Ink Chromatography
Separate the hidden colours inside marker ink and discover how chromatography is used in forensic science and chemistry.
Adult supervision recommended for younger children.
This experiment is safe and low-mess. Marker ink may stain fingers and surfaces — work on newspaper or a protected surface.
You Will Need
Step-by-Step Instructions
Prepare Your Paper Strips
Cut a coffee filter into strips about 3cm wide and 12-15cm long. You need one strip per colour you want to test. Use a pencil (not pen) to label each strip with the colour and brand of marker you will test.
Draw Ink Dots
About 2cm from the bottom of each strip, draw a thick dot or short line with your marker. Make it bold so there is plenty of ink. The dot must be above the waterline when you place it in the cup.
Set Up the Glass
Pour about 1cm of water into the bottom of the glass. Tape the top of each paper strip to a pencil laid across the top of the glass, so the strip hangs down into the water. The bottom of the strip should just touch the water, but the ink dot must stay above the waterline.
Watch and Wait
Watch as the water slowly creeps up the paper strip. When it reaches the ink dot, the water will start carrying the ink upward. Different pigments in the ink will travel at different speeds, separating into distinct colour bands. Wait about 15-20 minutes.
Remove and Observe
Once the water has travelled close to the top of the strip (or the colours have clearly separated), remove the strips and lay them on a paper towel to dry. Observe the colour bands — you may be surprised at what colours were hiding inside your markers!
Record Your Results
For each marker, record: the original colour, the individual colours you can see separated on the strip, and how far each colour travelled. You can tape your dried strips into a notebook.
Extension: Forensic Challenge
Have someone write a "secret message" using one of several black markers. Then run chromatography on a sample of the mystery ink and on all the candidate markers. Can you match the mystery ink to the pen that wrote the message? This is how forensic scientists can identify which pen was used to write a document!
What Happened?
The water crept up the paper strip through a process called capillary action — the same force that pulls water up through the roots and stems of plants. As the water reached the ink dot, it dissolved the ink and carried the pigments upward.
Most marker inks that appear to be a single colour are actually mixtures of several different pigments. Black ink, for example, often contains blue, purple, red, and yellow pigments mixed together. Green might contain blue and yellow.
Different pigment molecules have different properties. Some are more attracted to the water (the mobile phase) and travel further up the paper. Others are more attracted to the paper (the stationary phase) and stay closer to the starting point. This difference in travel speed is what separates the colours into distinct bands.
The Science Behind It
Chromatography (from the Greek "chroma" meaning colour, and "graphein" meaning to write) is a laboratory technique used to separate mixtures into their individual components.
In paper chromatography, there are two phases:
- • Stationary phase: the paper strip (stays still)
- • Mobile phase: the water (moves up the paper)
Different molecules interact differently with these two phases. Molecules that are more soluble in water travel further (they "prefer" being in the mobile phase). Molecules that are more attracted to the paper fibre travel less far (they "prefer" the stationary phase).
The distance a substance travels relative to the distance the solvent (water) travels is called the Rf value (retention factor). Each substance has a characteristic Rf value under the same conditions, which is how scientists identify unknown substances.
Chromatography is used in: forensic science (analysing inks, drugs, blood), food science (testing for additives), environmental science (detecting pollutants), and pharmaceutical research (purifying drugs).
Think About It
Why do you think we use a pencil to label the paper strips and draw the starting line, rather than a pen?
If you tested the same colour marker from two different brands and got different chromatography patterns, what does that tell you about how those inks are made?
Why is it important that the ink dot starts above the water level? What would happen if the dot was submerged?
Forensic scientists use chromatography to analyse substances found at crime scenes. Can you think of situations where this technique would help solve a crime?
Knowledge Check
Test what you have learned! Select the correct answer for each question.
Question 1
What is chromatography used for?
Question 2
In paper chromatography, the "mobile phase" is...
Question 3
Why does a black marker often show many different colours during chromatography?
Question 4
A pigment that travels a long way up the paper strip is more attracted to...
Question 5
Which of these is a real-world application of chromatography?
Key Concepts Summary
- ● Chromatography is a technique for separating mixtures based on how fast each component travels through a material.
- ● It involves a stationary phase (paper) and a mobile phase (water/solvent) that carries the mixture.
- ● Many inks that look like a single colour are actually mixtures of several pigments.
- ● Components separate because they have different attractions to the mobile phase versus the stationary phase.
- ● Chromatography is used in forensics, food science, environmental monitoring, and pharmaceutical research.