Chromatography — AQA GCSE Combined Science
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Chromatography explained
Chromatography separates the components of a mixture as they move between two phases.
Read the full explanation
The stationary phase is fixed, for example the water held in paper or a solid coating on a plate. The mobile phase moves, for example a solvent rising up paper or a gas carrying vapour through a column. A sample is placed at the start line and the mobile phase travels through the stationary phase. Each substance distributes itself between the phases to a different extent, so components travel different distances and separate. The distance moved relative to the solvent front gives an Rf value, which can be compared with known values to help identify a substance. A pure substance usually gives a single spot, while a mixture gives more than one spot.
The ratio of the distance moved by a compound (centre of spot from origin) to the distance moved by the solvent can be expressed as its R_f value:
Chromatography separates dissolved substances as they travel at different speeds through paper or a thin layer of silica. You measure two lengths with a ruler, both starting at the pencil origin line: the distance the solvent front travelled, and the distance from the origin to the centre of each coloured spot. Dividing the spot distance by the solvent-front distance gives the R_f value, a number between 0 and 1 with no units. For example, if the solvent front moves 8.0 cm and a spot centre moves 3.2 cm, R_f = 3.2 ÷ 8.0 = 0.40. Because R_f depends on the solvent and conditions, you must run a known reference substance alongside the unknown on the same chromatogram to identify a compound by matching R_f values.
R_f = distance moved by substance / distance moved by solvent
This equation turns a chromatogram into a number. Distance moved by substance means from the pencil origin line to the centre of the spot; distance moved by solvent means from the same origin line to the solvent front. Suppose the solvent front is 10.0 cm from the origin and a spot centre is 4.5 cm from the origin: R_f = 4.5 ÷ 10.0 = 0.45. The value has no units because it is a length divided by a length, and it is always 1 or less. Rearranging helps too: distance moved by substance = R_f × distance moved by solvent. Use the equation with a reference substance on the same chromatogram to identify an unknown, since R_f changes with solvent and conditions.
distaₙce moved by solveₙt
In chromatography, the R_f value compares how far a dissolved substance travels with how far the solvent front travels. You measure the distance moved by the solvent front from the pencil baseline to the highest point the solvent reached, and the distance moved by the solute from the pencil baseline to the centre of its spot. Both distances must be measured in the same units, usually millimetres or centimetres, and the baseline must be drawn in pencil so it does not dissolve. For example, if a dye spot moves 3.0 cm and the solvent front moves 6.0 cm, R_f = 3.0 ÷ 6.0 = 0.50. R_f has no units and is always between 0 and 1. This measurement underpins identification because a compound's R_f under fixed conditions is characteristic.
Different compounds have different R_f values in different solvents, which can be used to help identify the compounds. The compounds in a mixture may separate into different spots depending on the solvent but a pure compound will produce a single spot in all solvents.
Chromatography separates dissolved substances as they travel at different rates through paper or a thin layer. A compound's R_f value depends on its solubility in the solvent and its attraction to the stationary phase, so the same compound can give different R_f values in different solvents. This means R_f alone does not prove identity; it must be compared with the R_f of a known pure sample run under the same conditions. A mixture may produce several spots because its components separate, and the pattern of spots can change with the solvent. A pure compound produces a single spot in every solvent, although the spot's position may differ between solvents. For example, a pure dye gives one spot in water and one spot in ethanol, whereas an impure dye may give two spots in one solvent and three in another.
explain how paper chromatography separates mixtures
Paper chromatography separates mixtures because each component distributes differently between two phases. The stationary phase is the chromatography paper; the mobile phase is the solvent that moves up the paper. A component that is very soluble in the solvent and only weakly attracted to the paper travels far with the solvent front. A component that is strongly attracted to the paper and less soluble in the solvent moves only a short distance. Because each substance has its own balance of solubility and attraction, the components separate into distinct spots at different heights. For example, a mixture of coloured inks may produce several spots, and comparing these with spots from known dyes can identify the components.
suggest how chromatographic methods can be used for distinguishing pure substances from impure substances
Chromatography separates dissolved substances as a solvent front moves through a stationary phase. A pure substance produces a single spot in every solvent system because it has one fixed composition and one R_f value. An impure substance contains two or more substances, so it produces two or more spots, or a smeared streak, under the same conditions. To distinguish them, run a chromatogram of the sample alongside a known pure reference under identical conditions. If the sample gives one spot matching the reference, it is pure; if it gives extra spots or a streak, it is impure. For example, a food colouring that gives one spot is a pure dye, while one giving three spots is a mixture of dyes.
