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Year 8 UK KS3 Chemistry Separating Mixtures 🌈 Interactive
Chromatography
Separating mixtures using colour — and the science behind it
🌈 Live Chromatography Strip Simulator
Each strip shows how different dyes travel different distances up paper. The further a dye travels, the more soluble it is in the solvent.
Rf = 0.55
Red dye
Red ink
2 dyes!
Blue Rf=0.73
Red Rf=0.48
Purple ink
3 dyes!
Green ink
Green ink
Rf = 0.73
Purple dye
Reference
💡 The coloured spots are separated dyes. Each dye has a unique Rf value that identifies it like a fingerprint.
📋 Table of Contents
🎬 Teacher Video Placeholder
Add a YouTube video demonstration of paper chromatography here
<iframe width="100%" height="360" src="https://www.youtube.com/embed/YOUR_VIDEO_ID"
frameborder="0" allow="autoplay; encrypted-media" allowfullscreen></iframe>🔬 What is Chromatography?
Section 1 · Separating Mixtures · KS3 Chemistry
Chromatography is a technique used to separate mixtures into their individual components. The word comes from the Greek words chroma (colour) and graphein (to write) — because it was first used to separate coloured plant pigments. Today, it is one of the most important separation techniques in science.
All types of chromatography work on the same basic principle: the components of a mixture travel at different speeds through a material, causing them to separate. The faster a substance travels, the further it moves — and we can use this to identify what is in a mixture.
🌟 Key Definition: Chromatography is a separation technique that uses the different speeds at which substances move through a material to separate them from a mixture.
💡 Did you know? Chromatography was invented in 1903 by Russian botanist Mikhail Tsvet, who used it to separate plant pigments in leaves — including the green, yellow, and orange pigments we now know from autumn!
🧪Separates mixtures into pure components
🎨Originally used with coloured plant dyes
⚡Works because substances travel at different speeds
🔎Used to identify unknown substances
Add an image here: A photograph showing Mikhail Tsvet’s original chromatography experiment, OR a diagram comparing a mixed ink spot to separated colour bands on paper
Recommended size: 600 × 350px | Label: “Chromatography: Separating a mixture”⚡ Quick Check — Section 1
1
What does the word “chromatography” literally mean?
“Colour writing” — from the Greek chroma (colour) and graphein (to write).
2
What is the basic principle behind all types of chromatography?
Components of a mixture travel at different speeds through a material, causing them to separate. Faster-moving substances travel further.
3
Who invented chromatography, and what did they use it for?
Mikhail Tsvet, a Russian botanist, invented it in 1903 to separate coloured plant pigments.
⚗️ How Paper Chromatography Works
Section 2 · Stationary & Mobile Phases · KS3 Chemistry
In paper chromatography, a spot of the mixture is placed near the bottom of a strip of chromatography paper. The paper is then placed in a solvent (such as water or ethanol), making sure the spot is above the solvent level. The solvent slowly travels up the paper, carrying the components of the mixture with it.
The reason different substances separate is linked to two key ideas: the stationary phase and the mobile phase. The stationary phase is the paper — it stays still. The mobile phase is the solvent — it moves. Each substance in the mixture has a different attraction to the paper and to the solvent, so each one travels a different distance.
🔑 Key Fact — Stationary Phase: The stationary phase is the chromatography paper. Substances that are strongly attracted to the paper will stick to it more and travel a shorter distance up the strip.
🔑 Key Fact — Mobile Phase: The mobile phase is the solvent. Substances that are more soluble in the solvent (attracted to it more strongly) will be carried further up the paper.
| Phase | What it is | Movement | Example |
|---|---|---|---|
| Stationary phase | The solid or liquid that stays still | Does not move | Chromatography paper |
| Mobile phase | The liquid or gas that moves | Moves through stationary phase | Water or ethanol solvent |
⚠️ Common Mistake: Students often place the ink spot IN the solvent. This washes the ink away immediately! The spot must be placed above the solvent level so the solvent can carry it up slowly.
