Hidden colours in ink

What they will learn

• That many colours (especially the black and brown of felt-tip pens) are mixtures of several inks.
• That water climbs up paper on its own by capillary action, through the gaps between the fibres.
• That each ink moves at a different speed depending on how much it dissolves in water and how much it sticks to the paper, and that is how they separate.
• How to measure the distance travelled by each colour and compare it with the water's (Rf).

Prepare beforehand

• Print template A on ordinary paper and cut it out: it will be your guide for cutting the strips from the filter.
• Open the coffee filters along the seams to get flat sheets.
• Check that the pens have water-based ink: draw a line on some paper and wet it with a finger; if it runs, it will work.
• Mark a line on the outside of each glass, 10 mm above the inside bottom (water level).

Materials and tools

• Coffee filters — 3–4 filters (2 strips per side)
  white paper, size 1×4 or similar; the sides must be ≥ 12 cm
• Water-based felt-tip pens — 3 black ones of different brands + brown, green and purple
  school pens with washable water-based ink (not permanent)
• Clear glasses — 3–6
  about 10.5 cm tall with a mouth ≥ 6 cm wide (jam jar, drinking glass); if it is shorter, roll more of the strip round the pencil; if it is taller, hang the strip from a peg
• Tap water — ≈ 20–30 ml per glass
  up to 10 mm deep
• Pencils or wooden skewers — 1 per glass
  longer than the mouth of the glass
• Sticky tape or clothes pegs — 1 per strip
  to hold the tab on the pencil
• Surgical spirit / rubbing alcohol 70–96 % — 20 ml (optional)
  only for the permanent marker experiment

Tools

• Scissors — cutting the strips
• Ruler — measuring strips, water level and colour distances
• Graphite pencil — marking the start line and writing the pen's name
• Stopwatch — timing the 15–20 min

Pieces and templates

• A · Chromatography strip (guide) ×6 — coffee filter or absorbent paper

Step by step

1. Cut the strips

   Lay template A on the opened coffee filter, hold it down with two fingers and cut around it. Make 6 strips of 30 × 120 mm.

   Check: Each strip measures 30 × 120 mm and has no creases.

2. Mark the start line

   With a graphite pencil, draw a thin line 20 mm from the bottom end of each strip. At the top, also in pencil, write the name of the pen you are going to test.

   Tip: Never use a biro or felt-tip: its ink would move too.

3. Make the ink dot

   On the line, in the centre, make a 3–4 mm dot with the felt-tip. Let it dry for 30 seconds and go over it 2 more times so it holds plenty of ink without getting bigger.

   Check: Dark, round dot no larger than 4 mm.

4. Fold the tab

   Fold the top 20 mm of the strip over the pencil and hold it with sticky tape or a clothes peg.

5. Set the height while dry

   With the glass still empty, rest the pencil on the rim: the tip of the strip should be 5 mm from the bottom (below the 10 mm mark). If it is too long, slide the strip by rolling more of it round the pencil.

   Check: The strip hangs straight, without touching the sides of the glass.

6. Pour in the water

   Lift out the pencil with the strip. Pour water into the glass up to the 10 mm mark (one finger's depth, roughly). As the tip sits 5 mm from the bottom, the dot (20 mm higher up) will be 15 mm above the water.

7. Hang the strip

   Put the pencil back across the glass, slowly and without splashing. The tip of the strip dips 5 mm into the water and the ink dot stays out of the water.

   Check: The dot does NOT touch the water. If it does, take the strip out and start another.

8. Wait and watch

   Do not move the glass. The water climbs up the paper: in 10 minutes it travels about 6 cm and then moves more and more slowly. As it passes the dot, it carries the inks along and they separate into coloured bands.

   Tip: Set up several strips at once, one per glass, with different pens.

9. Take the strip out in time

   When the water front (the wet line) reaches the STOP mark, 1 cm from the pencil and about 8.5 cm above the water (15–20 min; if it has not got there after 25 min, take it out anyway), take the strip out and mark in pencil how far the water reached.

10. Dry and measure

    Leave the strip to dry for 10 minutes, hanging or on kitchen roll. Measure in millimetres from the start line to the centre of each coloured band and to the water front.

    Check: At least one black pen has separated into 2 or more colours.

Does it work? The test

Coffee filter strip with a dot of water-based black felt-tip, tip dipped 5 mm into 10 mm of water, for 15–20 minutes.