interpret chromatograms and determine R_f values from chromatograms
A chromatogram shows spots produced by substances carried different distances by a solvent. To interpret it, identify the solvent front, the start line and each spot. The R_f value of a spot is calculated using R_f = distance moved by substance ÷ distance moved by solvent front. Both distances are measured from the start line to the centre of the spot and to the solvent front, in the same units, so R_f has no units and is always between 0 and 1. For example, if a spot moves 3.0 cm and the solvent front moves 6.0 cm, R_f = 3.0 ÷ 6.0 = 0.50. Compare R_f values with known values or reference spots to identify substances, and count spots to judge purity.
provide answers to an appropriate number of significant figures.
In chromatography, measured distances and calculated R_f values must be reported with a sensible number of significant figures. The number of significant figures is the count of meaningful digits in a value, starting from the first non-zero digit; for example, 4.62 cm has three significant figures, and 0.0580 has three because the zeros after the 5 and 8 are significant. When you measure a solvent front of 8.0 cm and a spot distance of 3.2 cm, the R_f is 3.2 ÷ 8.0 = 0.40, which is appropriately given to two significant figures, matching the precision of the measurements. Do not overstate precision by writing many digits from a calculator display, and do not round so heavily that the value becomes misleading. The appropriate number depends on the precision of the measurements and the context of the calculation.
Required practical activity 12: investigate how paper chromatography can be used to separate and tell the difference between coloured substances. Students should calculate R_f values.
Paper chromatography separates coloured substances because they travel at different rates through the paper as the solvent moves. Draw a pencil start line on chromatography paper, place small spots of each coloured substance on it, and lower the paper into a solvent so the solvent level is below the start line. Allow the solvent to rise, then mark the solvent front in pencil before it dries. Each coloured component forms a spot at a characteristic distance. Calculate R_f using R_f = distance travelled by the substance ÷ distance travelled by the solvent front. Compare R_f values of unknown spots with those of known substances to identify them; matching values suggest the same substance. A pure coloured substance usually gives one spot, while a mixture gives more than one spot.
Your focus
- Describe chromatography as a separation technique involving a stationary phase and a mobile phase.
- Explain how differences in distribution between the phases cause components of a mixture to separate.
- Calculate and use Rf values to help identify substances from a chromatogram.
Show all 30 objectives
- Measure the distance moved by the solvent front and by the centre of a spot from the origin line.
- Calculate an R_f value by dividing the distance moved by the substance by the distance moved by the solvent.
- Use R_f values from the same chromatogram to identify an unknown compound by comparison with a reference.
- Apply the equation R_f = distance moved by substance ÷ distance moved by solvent to measured distances.
- Interpret an R_f value as a unitless ratio between 0 and 1.
- Rearrange the equation to calculate a missing distance from a known R_f value.
- Measure the distance moved by the solvent front and by a solute from the pencil baseline.
- Calculate an R_f value using the correct distances and no units.
- Explain why the baseline is drawn in pencil and why the solvent front must be marked promptly.
- Explain why a compound can have different R_f values in different solvents.
- Describe how the number of spots relates to whether a sample is pure or a mixture.
- Compare chromatograms of an unknown sample with a known pure sample to support identification.
- Explain the roles of the mobile and stationary phases in paper chromatography.
- Relate the distance moved by a component to its solubility and attraction to the paper.
- Use chromatographic results to identify components of a mixture.
- Describe how a chromatogram can show whether a substance is pure or impure.
- Explain why a pure substance gives one spot and an impure substance gives more than one spot.
- Compare a sample with a pure reference run under the same conditions to decide whether the sample is pure.
- Measure the distance moved by a substance and by the solvent front from the start line.
- Calculate R_f values using the correct formula and give them without units.
- Interpret chromatograms to identify substances and to deduce the number of substances in a mixture.
- Identify the number of significant figures in a measured or calculated value.
- Calculate an R_f value and round it to an appropriate number of significant figures.
- Justify the number of significant figures used in a chromatography answer.
- Carry out paper chromatography to separate coloured substances and record the chromatogram accurately.
- Calculate R_f values from measured distances and use them to compare coloured substances.