Add a diagram here: A step-by-step setup diagram showing (1) chromatography paper with ink spot above solvent, (2) paper in beaker with solvent, (3) solvent rising with arrows, (4) final separated spots with labels for stationary phase and mobile phase
Recommended size: 700 × 400px | Label: “Paper Chromatography — Setup Diagram”⚡ Quick Check — Section 2
1
What is the stationary phase and the mobile phase in paper chromatography?
Stationary phase = chromatography paper. Mobile phase = the solvent (e.g. water or ethanol).
2
Why does a substance that is more soluble in the solvent travel further up the paper?
Because it has a stronger attraction to the mobile phase (solvent) and is carried further as the solvent moves up the paper.
3
Why must the ink spot be placed above the level of the solvent?
If the spot is in the solvent, the ink will dissolve and wash away rather than being carried up the paper slowly.
📐 Calculating Rf Values
Section 3 · Retention Factor · KS3 Chemistry
Once the chromatography experiment is complete, we can measure how far each substance has moved relative to the solvent. This gives us a number called the Rf value (Retention Factor). The Rf value is unique to each substance under the same conditions, so it can be used to identify unknown substances.
The Rf value is always between 0 and 1. An Rf of 0 means the substance didn’t move at all (it stuck to the paper completely), while an Rf of 1 means it moved exactly as far as the solvent front (very soluble in the solvent).
Rf = Distance moved by substance (cm) ÷ Distance moved by solvent (cm)
🎯 Tip: Always measure from the original spot (the pencil line at the bottom) to the centre of each coloured spot. Measure the solvent front from the same starting line.
Worked Example — Calculate the Rf value for a blue dye:
Step 1: Distance moved by solvent = 12.0 cm
Step 2: Distance moved by blue dye = 8.4 cm
Step 3: Rf = distance by substance ÷ distance by solvent
Step 4: Rf = 8.4 ÷ 12.0
✅ Rf = 0.70
⚠️ Common Mistake: Rf values have NO UNITS — it is a pure ratio. Do not write “0.70 cm”. If your Rf value is greater than 1, you have made an error in your measurement!
| Dye Colour | Distance by Dye (cm) | Distance by Solvent (cm) | Rf Value |
|---|---|---|---|
| Red | 5.6 | 12.0 | 0.47 |
| Yellow | 9.2 | 12.0 | 0.77 |
| Blue | 8.4 | 12.0 | 0.70 |
| Orange | 6.8 | 12.0 | 0.57 |
Add an annotated diagram here: A completed chromatogram strip with arrows and measurements labelled — showing the pencil baseline at the bottom, the solvent front line at the top, and two coloured spots with their distances measured. Include the Rf calculation written out beside each spot.
Recommended size: 500 × 400px | Label: “Measuring Rf Values on a Chromatogram”⚡ Quick Check — Section 3
1
A dye travels 6.5 cm and the solvent travels 13.0 cm. What is the Rf value?
Rf = 6.5 ÷ 13.0 = 0.50 (no units)
2
What does an Rf value close to 1 tell us about a substance?
It is very soluble in the solvent (mobile phase) and has little attraction to the paper (stationary phase), so it travels almost as far as the solvent front.
3
A student calculates an Rf of 1.3. What has gone wrong?
Rf values cannot be greater than 1. The student has made a measurement error — likely measuring the substance distance as greater than the solvent distance.
🔍 Interpreting Chromatograms
Section 4 · Reading Results · KS3 Chemistry
A chromatogram is the result of a chromatography experiment — it shows the separated spots on the paper. Being able to read a chromatogram accurately is an important scientific skill. We can use chromatograms to answer questions such as: How many dyes are in this ink? Is this sample pure? Which substances are in this mixture?
The number of spots on a chromatogram tells us the minimum number of components in the original mixture. If a substance produces only one spot in chromatography, it is likely to be a pure substance. If it produces multiple spots, it is a mixture.
🌟 Key Definition: A chromatogram is the pattern of separated spots produced after running a chromatography experiment. Each spot represents one component of the mixture.
✅ Remember: To identify an unknown substance, compare its Rf value to a reference table of known Rf values. If the Rf values match under the same conditions, the substances are likely identical.
