Oil and water

The lab set we have has a suggested experiment of showing how oil floats on water.  It didn't sound like it would engage a two year old for very long, but I decided to try this with the small one.  I thought I could make it more fun by increasing the amount of pouring and shaking involved, and extend the science a bit to show how washing up liquid works.

We added a little blue food colouring to some water in one of our big test tubes.  The food colouring makes the water more visible (and he generally likes things that are brightly coloured).  You could easily do this activity without colouring, particularly if you used a more strongly coloured cooking oil e.g. olive oil. Normal food colouring is water soluble, so it stays in the water rather than mixing with the oil later in the experiment. If you don't have test tubes, a clear plastic drinks bottle with a lid would substitute here.

Blue water, sunflower oil and washing up liquid ready to mix

The small boy knows about using oil in cooking as he often 'helps' me in the kitchen, but I don't think we have ever talked about how it is different from water or why we add it to recipes. We had a quick chat about the oil in our big bottle comes from squeezing sunflower seeds, and he happily poured some sunflower oil from a small test tube into the big tube of water.  Pouring things seems to be a large part of the fun of 'doing science' (as he calls every activity with our test tubes and his lab coat), but I drew the line at giving him a full big bottle of oil to pour from because of the potential to be cleaning it up off the floor when he is trying to tip a little into a fairly small tube!  However, it doesn't matter if the pouring is a little overenthusiastic and some mixing happens, unlike when making the traffic light or rainbow rocket sugar gradients, so this one is more suitable for slightly clumsy little hands.

The oil separates and sits on the top of the water because it is less dense than the water. I asked him to look at where the oil had gone, and after an initial answer of 'into the tube' (which I guess is better than 'all over the table and the floor', and not wrong!), he looked more carefully and told me it was on top of the water. I explained that oil floats on water, like some of the objects in his sinking or floating game.

Oil layer on top of the blue coloured water

I suggested seeing what happens the tube is shaken, and tightened the lid a little more after he'd put it on. After some enthusiastic shaking with the lid firmly on, we looked and saw everything had mixed, and all the liquid in the tube was blue. I asked what he thought would happen if we put it back in the rack and left it for a minute. He didn't answer but put the tube down, and we watched as the oil reappeared at the top in front of our eyes which he noticed and told me about! He wanted to do it again a few times, and found the same thing happened.

...and a minute later the oil is back at the top
Once he stopped wanting to shake the tube, I suggested adding some washing up liquid. I had put some in a little water in a small tube and mixed it to avoid both giving him a huge bottle and having something too thick to pour quickly.  You do need quite a lot compared to the amount you'd put in a washing up bowl as there's a lot of oil in comparison to the grease you'd be washing up off a few plates. He added this to his big tube and then (with the lid firmly on again) gave it another shake to mix everything.

He put the tube back down in the rack and waited, but only a tiny layer of oil reappeared after 10 minutes or so. We talked about what you use the washing up liquid for (e.g. cleaning dirty plates) and how some of the 'dirt' on plates is grease/oil.  The washing up liquid helps the oil to mix with water and be washed away off the plates.

Water, oil and detergent mix with some foam on top from vigorous shaking

If you want a better explanation for yourself about how the washing up liquid/detergent works, it's because the detergent molecules have a hydrophilic ('water loving') end which is attracted to water molecules, and a hydrophobic ('water hating') end which is repelled by the water, but attracted to oil molecules, and therefore they allow the water and oil to mix.  I didn't attempt to explain it like this to the small boy as he doesn't really have the concept of molecules.

This wasn't a long activity, but he did enjoy the mixing, and liked seeing the oil reappear on top after shaking.  I don't think this is one of the 'science experiments' that he will ask to do over and over again like the red cabbage indicator as there's less pouring and pipetting which are his favourite parts of 'doing science', but it was still a fun thing to do and he probably learnt a little.