- Explain how chromatography can tell the difference between pure substances and mixtures.
Chromatography exam tips
Marking Points
- State that chromatography separates the components of a mixture, not a compound.
- Identify the stationary phase as the fixed phase and the mobile phase as the moving phase in a named example such as paper chromatography.
- Explain that separation depends on how each substance distributes itself between the stationary and mobile phases.
- Describe how to obtain and use an Rf value, calculated as distance moved by the substance divided by distance moved by the solvent front, to help identify a substance.
- Interpret a chromatogram: a single spot suggests a pure substance, while multiple spots indicate a mixture.
- State that both distances are measured from the pencil origin line, not from the bottom edge of the paper or plate.
- Measure the distance moved by the solvent to the solvent front, the furthest point reached by the liquid.
- Measure the distance moved by the compound to the centre of the spot, not to its top or bottom edge.
- Calculate R_f by dividing the distance moved by the substance by the distance moved by the solvent.
- Recognise that R_f has no units and normally lies between 0 and 1 because the spot cannot travel further than the solvent front.
- Explain that a reference substance must be run on the same chromatogram under the same conditions for a valid comparison of R_f values.
- Substitute the distance moved by the substance and the distance moved by the solvent into the equation in the correct positions.
- Keep the substance distance as the numerator and the solvent distance as the denominator.
- Measure both distances from the origin line to the centre of the spot and to the solvent front respectively.
- Report R_f as a unitless value, usually between 0 and 1, to an appropriate number of significant figures.
- Rearrange the equation to find an unknown distance when the R_f value and the other distance are given.
- Use R_f values only for comparison when the reference and unknown were run on the same chromatogram with the same solvent.
- State that the distance moved by the solvent is measured from the pencil baseline to the solvent front, not from the bottom of the paper.
- State that the distance moved by the solute is measured from the pencil baseline to the centre of the spot, not to its top or bottom edge.
- Explain that both distances must be measured in the same units so that the ratio is dimensionless.
- Calculate R_f using R_f = distance moved by solute ÷ distance moved by solvent, giving a value between 0 and 1.
- Explain that the baseline is drawn in pencil because pencil is insoluble and will not travel with the solvent.
- Describe how the solvent front is marked immediately before the solvent reaches the top of the paper, because it becomes difficult to locate once the paper dries.
- State that different compounds have different R_f values in a given solvent, so R_f can help identify a compound when compared with a known sample.
- Explain that R_f values depend on the solvent used, so a compound can have different R_f values in different solvents.
- Describe how a mixture separates into different spots because its components travel at different rates.
- State that a pure compound produces a single spot in all solvents, while a mixture may produce more than one spot.
- Explain that identification requires running a known pure sample under the same conditions and comparing R_f values or spot positions.
- Recognise that a single spot in one solvent does not by itself prove purity; the test should be repeated in a different solvent.
- State that the stationary phase is the paper and the mobile phase is the solvent.
- Explain that components dissolve in the mobile phase and are carried up the paper at different rates.
- Explain that components more strongly attracted to the stationary phase move more slowly and less far.
- Explain that components more soluble in the mobile phase move faster and further up the paper.
- Use the different distances moved to identify components by comparison with known substances or by Rf values.
- A pure substance gives a single spot on a chromatogram because it contains only one substance with one R_f value.
- An impure substance gives two or more spots, or a streak, because it contains two or more substances that separate differently.
- The same chromatographic conditions, including solvent, stationary phase and temperature, must be used for a valid comparison.
- A known pure reference substance can be run alongside the sample; a matching single spot supports purity, while extra spots show impurity.
- Chromatography separates substances by their different solubilities in the mobile phase and attractions to the stationary phase, so each substance travels a characteristic distance.
- The number of spots indicates the minimum number of substances present, but two substances with similar R_f values may overlap.
- Identify the start line and the solvent front on the chromatogram before taking any measurements.
- Measure the distance moved by the substance from the start line to the centre of the spot.
- Measure the distance moved by the solvent front from the start line to the solvent front.
- Calculate R_f using R_f = distance moved by substance ÷ distance moved by solvent front.
- Give R_f as a value between 0 and 1 with no units, since it is a ratio of two lengths.
- Use R_f values and spot positions to identify substances by comparison with known or reference values obtained under the same conditions.
- Interpret the number of spots to deduce the minimum number of substances present in a mixture.