1️⃣One spot = pure substance (or only one component in this solvent)
✳️Multiple spots = mixture of different substances
📊Compare Rf values to identify unknowns
🔄Same Rf = likely the same substance
⚠️ Common Mistake: Students sometimes say a substance is “pure” just because it produces one spot. This is only true if tested under multiple different conditions. In one solvent, two different substances could have the same Rf value by coincidence!
Add an image here: Side-by-side comparison of three chromatogram strips — one showing a single spot (pure substance), one showing two spots (mixture of 2), and one showing three spots (mixture of 3). Each strip should be clearly labelled with spot distances and Rf values.
Recommended size: 650 × 350px | Label: “Interpreting Chromatograms — Pure vs. Mixtures”⚡ Quick Check — Section 4
1
A chromatogram shows three spots. What does this tell us about the original substance?
The original substance is a mixture containing at least three different components.
2
Unknown substance X has an Rf of 0.62. Reference table shows compound A (Rf = 0.62) and compound B (Rf = 0.45). What can you conclude?
Substance X is likely compound A, since their Rf values match under the same conditions.
3
Why might a single spot on a chromatogram not always prove a substance is pure?
Two different substances could have the same Rf value in a particular solvent. Testing in several different solvents is needed to be more certain of purity.
🌍 Real-World Applications
Section 5 · Science in Society · KS3 Chemistry
Chromatography is not just a classroom technique — it has hugely important real-world applications across medicine, food science, forensics, and environmental monitoring. It is one of the most widely-used analytical techniques in the world.
In forensic science, chromatography is used to analyse ink on questioned documents, identify drugs and poisons in blood samples, and match fibres or dyes at crime scenes. Drug testing in sports also relies heavily on chromatography to detect banned substances in urine or blood samples of athletes.
💡 Did you know? The ink in a forged document can be identified by chromatography. Even if two black pens look identical, their ink may contain different dye mixtures — giving different chromatogram patterns that can be matched back to a specific manufacturer.
🔬Medicine: Identifying drugs and testing blood samples
⚖️Forensics: Matching inks, dyes, and fibres at crime scenes
🌿Food science: Detecting artificial colourings and contaminants
🏅Sports: Testing athletes for performance-enhancing drugs
🌊Environment: Detecting pollutants in water and air
💊Pharmaceuticals: Checking purity of medicines
🧪 Experiment Link: Food dye chromatography is a classic practical — try spotting different coloured sweets (e.g. Smarties or Skittles) dissolved in water onto chromatography paper. You’ll often find that one “colour” is actually made of several dyes!
Add an image here: A collage or split-image showing chromatography in real-world settings — e.g. a forensic scientist analysing document ink, a laboratory HPLC machine, and a food testing laboratory. Alternatively, a photo of the food dye chromatography practical using coloured sweets.
Recommended size: 700 × 380px | Label: “Chromatography in the Real World”⚡ Quick Check — Section 5
1
Name two real-world uses of chromatography.
Any two from: forensic ink analysis, drug testing in sports, food dye testing, detecting environmental pollutants, checking purity of medicines, identifying substances in blood.
2
How could chromatography help identify a forged document?
The ink can be analysed to reveal its dye mixture. Different pens produce different chromatogram patterns, so the ink can be matched back to a specific pen or manufacturer.
3
Why might chromatography be used in food science?
To identify artificial colourings, detect contaminants, or confirm that food contains only approved additives.
🧫 Types of Chromatography
Section 6 · Beyond Paper · KS3 Chemistry
While paper chromatography is the most common type studied at KS3, scientists use many different types of chromatography depending on the substances they need to separate. All types share the same principle — a mobile phase moves through a stationary phase — but the materials and methods vary greatly.
Thin Layer Chromatography (TLC) uses a thin layer of silica or aluminium oxide coated onto glass or plastic instead of paper. It gives sharper, clearer spots than paper chromatography and is used widely in pharmaceutical research. Gas Chromatography (GC) uses a gas as the mobile phase and can separate extremely small amounts of substance — it is used in forensics, food analysis, and drug testing.
🌟 Key Fact: High Performance Liquid Chromatography (HPLC) is one of the most powerful modern chromatography techniques. It can separate thousands of different compounds and is used to test the purity of medicines and detect pollutants in water at concentrations as low as parts per billion!
