: Rf = distance moved by spot / distance moved by solvent

Question 5

(a) M1 Rf = 5.6 / 8.4, oe correct substitution into the Rf equation
(a) A1 0.67 cao (allow 0.666 to 0.667 before rounding)
(a) Answer: 0.67

Question 6

(a) B1 0.30 (2.7 / 9.0) cao
(a) B1 0.70 (6.3 / 9.0) cao
(a) B1 0.90 (8.1 / 9.0) cao
(a) Answer: 0.30, 0.70 and 0.90
(b) B1 Allura Red AC (Rf 0.30 matches)
(b) B1 Tartrazine (Rf 0.70 matches)
(b) B1 Brilliant Blue (Rf 0.90 matches) (ecf from part a: match whichever Rf values the student calculated to the closest reference dye)
(b) Answer: Allura Red AC, Tartrazine and Brilliant Blue.
(c) B1 formulation (a mixture)
(c) B1 because it separates into more than one spot on the chromatogram, showing it contains more than one dye, oe
(c) Answer: It is a formulation. It produces three separate spots on the chromatogram, showing it is a mixture of three different dyes rather than one pure substance.

Question 7

Level 3 (5-6): A clear, logically ordered method is described, covering setting up the chromatogram, running it correctly, and how the results (Rf values and/or colours, compared with the reference dyes) would be used to identify the dyes present in each squash sample.
Level 2 (3-4): The method is partly described, covering some key practical steps, such as setting up or running the chromatogram, but with gaps in either the running of the experiment or the analysis of results. There is some logical structure.
Level 1 (1-2): Simple, isolated statements are made about chromatography, with little or no reference to a full practical method or to how the dyes would be identified.
Level 0 (0): No relevant content.
Indicative content:
- draw a baseline in pencil (not pen, as pen ink would dissolve and run), near the bottom of the chromatography paper
- use a fine pipette or capillary tube to place a small spot of each squash sample, and of each reference dye, on the baseline; label each spot
- let each spot dry before adding more solution on top of it, to keep the spots small and concentrated
- pour a shallow layer of solvent into a beaker, to a depth below the pencil baseline
- suspend or hang the paper in the beaker so the baseline sits above the solvent surface (the spots must not be submerged in the solvent)
- cover the beaker with a lid, to keep the atmosphere saturated with solvent vapour and reduce evaporation of the solvent
- leave until the solvent has risen close to the top of the paper, then remove the paper and immediately mark the solvent front in pencil, before it evaporates
- allow the chromatogram to dry, then measure the distance travelled by the solvent front and by each spot, from the baseline
- calculate the Rf value of each spot using Rf = distance moved by spot / distance moved by solvent
- compare the Rf value (and colour) of each spot from a squash sample with the Rf values of the reference dyes run on the same chromatogram, under the same conditions
- a dye is identified as present in a squash sample if a spot's Rf value (and colour) matches a reference dye's Rf value

Question 8

(a) B1 pencil is insoluble in the solvent, so it does not dissolve and move up the paper (and so cannot be mistaken for a dye spot); ink would dissolve and run, oe
(a) Answer: Pencil does not dissolve in the solvent, so the baseline stays in place and is not confused with a dye spot; ink would dissolve and run up the paper with the solvent.
(b) B1 if the solvent covered the baseline, the spots would dissolve directly into the solvent bath rather than being carried up the paper, so no separation would take place, oe
(b) Answer: If the solvent were above the baseline, it would dissolve the spots straight into the solvent instead of carrying them up the paper, so the substances could not be separated.
(c) B1 it keeps the atmosphere inside the beaker saturated with solvent vapour, which stops the solvent evaporating, so the chromatogram runs consistently/reliably, oe
(c) Answer: The lid keeps the atmosphere inside saturated with solvent vapour, which stops the solvent evaporating and helps give reliable, repeatable results.

Question 9

(a) B1 any one of: faster than chemical tests
(a) B1 any other one of: more sensitive (can detect very small amounts/low concentrations)
(a) B1 any other one of: more accurate; needs only a very small sample; less reliant on the skill or subjective judgement of the person carrying out the test (accept any three distinct correct advantages)
(a) Answer: Any three of: faster; more sensitive; more accurate; needs a smaller sample; less reliant on the skill or judgement of the analyst.
(b) B1 the equipment is expensive to buy and maintain, and specially trained staff are needed to operate it and interpret the results, oe
(b) Answer: The equipment is expensive to buy and maintain, and needs trained staff to operate it and interpret the data.

Question 10

(a) B1 a small sample of the solution is put into a flame, oe
(a) B1 the light given out (emitted) by the flame is passed through a spectroscope, oe
(a) B1 the spectroscope produces a line spectrum, a pattern of coloured lines at specific wavelengths, oe
(a) B1 the line spectrum produced is compared with reference spectra of known ions, to identify which metal ion(s) are present in the sample, oe
(a) Answer: A sample is placed in a flame; the light emitted is passed through a spectroscope, which produces a line spectrum. This spectrum is compared with reference spectra to identify the metal ion(s) present.

Question 11

(a) B1 sodium, Na+
(a) B1 because the spectrum shows a line at 589 nm, which matches the reference wavelength for sodium, oe
(a) Answer: Sodium (Na+). The line at 589 nm matches the reference emission wavelength for sodium ions.
(b) B1 the solution contains a mixture of (at least) two different metal ions, oe
(b) B1 sodium ions, Na+ (line at 589 nm), and lithium ions, Li+ (line at 671 nm), are both present
(b) Answer: The solution is a mixture containing both sodium ions (589 nm) and lithium ions (671 nm).

