Chromatography rainbow

After a chromatography activity was moderately successful last week, I thought we'd have a go at another one, this time rainbow themed.  The idea of putting rainbows in windows is popular around us and both my boys enjoy spotting them on our walks (the littlest has learnt to say "rainbow" during lockdown and the big one observes when the colours aren't in the correct order...). I therefore thought making a rainbow would go down well with my eldest.  As it happened, he wasn't as keen on it as I thought he'd be, possibly because we had to try twice to make it work.  However, I thought I'd post it here as it produces a very pretty result!

Rainbow colours on kitchen roll

The idea behind this is that water carries water-soluble coloured inks up paper, and if you do this from both ends, the colours move together.  If you use coloured inks which are made of single colours rather than mixtures, you will get a more rainbow-like result; if they are made of multiple colours, they separate as they travel up the paper due to the different solubility of the colours.

Kitchen roll with coloured spots

We used kitchen roll which is strong but absorbent.  I suggest using a thin strip and not making it too long - our first attempt was too long and the water didn't reach the middle, so the colours didn't merge and create a nice rainbow effect.  The one which worked better was 16 cm long and about 3 cm wide.

Rainbow colours starting to travel along the paper

My son put spots of coloured pen in the order of the rainbow across the kitchen roll, with it folded in half so it soaked through to the other end.  He's a stickler for rainbows having all the colours in the right order (there are a lot on windows around us which don't...) so he wasn't very impressed that we only had blue and purple in our Crayola pens rather than blue, indigo and violet, but he eventually accepted that we didn't have 7 colours to use!  

To make the colours travel up the kitchen roll rather than soaking into the water, you need to put the coloured spots so they will be positioned above the water level.  We used shot glasses with about 1.5cm water depth and put our spots around 2cm from the ends of the kitchen roll.

Almost at the top

Put the two ends of the strip of kitchen roll into separate containers of water at the same time, such that the kitchen roll forms a bridge between the two glasses.  It's the same idea that we used when doing our colour walking experiment, with capillary action meaning water travels up the kitchen roll.  As the water moves, the coloured inks dissolve in the water and travel with it up the paper, with some left behind in a rainbow-coloured trail.

Finished rainbow pattern

If your paper isn't too long, the rainbow will meet in the middle, and you can take it out and dry it.  We draped ours across the empty water glasses for an hour or so on a warm day. My son was pretty happy with the result second time around (the first time the paper was too long - the full height of a piece of the kitchen roll - and even though we left it for ages it never reached the middle), although he still reminded me about the missing indigo...