What is chromatography and how is it used in the study of pure and impure substances?

Chromatography is a laboratory technique used to separate mixtures into their individual components. It is particularly useful in the study of pure and impure substances as it allows scientists to identify and analyze the different substances present in a mixture. This technique is commonly used in IB MYP Science to demonstrate how mixtures can be separated based on the different properties of their components, such as solubility and molecular size.

How does paper chromatography work, and what are its applications in science?

Paper chromatography is a simple and effective method used to separate and identify mixtures of soluble substances. In this technique, a small sample of the mixture is placed on a piece of chromatography paper, which is then placed in a solvent. As the solvent travels up the paper, it carries the different components of the mixture with it at different rates, allowing them to be separated. This method is widely used in educational settings, including the IB MYP curriculum, to analyze pigments in plants and inks in pens.

What are the key factors that affect the separation of substances in chromatography?

The separation of substances in chromatography is influenced by several key factors, including the polarity of the molecules, the type of solvent used, and the nature of the stationary phase (such as paper or gel). The interaction between the molecules of the substances being separated and the stationary and mobile phases determines how quickly each component moves, leading to their separation. Understanding these factors is crucial for students studying chromatography in the IB MYP Science curriculum.

Why is chromatography important in scientific research and industry?

Chromatography is essential in both scientific research and industry because it provides a reliable method for analyzing complex mixtures. In research, it helps scientists identify and quantify the components of chemical mixtures, which is vital for developing new materials and medicines. In industry, chromatography is used for quality control, ensuring that products meet required standards by analyzing their chemical composition. This technique's versatility and precision make it a fundamental tool in the study of pure and impure substances.

What are the differences between paper chromatography and thin-layer chromatography (TLC)?

While both paper chromatography and thin-layer chromatography (TLC) are used to separate mixtures, they differ in their stationary phases. Paper chromatography uses paper as the stationary phase, whereas TLC uses a thin layer of material like silica gel coated on a glass or plastic plate. TLC generally provides better resolution and faster results than paper chromatography, making it more suitable for analyzing complex mixtures. These differences are important for students to understand when exploring chromatography techniques in the IB MYP Science curriculum.

Why is "Drawing the baseline in pen instead of pencil." a common mistake in Chromatography?

Why it happens: It's the obvious thing to draw with. How to avoid it: Pencil graphite doesn't dissolve in the solvent. Pen ink WOULD dissolve and run, ruining the experiment.

Why is "Getting an Rf value greater than 1." a common mistake in Chromatography?

Why it happens: Arithmetic slip or mismeasurement. How to avoid it: Rf is ALWAYS between 0 and 1 (a component can't travel further than the solvent that carries it). If you get > 1, you've measured something wrong or divided the wrong way.

Pure and impure substancesIB MYP Sciences (Years 1-5)

Chromatography

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Chromatography — IB MYP Sciences: separating coloured mixtures and the Rf value

Paper chromatography separates dyes and pigments by exploiting how strongly each component sticks to the paper vs the solvent. This MYP Sciences note covers the technique, the meaning of the Rf value, and applications in forensics, food testing and drug analysis.

What you’ll learn

Mapped to the IB MYP Sciences subject guide (2026 onwards).

MYP Sciences A — Describe the steps of paper chromatography.
MYP Sciences A — Define and calculate Rf.
MYP Sciences C — Interpret a chromatogram to identify or compare substances.
MYP Sciences D — Evaluate a real-world use of chromatography.

How paper chromatography works

Spots travel up the paper as a solvent rises; different components travel different distances.

Setup:

Draw a horizontal pencil baseline on chromatography paper, just above the bottom edge. (Use pencil, not pen — pen ink would dissolve and run.)
Spot small drops of the mixture(s) onto the line. Let them dry.
Stand the paper in a beaker with a SHALLOW layer of solvent BELOW the baseline. The solvent rises up the paper by capillary action.
As it passes through each spot, the components dissolve and travel UP with the solvent.
Components that prefer the SOLVENT (mobile phase) travel further.
Components that prefer the PAPER (stationary phase) travel less.
After enough time, mark the solvent front (the highest the solvent reached) before it evaporates.

A mixture spotted on the baseline separates into different coloured spots as the solvent rises. Each component moves at its own characteristic rate.

Pencil baseline (pen ink would run).
Solvent rises by capillary action.
Stronger attraction to paper → moves less.
Stronger attraction to solvent → moves more.

The Rf value

Rf = distance moved by spot ÷ distance moved by solvent.

Each component has a CHARACTERISTIC Rf value (under the same solvent and paper conditions). Calculate as:

$R_f = \frac{\text{distance moved by spot from baseline}}{\text{distance moved by solvent front from baseline}}$

Rf is ALWAYS between 0 and 1:

Rf close to 1 → travelled with the solvent (low attraction to paper).
Rf close to 0 → barely moved (high attraction to paper).

Worked example. Spot moves 4.5 cm; solvent front moves 9.0 cm. Rf = 4.5 / 9.0 = 0.50.

You can identify an unknown by comparing its Rf to known reference values (under the same conditions). For example, food dye tests compare unknown dyes against legal reference dyes.