- State that significant figures are the meaningful digits in a measured or calculated value, counted from the first non-zero digit.
- Apply the rule that leading zeros are never significant, while zeros between non-zero digits and trailing zeros after a decimal point are significant.
- Calculate R_f as the distance travelled by the substance divided by the distance travelled by the solvent front, then round the result to a sensible number of significant figures.
- Match the number of significant figures in a reported answer to the precision of the measurements, avoiding excessive digits copied from a calculator.
- Recognise that a value such as 0.40 has two significant figures, whereas 0.4 has one, and choose the form that reflects the measurement precision.
- Describe setting up paper chromatography with a pencil start line, small spots of coloured substances, and a solvent below the start line.
- Explain that components separate because they have different solubilities in the solvent and different attractions to the paper, so they move different distances.
- Measure the distance travelled by each coloured spot from the start line to the centre of the spot, and measure the distance travelled by the solvent front from the start line.
- Calculate R_f values using R_f = distance travelled by the substance ÷ distance travelled by the solvent front, and use the values to compare or identify substances.
- Interpret chromatograms: a single spot indicates a pure substance, while multiple spots indicate a mixture, and matching R_f values suggest the same substance.
Examiner Tips
- 💡Always label the stationary phase and mobile phase in a diagram before explaining separation.
- 💡Show the Rf calculation with both distances and keep the solvent front distance as the denominator.
- 💡Use the phrase 'distribution between the phases' to explain why components move different distances.
- 💡Write the equation in words first, then substitute the two measured lengths, so the examiner can see your method clearly.
- 💡Give R_f to two significant figures or as a decimal, and never attach a unit such as cm to the answer.
- 💡When identifying an unknown, compare its R_f with a reference value obtained on the same chromatogram and state that conditions such as solvent must match.
- 💡Show the substitution line, for example R_f = 4.5 ÷ 10.0, before giving the final value so method marks can be awarded.
- 💡Round the final R_f sensibly, such as two decimal places, and avoid over-long calculator displays.
- 💡If asked to identify a substance, quote both the calculated R_f and the matching reference R_f from the same chromatogram.
- 💡Write the equation R_f = distance moved by solute ÷ distance moved by solvent before substituting values.
- 💡Check that the calculated R_f lies between 0 and 1; a value outside this range indicates a measurement error.
- 💡When comparing spots, use the same baseline and solvent front for every spot on that chromatogram.
- 💡Use the phrase 'under the same conditions' when comparing R_f values with a known compound.
- 💡Link the number of spots to the number of components: one spot suggests a pure compound, multiple spots indicate a mixture.
- 💡When a question mentions different solvents, state that the R_f value may change but a pure compound still gives one spot in each solvent.
- 💡Use the phrases mobile phase and stationary phase correctly and link each to its role in the separation.
- 💡When explaining a chromatogram, compare the position of spots from the mixture with spots from known reference substances.
- 💡If asked why the solvent must not cover the baseline, state that the mixture would dissolve into the solvent instead of travelling up the paper.
- 💡State clearly that a pure substance gives one spot and an impure substance gives more than one spot or a streak.
- 💡Mention that the same solvent and conditions must be used when comparing a sample with a pure reference.
- 💡Use the term R_f value correctly and link it to the distance travelled by the substance relative to the solvent front.
- 💡If asked to suggest a method, describe running a chromatogram of the sample and a known pure sample side by side, then compare the spots.
- 💡Show the measurements and the division clearly so that the method is visible.
- 💡Check that the R_f value is between 0 and 1; if it is greater than 1, the calculation has been inverted.
- 💡Use the same unit for both distances, or convert them before dividing.
- 💡When identifying a substance, compare its R_f value with known values obtained using the same solvent and conditions.
- 💡Write down the measured distances with their units before dividing, so the significant figures in the final R_f value are easy to justify.
- 💡Check whether the question asks for significant figures rather than decimal places, and count digits from the first non-zero digit.
- 💡If a calculated R_f is exactly 0.4, consider whether reporting 0.40 better reflects the precision of measurements such as 3.2 cm and 8.0 cm.
- 💡State clearly that the solvent front must be marked before it evaporates, because the R_f calculation depends on that measurement.
- 💡Show the R_f equation and substitute the measured distances with units before calculating, so your method is clear.
- 💡When identifying an unknown substance, compare its R_f value with the R_f values of known substances run on the same chromatogram under the same conditions.