🎯 Tip for exams: At KS3, you mainly need to know paper chromatography. But knowing the names of TLC and GC will impress your teacher and prepare you for GCSE!
| Type | Stationary Phase | Mobile Phase | Common Use |
|---|---|---|---|
| Paper Chromatography | Paper fibres | Solvent (liquid) | Separating dyes, food colours |
| Thin Layer Chromatography (TLC) | Silica on glass/plastic | Solvent (liquid) | Drug identification, pharmaceuticals |
| Gas Chromatography (GC) | Solid particles in a column | Carrier gas | Forensics, food analysis, perfume testing |
| HPLC | Fine particles in a column | Liquid under high pressure | Medicine purity, pollution testing |
Add an image here: A comparison diagram showing paper chromatography strips next to a TLC plate and a gas chromatography machine (GC instrument). Label each type clearly, or use three photographs side by side with captions.
Recommended size: 700 × 320px | Label: “Types of Chromatography Compared”⚡ Quick Check — Section 6
1
What is the mobile phase in gas chromatography?
A carrier gas (such as nitrogen or helium) acts as the mobile phase in gas chromatography.
2
Give one advantage of TLC over paper chromatography.
TLC gives sharper, clearer spots and is more accurate; the silica stationary phase provides a more consistent surface than paper fibres.
3
State one common principle shared by ALL types of chromatography.
All types use a mobile phase moving through a stationary phase. Components separate because they travel at different speeds depending on their attraction to each phase.
⚠️ Required Practical — Paper Chromatography
Section 7 · Practical Skills · KS3 Chemistry
The paper chromatography practical is a common required practical at KS3 and GCSE. You need to know not only how to carry it out, but also why each step is important. Following the correct method ensures your results are accurate, fair, and repeatable.
The key steps are: draw a pencil baseline (not pen!), spot the samples above the baseline, place the paper in solvent (below the baseline), allow the solvent to run almost to the top, remove and mark the solvent front immediately, then measure and calculate Rf values. Each step has a scientific reason — and examiners love to ask about these!
⚠️ Common Mistake — Use PENCIL not PEN: The baseline must always be drawn in pencil, never pen. Pen ink would dissolve in the solvent and mix with your samples, ruining the results. Pencil marks do not dissolve and won’t interfere.
✅ Remember — Mark the Solvent Front Immediately: The solvent evaporates quickly when you remove the paper. If you wait before marking the solvent front, you will not know how far the solvent actually travelled, making Rf calculations impossible.
✏️Draw pencil baseline — pencil doesn’t dissolve in solvent
📍Spot samples above baseline — use a capillary tube
💧Solvent level below baseline — ink must not touch solvent
⏱️Wait until solvent nears the top — for best separation
📏Mark solvent front immediately on removal
🧮Measure and calculate Rf values from pencil marks
🔑 Key Fact — Fair Test: To compare Rf values fairly, all samples must be run on the same piece of paper in the same solvent at the same time. Rf values can change if a different solvent or different paper is used!
Add an image here: A numbered step-by-step practical guide (1–6) with photos or clear diagrams showing: (1) drawing pencil baseline with ruler, (2) spotting ink with capillary tube, (3) paper placed in beaker with solvent, (4) solvent rising up paper, (5) marking solvent front, (6) completed chromatogram with measurements.
Recommended size: 700 × 420px | Label: “Required Practical — Step-by-Step Method”⚡ Quick Check — Section 7
1
Why must the baseline be drawn in pencil, not pen?
Pen ink dissolves in the solvent and would mix with the samples, contaminating results. Pencil marks do not dissolve so they won’t interfere with the chromatography.
2
Why must you mark the solvent front as soon as you remove the paper from the solvent?
The solvent evaporates quickly. If you don’t mark it immediately, you won’t know how far the solvent travelled, making accurate Rf calculations impossible.
3
What must you do to compare Rf values fairly between different samples?
All samples must be run on the same piece of paper, using the same solvent, at the same time. This ensures the conditions are identical for a fair comparison.
🧠 Multiple Choice Quiz
Test your knowledge — 10 questions covering the whole topic
0 of 10 answered
✍️ Long Answer Questions
Write detailed answers — model answers and mark schemes included
⚡ One Word Quiz
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