Moving liquids with siphons

I was watching a video of a siphon demo on my phone when I thought my son was doing something else, and he wandered over to ask me a question and caught a glimpse of the video.  He asked to watch it, so I showed it to him, and he was captivated.  He asked if we could try it, and I promised I'd have a think about how we could set up something similar.  He was unconvinced and asked if we could do it straight away.  I like to try and build on his interests, and as he'd already shunned my suggested painting activity, I decided we would give it a go.

Siphon setup

We found two empty plastic yoghurt pots and some plastic straws.  I put holes in the pots (you need a hole to fit the straw through).  These need to be at a height which allows the straw to be bent in an L-shape with the long end angled slightly downwards, protruding out of the pot so the liquid will drain out into another container below.  We sealed around the holes with the play dough I'd made for his brother which has gone gooey; blu tack would probably be better, but I couldn't find our pack quickly so I settled for what we had to hand.

I found containers to balance the pots on such that liquid siphoning out of one would drain into the second, and liquid siphoning out of the second would drain into a tub at the bottom.  There's no reason that you couldn't put more in a row, but I wanted to make something quickly and we only had two of the plastic pots.  The video had food colouring in the containers, and I decided that - whilst this carried a risk of being split everywhere - it made it easier to see the liquid moving and some colour mixing could happen as the siphons worked to add a bit more fun science.  We therefore put a few drops of liquid food colouring into each pot.

Just starting

So what's a siphon?  You need a tube of some sort, with the bottom below the level of the liquid in the container you are trying to drain.  To get it started, you either need the liquid level to be higher than the top of the siphon tube (as in this experiment) or to apply some suction to get the liquid moving.  Once it's going, a combination of cohesion (as in the capillary action in our walking colours experiment) and gravity keep the water moving until the container is drained.  In the case of this experiment, a second siphon starts to work once the water level goes above the bend in the straw, and this in turn drains the second pot into the final container.

Siphon running

Our setup was a bit hastily improvised and wobbly, with one tub sat on a pint glass which was balanced on a plastic bowl and the other on a plastic cup.  I made it a little more stable by sticking the plastic pots to their stands with some of the gooey play dough, but I strongly suggest that if time is on your side, you use a less wobbly and breakable combination of items to put the containers at the right height!

Anyway, it worked when I quickly tested it without food colouring to make sure it was lined up properly and wasn't going to leak everywhere.  When my three year old started it off by pouring water into the first container, it filled until it got above the level of the bend in the straw and the siphon started to pour, and then when the water level was high enough in the second container, the second siphon started to work too.  I'd added red to the first container and blue to the second, which gave a lovely purple.  I put yellow in the final tub and this turned the whole mix a rather swampy shade of greeny-brown at the end.  You could no doubt make a prettier combination of colours!

We ended up doing it 5 or so times before the small boy decided he wanted to change the setup so he caught the liquid at the end with the beaker he poured water from at the start.  This meant raising the level of the first two pots, so we ended up with it balanced even more precariously.  It worked nicely, but I did have to catch the taller tower from tumbling once when he caught the edge of the pot as he was pouring the water in!  He was really taken by it, and kept coming back and wanting another go.

Small boy's variant with the initial beaker used to catch the liquid at the end

We talked a little about it being called a siphon, and he noticed that the level of the water had to go just above the bent straw before water would pour through it, but that the whole pot emptied once it started going.  I didn't attempt to explain the science behind it beyond saying that once it got moving, gravity causes the water to flow downwards, but he got to observe some siphons in action so he's now aware that they exist. We've since done this a few times at his request, he clearly finds it fascinating!