Common Mistakes
- Confusing the phases by calling the solvent the stationary phase; correct this by stating the solvent that moves is the mobile phase and the paper or plate is the stationary phase.
- Measuring Rf from the wrong baseline or forgetting to measure to the centre of the spot; correct this by measuring from the start line to the centre of the spot and from the start line to the solvent front.
- Claiming chromatography identifies a substance on its own; correct this by saying Rf values must be compared with known values under the same conditions.
- Measuring the spot distance to the top or bottom edge of the spot instead of its centre; correct this by marking the centre of the spot before measuring.
- Measuring from the bottom edge of the paper rather than the pencil origin line; correct this by always using the origin line as the zero point for both distances.
- Dividing the solvent distance by the spot distance; correct this by keeping the substance distance on top, so R_f = distance moved by substance ÷ distance moved by solvent.
- Inverting the fraction and dividing the solvent distance by the substance distance; correct this by writing the equation in words before substituting numbers.
- Adding units such as cm to the R_f answer; correct this by cancelling the length units, since a length is divided by a length.
- Using a spot distance greater than the solvent distance and reporting R_f above 1; correct this by re-measuring, as the spot cannot move further than the solvent front.
- Measuring the solute distance to the top edge of the spot rather than its centre; correct by marking and measuring to the centre of the spot.
- Measuring the solvent distance from the bottom edge of the paper rather than the pencil baseline; correct by always measuring from the baseline.
- Recording R_f with units such as cm; correct by cancelling the units because it is a ratio of two lengths.
- Claiming that a single spot in one solvent proves a compound is pure; correct by stating that purity is confirmed only if a single spot appears in all solvents tested.
- Assuming R_f is a fixed constant for a compound regardless of solvent; correct by stating that R_f varies with the solvent and conditions.
- Treating a matching R_f as proof of identity; correct by explaining that it supports identification when compared with a known sample under the same conditions.
- Confusing the mobile and stationary phases: the error is stating the paper moves or the solvent is stationary; the correction is that the solvent is the mobile phase and the paper is the stationary phase.
- Claiming that larger molecules always travel further: the error is using size as the deciding factor; the correction is that travel depends on solubility in the mobile phase and attraction to the stationary phase.
- Thinking that the solvent front and the component spot are the same thing: the error is confusing the moving solvent boundary with a separated component; the correction is that the solvent front is the furthest point reached by the solvent, while spots are the separated components.
- Error: thinking that a pure substance always gives a spot at the solvent front. Correction: a pure substance gives one spot at a characteristic R_f value, which is usually less than 1.
- Error: comparing spots from chromatograms run in different solvents. Correction: R_f values and spot patterns are only comparable when the same solvent, stationary phase and conditions are used.
- Error: assuming that one spot always proves purity. Correction: one spot shows only one detectable substance under those conditions; two substances with the same R_f value could overlap, so additional evidence may be needed.
- Error: confusing the mobile phase and stationary phase. Correction: the mobile phase is the solvent that moves, and the stationary phase is the paper or plate that does not move.
- Error: measuring the distance moved by the substance from the bottom of the paper rather than from the start line. Correction: always measure from the start line to the centre of the spot.
- Error: dividing the distance moved by the solvent front by the distance moved by the substance. Correction: use R_f = distance moved by substance ÷ distance moved by solvent front.
- Error: giving R_f a unit such as cm. Correction: R_f is a ratio of two lengths, so it has no units.
- Error: measuring to the top edge of a spot instead of its centre. Correction: measure to the centre of the spot for a consistent and accurate value.
- Counting leading zeros as significant, for example treating 0.058 as having five significant figures; correct this by starting the count at the first non-zero digit, giving two significant figures.
- Copying every digit from a calculator display, such as writing an R_f of 0.3970588; correct this by rounding to a sensible number of significant figures, for example 0.40.
- Confusing significant figures with decimal places, for example giving 0.40 as two decimal places rather than two significant figures; correct this by identifying the first significant digit and counting from there.
- Using a pen instead of a pencil for the start line or solvent front; correct this by using pencil because pen ink would dissolve and run in the solvent.
- Placing the solvent level above the start line; correct this by keeping the solvent below the start line so the spots do not wash into the solvent reservoir.
- Measuring the spot distance to its top edge rather than its centre; correct this by measuring from the start line to the centre of the spot for a consistent R_f value.