What should happen: The water climbs about 8.5 cm, up to the STOP mark (7 cm above the start line). The black dot separates into 2–4 bands: usually a blue or violet appears near the start and pinks, yellows or turquoises higher up (depending on the brand). A pen with a single dye (yellow, for example) climbs as a single spot.

If something goes wrong

• The ink goes into the water and the glass turns coloured — The dot was below the water level. Solution: Use less water (10 mm) or raise the strip: the dot must be at least 10 mm above the water.
• The dot does not move — Permanent marker (ink that does not dissolve in water). Solution: Use washable pens, or try the same strip with alcohol instead of water (experiment 3).
• The water does not climb, or climbs crookedly — The strip does not touch the water, or it touches the sides of the glass. Solution: Adjust the height so it dips 5 mm and hangs straight in the centre of the glass.
• The colours come out blurry and mixed — Dot too big, or two-ply kitchen roll. Solution: Make a small dot (3–4 mm) gone over several times and use coffee filter.
• All the colours bunch up at the very top — The strip was left in too long and the water reached the pencil. Solution: Take the strip out when the front is 1 cm from the pencil.
• The black comes out as a single colour — That pen uses a single black ink. Solution: It is not a failure: try other brands; it is a good result for comparing.

Safety

• Alcohol is flammable: only with an adult, in a small glass, away from flames and with the window open. It is not for drinking.
• Inks stain clothes: wear an apron or old clothes.
• The inky water can be poured down the sink.
• Keep glass tumblers on the table, not at the edge; plastic cups are better with little ones.

The science behind it

For children

Paper is made of tiny threads with little gaps between them. Water squeezes into those little gaps and climbs up on its own, like when you dip the corner of a napkin in water. When the water reaches the black dot, it takes the inks for a ride. But every ink is different: some let the water carry them very easily and go a long way, and others cling to the paper and stay behind. That is how we find out that the black was really a mixture of blue, pink, yellow...

For the adult

This is paper chromatography. The stationary phase is the cellulose of the paper (with water held between its fibres) and the mobile phase is the water rising by capillary action: surface tension pulls the water into the pores, which behave like tubes a few tens of microns wide (the advance slows over time, roughly as the square root of time). As it passes the spot, the water dissolves the dyes in the ink. Each dye is continuously shared between the moving water and the surface of the cellulose: those that are very soluble in water and have little affinity for paper move almost with the front; those that are adsorbed more strongly (through hydrogen bonds or their charge) lag behind. The ratio Rf = dye distance ÷ front distance is characteristic of each dye in a given system (paper, solvent, temperature), and does not depend on the “weight” of the molecule. The black inks of water-based felt-tips are usually mixtures of blue, violet, red and yellow dyes. Permanent markers use dyes that are poorly soluble in water and need a less polar solvent such as alcohol. The technique is used in laboratories to identify substances (food colourings, inks in forensic examinations, plant pigments).

• chromatography: a technique for separating the substances in a mixture by making them travel through paper or another material.
• capillary action: a liquid rising through very fine gaps, like water through paper.
• mixture: several substances together that can be separated again.
• dissolve: to make a substance spread through a liquid until you can no longer see any bits.
• front: the line showing how far the water has reached on the paper.

By age

• 5–7 years: The adult cuts the strips and sets the height; the child makes the dot, pours the water up to the mark and counts the colours that appear. Message: “black is a mixture”.
• 8–10 years: They do the whole set-up with the template and compare 3 black pens. They measure with a ruler how far each colour gets.
• 11–12 years: They work out the Rf of each band, set up the “mystery note” case and compare water with alcohol (with an adult).

In a group or in class

Pairs, 3 plastic cups and 6 strips per pair (36 cups for 24 children, or shared cups with 2 strips each hung from two pencils). Collect pens from the class and give each pair different brands. Have the strips cut beforehand. 40 min: 10 setting up, 20 waiting (use it to explain capillary action by wetting a napkin) and 10 measuring and sticking the strips on a shared wall chart.

Challenges

• Invent a secret ink by mixing two pens and challenge a classmate to work out which ones they are with a strip.
• Manage to separate the colours of a colour-coated sweet (a drop of water on the sweet, and that drop as the dot).
• Do circular chromatography: a dot in the centre of a whole filter and water coming in through a wick of rolled-up paper. What pattern appears?
• Find a black pen that does NOT separate and another that gives at least 4 colours.

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