Releasing rainbows

I have had a run of unsuccessful experiments recently (since the new year, is that a sign of something?!). I haven’t written any of them up yet as I am working on turning the bad to good, adding new parts or just trying to figure out where they went wrong… However this one didn’t turn out too badly, it just wasn’t quite as pretty as I was promised. It is, however, a nice little activity that isn’t very messy and should make you child go “wow” or at least something like that!


What you need:

  • filter paper (essential – we used the little filters for aeropress coffee makers. However I have been thinking that if I’d used a larger filter – like those for filter coffee makers then it might have worked better. Maybe. Or you can use kitchen roll but it tends to have a textured surface so doesn’t look quite as neat).
  • Black felt tip pen (essential – got to be a felt tip, and a non permanent one at that too).
  • Water (essential – just a few drops are all you need).
  • Plate or something to do it on (not essential but does stop anything else getting wet).

What to do:

  • Put filter paper on plate and draw a dot in the middle using the black felt tip.
  • Put a few drops of water onto the dot and watch the rainbow appear (hopefully*)

Tips/extra bits:

*You should see lovely colourful rings of the rainbow (red, orange, yellow, green, blue, indigo and violet), but as you can see from the images ours were not exactly beautiful examples of this. However, we just turned it into a spotting game to see what colours from a rainbow we could find. When you really look at them you do see quite a few of the colours in there so don’t be disheartened if it’s not perfect, M still really enjoyed doing it.

I did experiment with different sized dots of pen, different black felt tips and different numbers of drops of water, but none of it seemed to make much difference (although soaking it in water is not a good idea!). I do think a bigger bit of filter paper would allow there to be more space for the colours to spread out, but I don’t guarantee this. This was the most successful one we did:


Which actually doesn’t look too bad here.

The science bit

The water dissolves the dried pigment in the black ink, and the filter paper allows the bands of colour to separate out from the dot in the middle. The molecules of each different colour are different weights and so they travel different distances and speeds, causing a rainbow effect (supposedly!). Remember chromatography from science lessons at school?! Well this is that. This is a form of paper chromatography, but it can also be done on gases and liquids and is used in a range of activities from forensic testing to separating colour pigments.

Explaining it to children

The black ink is made up of lots of different colours (think about what happens when you mix lots of different colours of paint or play doh together). The water (and special filter paper) help these colours to separate out. You could also explain that different colours travel different distances on the paper depending on their weight, like if you push cars along the floor some travel further than others (this is not always dependant on weight though).


Experiment based on an activity from the fantastic Cool Science Tricks book by Daniel Tatarsky.

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