Making secondary colours appear and disappear with oil and water

Following on from our oil and water experiment, where the oil floated on top of the water because it is less dense, I decided to try doing something similar but a little more colourful to make it more memorable for the small boy.

I wanted both the oil and water layers to be brightly coloured so we could watch the colours mix to form a different colour and then separate back into two distinctly coloured layers.  This is easy to achieve for the water layer as the food colourings we have are water-soluble.  For the top oil layer I decided to try baby oil which is colourless (it's a mineral oil product rather than a vegetable oil like we used before) and add marbling ink which is oil-soluble; we have a set of 6 Scola brand marbling inks which we used when making our planets.

As I wanted bright colours, but didn't want to waste too much food colouring, baby oil or marbling ink, we used small pots.  I used the clear film canisters that we used for our fizzes and bangs last month as they are small, easy to see through and - importantly - have lids that fasten tightly.

We made three separate mixes:
1. Yellow food colouring in water, blue marbling ink in baby oil
2. Blue food colouring in water, red marbling ink in baby oil
3. Red food colouring in water, yellow marbling ink in baby oil

We used the primary colours as I wanted to reinforce what the small boy has seen before when we've been mixing food colourings in water.  The volumes we used were a little variable as the small boy was in charge of pouring and squirting the liquids with some close supervision. 

With the first pot, I got him to give it a good shake with his thumb over the lid just in case, and we saw it mix to give a lovely bright green before separating back into yellow on the bottom and blue on the top in less than a minute.  I then asked him which colour we would get when we mixed blue and red and he correctly said purple, and that is what we saw.  For the third one, I asked what we'd get when we mixed red and yellow and he didn't seem to be able to tell me despite having made orange several times with food colourings in water.  Unfortunately he'd been quite heavy-handed with the red food dye and when shaken we didn't get a very good shade of orange - it still looked pretty red - so I think we might need to come back to this one another time and adjust the amount of food colouring.

Primary colour water and oil mixes

Photo taken just after shaking, with the secondary colours visible

This activity was a nice blend of a colour mixing activity which was quick and repeatable, and a bit of a reminder about how oil and water separate into distinct layers.  Our film canisters are still full of the coloured water and oil mix, and have had a few shakes since as it's quite fun to watch the colours mix and separate quickly in front of your eyes.  You could do this with other colour combinations, but I chose to mix primary colours to give secondary colours as I thought it'd link nicely to other activities we have done.