What is chromatography and how is it used in chemical analysis?

Chromatography is a technique used to separate mixtures into their individual components. It is widely used in chemical analysis to identify substances within a mixture and to purify chemicals. In the Cambridge IGCSE Chemistry curriculum, chromatography is an essential experimental technique for analyzing complex mixtures.

How does paper chromatography work in separating substances?

In paper chromatography, a small sample of the mixture is placed on a piece of chromatography paper. The paper is then placed in a solvent. As the solvent travels up the paper, it carries the different components of the mixture with it. Different substances move at different rates, allowing them to be separated and identified based on their distinct travel distances.

What factors affect the separation of substances in chromatography?

Several factors influence the separation of substances in chromatography, including the type of solvent used, the nature of the paper or stationary phase, and the size and polarity of the molecules in the mixture. Understanding these factors is crucial for optimizing the separation process in experimental techniques and chemical analysis.

Why is the Rf value important in chromatography?

The Rf value, or retention factor, is a numerical representation of how far a substance travels in chromatography relative to the solvent front. It is calculated by dividing the distance traveled by the substance by the distance traveled by the solvent. Rf values are important because they help identify substances based on their movement through the stationary phase, which is a key concept in the Cambridge IGCSE Chemistry syllabus.

What are some common applications of chromatography in real-world scenarios?

Chromatography is used in various real-world applications, such as drug testing, food safety analysis, and environmental monitoring. It helps in detecting contaminants, analyzing complex mixtures, and ensuring the purity of pharmaceuticals. These applications highlight the importance of chromatography in both academic studies and practical chemical analysis.

Why is "Drawing the baseline in pen" a common mistake in Chromatography?

Why it happens: Habit. How to avoid it: Use PENCIL — pen ink dissolves in the solvent. Source: 0620/62 — recurring

Why is "Placing the paper with the solvent ABOVE the baseline" a common mistake in Chromatography?

Why it happens: Misreading the diagram. How to avoid it: Solvent must be BELOW the baseline so spots are not washed off.

Why is "Calculating $R_{f} > 1$" a common mistake in Chromatography?

Why it happens: Inverting the fraction. How to avoid it: Substance distance ÷ solvent distance. Substance can never travel further than the solvent → R_f 1.

Experimental Techniques and Chemical AnalysisCambridge IGCSE Chemistry (0620)

Chromatography

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Chromatography — Cambridge IGCSE 0620 Chemistry Extended (2026)

Paper chromatography separates dissolved substances by how far they travel in a solvent. $R_f$ values identify components. Locating agents reveal colourless spots.

What you’ll learn

Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).

14.3 — Describe paper chromatography.
14.3 — Calculate $R_f$ values.
14.3 — Use chromatography to identify components and check purity.

How chromatography works

Spot, dip, wait, dry, measure. Sample components race up the paper at different rates.

Procedure.

Take a strip of FILTER PAPER (or chromatography paper).
Draw a START LINE in pencil, $\sim 1\,\text{cm}$ from the bottom. Use PENCIL — ink would dissolve in the solvent.
SPOT the mixture on the line. Allow to dry.
Place the bottom of the paper into solvent in a beaker. Solvent must be BELOW the start line.
Cover the beaker with a watch-glass to prevent evaporation.
Solvent rises up the paper by CAPILLARY ACTION.
Different components travel up at different rates → separate.
When the solvent front nears the top, REMOVE the paper.
Mark the SOLVENT FRONT (the highest level the solvent reached) immediately.
Allow to dry; locate spots if colourless (see locating agents).

Why do components separate? Each substance has a different attraction to:

The PAPER (stationary phase). Stronger → slower movement.
The SOLVENT (mobile phase). Stronger → faster movement.

The balance of these two attractions gives each substance its characteristic $R_f$ value.

Cambridge tip. Always specify the use of PENCIL for the start line — pen ink would smear and dissolve in the solvent.

Spot mixture on pencil line above solvent.
Solvent rises by capillary action.
Components separate by attraction balance.
Mark solvent front on removal.

$R_f$ values

$R_f = \dfrac{\text{distance moved by spot}}{\text{distance moved by solvent}}$ .

Definition. $R_f = \dfrac{\text{distance moved by spot}}{\text{distance moved by solvent front}}.$

Both distances measured from the START LINE.

Worked. Solvent front moved $8.0\,\text{cm}$ ; a spot moved $4.0\,\text{cm}$ . $R_f = 4.0/8.0 = 0.5$ .

Properties of $R_f$ .

ALWAYS between $0$ and $1$ (spot can't move further than the solvent).
SAME substance gives SAME $R_f$ in the same solvent at the same conditions.
$R_f$ values are LOOKED UP in tables to identify substances.

Identifying an unknown. Run the unknown alongside known reference substances. If the unknown spot has the same $R_f$ as a reference substance (and same colour), they're likely the same substance.

Worked qualitative. A black ink shows three spots on chromatography paper after running. Conclusion: the ink is a MIXTURE of three coloured pigments. Compare $R_f$ values to known dyes to identify which.

Cambridge tip. Always measure to the CENTRE of the spot, not the edge — gives a more accurate $R_f$ .

$R_f$ = spot distance / solvent distance.
Always between 0 and 1.
Same substance → same $R_f$ (same conditions).
Identify by comparing with known $R_f$ values.

Applications and locating agents

Identify dyes, drugs, amino acids. Use locating agents (ninhydrin) for colourless spots.

Common uses.