A given dye gives different Rf values in different solvents (water vs ethanol etc.) and different papers. Always specify the conditions when quoting Rf.

Rf = spot distance ÷ solvent distance.
Always 0 < Rf ≤ 1.
Identify by matching Rf to a reference under the same conditions.

Real-world uses

Forensics, food, drugs and art — chromatography is everywhere.

Forensics: separating dyes in pen inks to match a ransom note to a specific pen.

Food chemistry: checking that food dyes are legal (e.g. red 40 vs banned colourants). Testing pesticide residues.

Pharmaceutical industry: checking the purity of a drug — a pure drug gives a single spot; impurities give extra spots.

Art conservation: identifying pigments in old paintings to date or authenticate them.

Sports doping testing: more advanced versions (HPLC, GC-MS) detect tiny traces of banned drugs in urine.

Medical labs: analysing blood samples for drugs, toxins, hormones.

MYP criterion D extension: discuss how chromatography contributes to consumer safety (detecting illegal additives in food), justice (forensic evidence) and medicine (drug purity testing) — and the limitations (results need careful interpretation; matching Rf isn't proof unless conditions match exactly).

Forensics: ink and dye matching.
Food: detecting illegal dyes and contaminants.
Pharma: purity testing.
Sport: doping detection (with advanced methods).

How it’s examined

Criterion C asks for Rf calculations and chromatogram interpretation. Criterion D often asks for a real-world evaluation of how chromatography helps society.

Sources: IB MYP Sciences guide (IBO, official subject guide). Last reviewed 2026-05-25.

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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.

1Calculating Rf
Getting started• Rf
Question
On a chromatogram, the solvent front moves 8.0 cm from the baseline. A green spot has moved 5.6 cm. Calculate its Rf.
Step-by-step solution
1. Step 1
  Rf = spot distance ÷ solvent front distance.
2. Step 2
  Rf = 5.6 / 8.0 = 0.70.
Answer
0.70.
2Which component went furthest?
Building confidence• mechanism
Question
Three dyes are spotted on the same line. After running, dye A is highest, B is middle, C lowest. Which dye has the WEAKEST attraction to the paper?
Step-by-step solution
1. Step 1
  Components that prefer the solvent travel further; components that prefer the paper travel less.
2. Step 2
  A went furthest → A prefers the solvent over the paper → A has the WEAKEST attraction to paper.
Answer
Dye A — it travelled furthest, so it had the weakest attraction to paper (and the strongest attraction to the solvent).
3Identifying an unknown
Stretch• forensic
Question
A ransom note is written in black ink. Forensic chemists run a chromatogram of the ink and compare it with three known pen inks (A, B, C). The pattern matches pen B exactly. What does this prove?
Step-by-step solution
1. Step 1
  If the chromatogram patterns are identical, the inks are very likely the same.
2. Step 2
  It DOESN'T uniquely identify the pen (the same brand of ink is in many pens). But it narrows the source considerably.
3. Step 3
  In a court, the result would be considered consistent with the suspect's pen — but not absolute proof.
Answer
Strong circumstantial evidence the note was written with the same kind of ink as pen B. Not absolute proof — many pens share the same dye mix.

Key Formulae — Chromatography

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

Retention factor (Rf)
$Rf = distance of spot / distance of solvent front$
$distance of spot$
from baseline to centre of spot (cm)
$distance of solvent front$
from baseline to highest solvent line (cm)
When to use
Identifying components by comparing Rf with reference values.

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 IB MYP Sciences sitting.

Chromatography
Examiner keyword
A separation technique using a stationary phase (e.g. paper) and a mobile phase (solvent).
Stationary phase
Examiner keyword
The non-moving medium in chromatography (e.g. the paper).
Mobile phase
Examiner keyword
The moving solvent that carries the components up the paper.
Rf value
Examiner keyword
Spot distance ÷ solvent-front distance. A characteristic of each substance under a given set of conditions.

Common Mistakes and Misconceptions — Chromatography

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

✕Drawing the baseline in pen instead of pencil.
Why it happens
It's the obvious thing to draw with.
How to avoid it
Pencil graphite doesn't dissolve in the solvent. Pen ink WOULD dissolve and run, ruining the experiment.
✕Getting an Rf value greater than 1.
Why it happens
Arithmetic slip or mismeasurement.
How to avoid it
Rf is ALWAYS between 0 and 1 (a component can't travel further than the solvent that carries it). If you get > 1, you've measured something wrong or divided the wrong way.

Practice questions

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

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Chromatography — frequently asked questions

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Chromatography

Short Study Notes in the form of Slides

Short Study Notes — Chromatography

Detailed Notes

What you’ll learn

How it’s examined

1Calculating Rf

2Which component went furthest?

3Identifying an unknown

Retention factor (Rf)

Chromatography

Stationary phase

Mobile phase

Rf value

✕Drawing the baseline in pen instead of pencil.

✕Getting an Rf value greater than 1.

Practice questions

Past paper style quiz

Assess your understanding

Chromatography — frequently asked questions