Colour walking experiment

I've seen this experiment done by several different people on Instagram and elsewhere, and I tried it out by myself ages ago but hadn't done it with the small boy.  We were stuck at home whilst the smallest had a stomach bug (which he thankfully doesn't seem to have passed on to the rest of the family), so I thought we could give it a go.

Setting up the coloured water

The idea is that you take containers of primary coloured liquid (we used food colouring) and adjacent empty containers.  Using strips of kitchen roll, the coloured liquid moves into the empty containers, with primary colours (red, yellow, blue) mixing to give secondary colours (green, orange, purple).  We've done lots of colour mixing before with food colouring, including simply mixing coloured water to see what colours are produced, melting combinations of coloured ice cubes and making oil and water mixes, so the small boy has a pretty good idea of the colours which will be made by mixing but there's something mesmerising about seeing water moving from one container to another without anyone touching them.

Around a minute after adding the kitchen roll strips

The science behind the liquid moving is capillary action, whereby liquids can flow in small spaces even against gravity due to adhesion (the molecules of the liquid interacting with the adjacent material) and cohesion (the water molecules attract each other).  If the gap is sufficiently small, the combination of cohesion and adhesion cause the water to move, and it's this same mechanism that plants use to move water from the ground to the leaves (assisted by evaporation of water from the leaves).

Colours starting to mix

I set out six glasses in a circle (well, he corrected me to note that it looked more like a hexagon...) and put food colouring in alternate glasses (red in one, yellow in one and blue in another).  I gave him a beaker of water and a pipette to dilute the colours and almost fill the glasses.  This is a particularly labour-intensive way of doing the experiment as you could just pour, but he likes pipetting!  If you're trying this, make the glasses fairly full with the coloured liquid as it's slower if the water has to climb higher out of the glass to get to the next one.

Secondary colours appearing

We then added a strip of kitchen roll (one piece, cut into six long strips) between adjacent glasses, and watched.  Very quickly the coloured liquid started to appear travelling upwards along the kitchen roll and then downwards into the empty glasses.  It took about 5 minutes for liquid to start dripping into the empty glasses and about 10 minutes to start to see the colour mixing.  He wasn't very interested to start with, but once the liquids started dripping obviously and he could see the colours starting to mix together, he thought it was quite interesting.  We went back to check on it several times over about 3 hours, although he was starting to lose interest towards the end.  By 3 hours, the level of the liquid in the formerly empty glasses was starting to get close to the same as in the original primary coloured glasses, with a nice green, orange and purple colour visible.

After almost 3 hours

It's not a quick experiment, nor one with a lot of hands-on participation as the idea is that capillary action does the work of transferring the liquid.  I also didn't try to explain capillary action to the small boy, but he did see it in action for himself, and it provided a bit of entertainment between doing other things.