Dye and ink analysis (forensic / textile).
Pigment separation in plant extracts.
Drug testing in athletes / hospitals.
Amino acid identification in proteins.
Food colouring analysis.
Purity testing — pure substance gives ONE spot only.

Locating agents. Many substances are colourless — wouldn't show as a spot. Use a "locating agent" to reveal them:

Ninhydrin for amino acids — turns spots PURPLE/BROWN.
UV light — many drugs and biological molecules fluoresce under UV.
Iodine vapour — stains organic compounds.

Worked qualitative. A student is given a mixture of two amino acids. After running paper chromatography, no spots are visible. What should they do? Spray with NINHYDRIN (or dip in ninhydrin solution) and warm gently. Spots appear purple/brown.

Purity check. A pure substance gives exactly ONE spot on chromatography. If multiple spots appear, the substance is impure (or a mixture).

Worked qualitative. A drug company is testing whether a new drug sample is pure. They run chromatography. The result shows ONE spot at $R_f = 0.62$ (matching pure drug standard). Conclusion: pure.

Locating agent: reveals colourless spots.
Ninhydrin: amino acids (purple).
UV: drugs, biological molecules.
Pure substance: ONE spot.
Mixture: multiple spots.

How it’s examined

Chromatography appears most years on Paper 4 (4-6 marks): describe procedure, calculate $R_f$ , interpret results. Examiner reports flag students using pen instead of pencil for the start line, and forgetting that the solvent must START BELOW the spots.

Sources: Cambridge IGCSE Chemistry 0620 syllabus 2026-2028 (14.3); 0620/22 May/Jun 2024 — Q21 (chromatography, $R_f$); 0620 Examiner Reports 2022-2024. Last reviewed 2026-05-07.

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Step-by-step worked examples — Chromatography

Step-by-step solutions to past-paper-style questions on chromatography, written exactly the way a tutor would explain them at the board.

1Standard chromatography procedure
Core• procedure
Question
Describe the steps for paper chromatography.
Step-by-step solution
1. Step 1
  Draw a PENCIL baseline above the solvent level.
2. Step 2
  Add a small spot of each sample on the baseline. Allow to dry.
3. Step 3
  Place paper in a beaker with solvent BELOW the baseline.
4. Step 4
  Cover and let solvent rise. Stop just before the solvent reaches the top.
5. Step 5
  Mark the solvent front in pencil. Calculate $R_{f}$ for each spot.
Answer
Pencil baseline → spot samples → solvent below baseline → cover → stop early → mark solvent front → R_f values.
2Calculate $R_{f}$
Extended• Adapted from 0620/62 May/Jun 2024 Q3• Rf
Question
A spot moves $4.5\,\text{cm}$ . The solvent front is $9.0\,\text{cm}$ above the baseline. Find the $R_{f}$ value.
Step-by-step solution
1. Step 1
  $R_{f} = \dfrac{\text{distance moved by spot}}{\text{distance moved by solvent}}$ .
  $R_{f} = \dfrac{4.5}{9.0} = 0.5$
Answer
$R_{f} = 0.5$
3Reveal a colourless spot
Extended• locating agent
Question
How are colourless amino acids located on a chromatogram?
Step-by-step solution
1. Step 1
  Spray with NINHYDRIN solution.
2. Step 2
  Heat — amino acid spots turn purple/brown.
Answer
Spray with ninhydrin and heat — amino acids show as purple spots.

Key Formulae — Chromatography

The formulae you need to memorise for chromatography on the Cambridge IGCSE 0620 paper, with every variable defined in plain English and a note on when to use it.

$R_{f}$ value
$R_{f} = \dfrac{\text{distance moved by substance}}{\text{distance moved by solvent front}}$
When to use
Identify a substance; characteristic of the substance and solvent.

Key Definitions and Keywords — Chromatography

Definitions to memorise and the exact keywords mark schemes credit for chromatography answers — sharpened from recent examiner reports for the 2026 0620 sitting.

Chromatography
Examiner keyword
Method to separate a mixture based on the components' different solubilities/affinities for the stationary and mobile phases.
$R_{f}$ value
Examiner keyword
Ratio of distance moved by the substance to distance moved by the solvent. Always between 0 and 1.
Locating agent
Examiner keyword
A reagent that reveals colourless spots on a chromatogram (e.g. ninhydrin for amino acids).

Common Mistakes and Misconceptions — Chromatography

The traps other students keep falling into on chromatography questions — taken from recent Cambridge IGCSE 0620 examiner reports and mark schemes — and how to avoid them.

✕Drawing the baseline in pen
0620/62 — recurring
Why it happens
Habit.
How to avoid it
Use PENCIL — pen ink dissolves in the solvent.
✕Placing the paper with the solvent ABOVE the baseline
Why it happens
Misreading the diagram.
How to avoid it
Solvent must be BELOW the baseline so spots are not washed off.
✕Calculating $R_{f} > 1$
Why it happens
Inverting the fraction.
How to avoid it
Substance distance ÷ solvent distance. Substance can never travel further than the solvent → $R_{f} \le 1$ .

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

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Video lesson

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Chromatography — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Chromatography

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Standard chromatography procedure

2Calculate RfR_{f}Rf​

3Reveal a colourless spot

RfR_{f}Rf​ value

Chromatography

RfR_{f}Rf​ value

Locating agent

✕Drawing the baseline in pen

✕Placing the paper with the solvent ABOVE the baseline

✕Calculating Rf>1R_{f} > 1Rf​>1

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Chromatography — frequently asked questions

2Calculate $R_{f}$

$R_{f}$ value

$R_{f}$ value

✕Calculating $R_{f} > 1$