Extreme Drawing

Paper chromatography for kids: the colours inside black

Kitchen science experiment

Black ink is hiding other colours

Washable black marker is a mix of dyes. Put water on a dot and the water creeps outward through the paper, carrying each dye at its own speed, so the black splits into rings of blue, pink and yellow.

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  • Ages 5+
  • 20 minutes
  • Low mess
Painted illustration of paper strips and a coffee filter in glasses of water, where black marker dots have spread into blue, pink and yellow bands, watched by the painter.

Quick facts

Age
5 to 10 (younger with help)
Time
About 20 minutes
Mess
Low: a little water and ink
Adult needed
Only to help cut and hang the strips
You need
Washable black felt-tip, coffee filters or kitchen paper, a glass of water (almost free)
On screen / for real
Both

In Extreme Drawing

Pick the Marker in the drawing bar, choose Black and make a fat dot. Then pick Water on the Bathroom tab, choose Drops and let one drop fall right on the dot.

The drop soaks out through the paper fibres and takes the ink with it. As it dries you see a bright cyan-blue ring at the water's edge, a pink band halfway and a yellow-orange centre, the way washable black marker does on real paper. None of the dye disappears: it has only been sorted.

It works because a marker's dyes cling to the paper fibres, each by its own amount: the blue lets go most easily and travels furthest, the pink less, and the yellow hardly moves. Food colouring and ink drops don't cling, so they spread with the water as one colour. Try a drop of food colouring next to your marker dot and compare.

More to try: other marker colours (which ones split?), a row of dots along a line, or a marker drawing on paper that's already wet. Crayon and coloured pencil don't run at all: their wax turns the water away.

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Painted illustration of the painter brushing a rainbow watercolour that bleeds into wet paper, with crayons, markers, a paint box and a child's rainbow drawing.

Do it for real: coffee-filter chromatography

You need

  • A washable black felt-tip (water-based, not permanent)

  • White coffee filters, kitchen paper or blotting paper

  • Scissors and a pencil

  • A glass or jar with a little water

  • Sticky tape, or a clothes peg

  • Other washable colours to compare (brown, green and purple are good)

  1. Cut a strip

    Cut a strip of coffee filter about 3 cm wide and a bit longer than your glass is tall.

  2. Mark the start

    Draw a pencil line 2 cm from the bottom. Pencil doesn't run, so it won't spoil the test. Make a fat marker dot on the line.

  3. Add a little water

    Pour about 1 cm of water into the glass. The water must stay below the dot.

  4. Hang the strip

    Tape the top of the strip to a pencil laid across the glass, so the bottom just dips into the water and the dot stays above it.

  5. Watch the colours climb

    Over 5 to 10 minutes the water creeps up and the colours pull apart. Take the strip out before the water reaches the top.

  6. Dry and compare

    Lay the strip flat to dry. Which colour went highest? Try different brands of black side by side.

  7. The circle version

    Like the app: put a dot in the middle of a round coffee filter on a plate and drip water onto the dot, one drop at a time. The colours spread out in rings.

Painted illustration of paper strips and a coffee filter in glasses of water, where black marker dots have spread into blue, pink and yellow bands, watched by the painter.

Why it works

For kids: black ink is really a few colours mixed together. Water pulls each colour along the paper, but some colours hold on to the paper more tightly than others, so they get left behind and the colours spread out.

For grown-ups: this is chromatography. The paper's cellulose fibres are the stationary phase and water is the mobile phase, drawn along by capillary action. Each dye divides its time between dissolving in the moving water and sticking to the fibres; a dye that sticks less travels further (it has a higher Rf value). Black washable inks are blends of dyes, often blue, red or magenta and yellow, chosen together to absorb all visible light.

Separation needs the dyes to differ. In the game the three marker dyes move at 1, 0.3 and 0.05 times the water's speed. When the speeds were closer together, the colours smeared into each other and only a pale blue ring showed, just as dyes that are too alike blur together on real paper.

Try next: predict, then test

Same black, different brands?

Do two black markers from different brands split into the same colours?

What hides in brown or green?

Guess the colours first, then test. Is yellow hiding in green?

Which paper is fastest?

Coffee filter, kitchen paper or printer paper: which one lets the water climb quickest?

Salt water instead?

Does a pinch of salt in the water change how far the colours go?

For teachers

Paper chromatography fits KS2 Year 5 "properties and changes of materials" as an enquiry into mixtures and separating (chromatography itself is named later, at KS3), and NGSS 5-PS1-3 (identifying materials by their properties). Drop water on a marker dot on the whiteboard, ask what colours will appear, then hand out strips.

Ideas for teachers

Questions about paper chromatography

What is paper chromatography for kids?

It is a way of sorting a mixture of colours with paper and water. The water carries each colour along the paper at a different speed, so they end up in separate bands.

Why does black marker turn blue, pink and yellow?

Black washable ink is made by mixing several dyes. The blue usually travels furthest, the pink less and the yellow least, though every brand mixes its black a little differently.

Which markers work for chromatography?

Washable, water-based felt-tips. Permanent markers don't move with water.

What paper is best for chromatography?

White coffee filters, kitchen paper or blotting paper. Printer paper works but slowly, because it is dense and coated.

Why did my colours not separate?

Usually the dot was under the water line, the marker was permanent, or the strip touched the side of the glass. Try again with the dot above the water.

More colour science

Painted illustration of the painter touching a dish of milk with a soapy cotton bud while red, blue, yellow and green food colouring swirls outwards.

Magic milk

Food colouring rushes away from a drop of soap.

Painted illustration of the painter brushing blue watercolour over paper, revealing a white crayon fish and bubbles.

Crayon resist

Why wax keeps water and paint away: the secret white crayon message.

Split a black dot on screen

One marker dot, one drop of water, three colours. Then try your own markers on a coffee filter.

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