Question 12

(a) M1 peak height / concentration calculated for at least two pairs of values, e.g. 10/2 = 5, 20/4 = 5, 30/6 = 5, 40/8 = 5, 50/10 = 5
(a) A1 the ratio is constant (= 5) for every pair of values, so peak height is directly proportional to concentration, oe
(a) Answer: Peak height / concentration = 5 for every pair of values in the table, so peak height is directly proportional to concentration.
(b) M1 concentration = peak height / 5, oe correct use of the proportional relationship from part a (ft)
(b) A1 7 mg/dm³ cao (ft)
(b) Answer: 7 mg/dm³

Question 13

(a) B1 any one of: it is more accurate/objective, since it does not rely on an observer judging a flame colour by eye
(a) B1 any other one of: it can also determine the concentration of the ion present, not just confirm which ion is present; it can identify ions in a mixture, even if their flame colours would overlap or mask each other when viewed by eye; it is more sensitive (can detect lower concentrations)
(a) Answer: Any two of: it is more accurate/objective than judging a flame colour by eye; it can determine the concentration of the ion, not just its identity; it can identify ions in a mixture whose flame colours might otherwise be masked or overlap.

Question 14

(a) M1 5.0 x 0.18, oe correct method
(a) A1 0.90 kg cao (allow 900 g)
(a) Answer: 0.90 kg (900 g)
(b) M1 5.0 x 0.30, oe correct method
(b) A1 1.5 kg cao
(b) Answer: 1.5 kg

Question 15

(a) M1 distance = Rf x distance moved by solvent = 0.75 x 12.0, oe correct rearrangement
(a) A1 9.0 cm cao
(a) Answer: 9.0 cm

Question 16

(a) M1 6.5 / 10.0, oe correct substitution
(a) A1 0.65 cao
(a) Answer: 0.65
(b) B1 Suspect 2
(b) B1 Suspect 2's ink also travelled 6.5 cm, giving the same Rf value (0.65) as the ransom note ink (ecf from part a)
(b) Answer: Suspect 2. Their ink spot also travelled 6.5 cm, giving the same Rf value (0.65) as the ransom note ink.
(c) B1 an Rf value depends on the solvent used (and other conditions, such as temperature and the type of paper), oe
(c) B1 so Rf values can only be validly compared if they were obtained under identical conditions, e.g. run together on the same chromatogram with the same solvent, oe
(c) Answer: Rf values depend on the solvent (and on conditions such as temperature and paper type), so samples can only be fairly compared if they are run together, under identical conditions.

Question 17

(a) B1 producing only one spot, in every solvent tested, suggests the powder is likely to be a pure substance rather than a mixture, oe
(a) B1 a mixture would be expected to separate into more than one spot in at least one solvent, since different substances usually move at different rates through the paper, oe
(a) B1 other test: measure the melting point of the powder; a pure substance melts sharply at one exact temperature, rather than over a range, oe (accept boiling point test)
(a) Answer: One spot in every solvent suggests the powder is pure, since a mixture would usually separate into more than one spot in at least one solvent. This could be confirmed with a melting point test: a pure substance melts sharply at a single, fixed temperature.

Question 18

(a) B1 a contaminated (impure) sample melts over a range of temperatures, usually starting below the true melting point of the pure drug, oe
(a) B1 so if the extracted drug melts over a range, rather than sharply at the known melting point of the pure drug, this shows it is contaminated, oe
(a) Answer: If the extracted drug melts over a range of temperatures, rather than sharply at the known melting point of the pure drug, this shows the sample is contaminated.
(b) B1 run a sample of the tablet coating alongside reference spots of pure dye X and pure dye Y, on the same chromatogram, using the same solvent, oe
(b) B1 calculate the Rf value(s) of the spot(s) produced by the tablet coating sample
(b) B1 if the tablet sample produces only one spot, matching the Rf value of dye X, it is not contaminated; a second spot matching the Rf value of dye Y would show contamination, oe
(b) Answer: Run the tablet coating alongside reference spots of dye X and dye Y on the same chromatogram. If only one spot appears, with the same Rf value as dye X, the coating is pure; an extra spot matching dye Y's Rf value would show contamination with dye Y.
(c) L3 (5-6): A well-reasoned evaluation is given, covering several advantages of flame emission spectroscopy (such as sensitivity, accuracy and the ability to measure concentration) applied clearly to why this matters for detecting trace metal contamination in tablets, leading to a justified conclusion.
(c) L2 (3-4): Some relevant advantages of flame emission spectroscopy are given and applied to the tablet quality-control context, but the answer lacks a balanced evaluation or clear reasoning throughout.
(c) L1 (1-2): A simple statement, or a short list, of one or two general points about instrumental methods is given, with little reference to the specific context of quality-controlling tablets for metal contamination.
(c) L0 (0): No relevant content.
(c) Answer: Levels of response part, no single correct answer; credit a balanced evaluation covering the high sensitivity of flame emission spectroscopy (able to detect trace/low concentrations of contaminating metal ions), its accuracy compared with chemical tests that rely on an analyst's judgement, its ability to measure the concentration of any contaminant (important for checking against a safety limit), and its speed (suiting high-throughput quality control), balanced against the cost of the equipment and staff training, leading to a justified conclusion. See indicative content.

Chromatography and Instrumental Analysis - Worksheets, Questions and Revision