Supercooled Water and Towers of Ice

Solid, liquid, gas. The three states of matter are something I first explored in primary school and water was the best example. You can easily see water frozen in your freezer, and it spews as a gas from your kettle. But if you mess with these normal states, you can do some fun and strange things with something as everyday as water.

We’re all taught in school that water boils at 100°C and freezes at 0°C. This range was defined by a Swedish astronomer(!) called Anders Celsius when he devised the temperature scale that bears his name and which is also known as ‘centigrade’. It’s a very useful way of thinking about temperature[1].

Rather awesomely, there is a way to cool water below 0°C without it becoming ice. The process is called ‘supercooling‘. To try it you’ll need a bucket, some bottled water, lots of salt (we used 700g of the stuff doing this), and a big bag of ice.

Supercooling involves lowering the temperature of a substance but preventing it taking a solid form. In the case of water you can supercool it way down (to about -45°C if you have the right kit) – just so long as you can stop ice crystals forming. Ice forms around ‘nucleation sites’ – places where ice crystals can get a foothold and start growing. These sites are usually impurities in the water or on the surface of some other material that its in contact with. So if you can get very pure water in a very smooth container, you can take it below freezing but keep it liquid. For that reason you should use plastic bottles of mineral water, but filtered water will work too. We used both here.

Instructions

1. Put some water into a bucket and pour all the salt in. Mix it up really well, and try and make sure the salt is all dissolved.
2. Stand the bottles of water in the bucket and pour in the ice cubes until they almost cover them. You want to cover the bottles just enough that you can still pick them up and also turn them.
3. Mix it up! Salt reduces the freezing point of water and so the ice will melt quite a bit as this goes along. You want to create a bucket of briney slush. This will let the temperature get very low.
4. Now you need to leave your bottles alone. Every ten minutes or so, go and rotate them gently. This is just to make sure the water cools down evenly and is cooled throughout. After about 45 minutes the process should be complete. You should be able to feel that the bucket is very cold to the touch – ice will be starting to form on the outside.
5. Now you can play with your supercooled water…

Playing with Supercooled Water

Here comes the fun part! Note that if this stuff isn’t working for you then you probably need more salt and ice – or to give it more time to cool down.

Supercooled water is just waiting to turn into ice. If you gently pick up one of the bottles and then violently shake it, it will almost instantly turn into ice. Shaking it creates air bubbles in the bottle on which ice crystals can form. Once a few have started, it’s a runaway process since the best place for ice crystals to form is on other ice crystals. This is pretty amazing and I couldn’t get a good video or photo of this happening – it was too quick.

What we did a lot of was make ice towers! It’s a seemingly magical process that my four-year-old thought was brilliant. To do this you simply take an ice cube and pour supercooled water onto it. (It’s best to place this on something to contain the excess water!). As the water touches the ice it quickly crystallises and starts to create a tower, building upward as you pour. It’s pretty cool (ahem!). Again: the best place for ice crystals to form is on other ice crystals.

The towers were quite beautiful too, made of pure, transparent crystals. They were quite solid for a minute or two and then quicky disintegrated (it was 22°C in our house).

Why Salt?

Salt is used to grit roads in the Winter because it lowers the freezing point if water and makes the ice melt. In the process it actually lowers the temperature further. If you have a thermometer handy then you can watch the water in the bucket drop in temperature as this experiment progresses. This is why this method works to supercool the bottles – the temperature quickly dropped to around -8°C in our bucket and the slushy mixture makes good contact with the outside of the bottles for quick cooling of the water inside. I suppose this might work in the freezer but opening the lid our of chest freezer would disrupt the cooling and frankly the slushy brine is half the fun!

Science at Home

My four-year-old loved doing this. We talked about molecules slowing down and ice forming. We talked about the Winter and frozen puddles, road gritters, and cold drinks. Mixing in the salt and ice is fun and making ice towers blew her away. The 45-minute wait was the hardest part for her – so we made glowing jelly at the same time!

I’m not sure if supercooling is necessarily something that young kids will take away from this but there’s lots to understand about ice and freezing. The ice towers were very exciting and next time I do this I’ll have some gloves ready for the kids so they can do the pouring and shaking – the bottles are too cold otherwise.

I’ll leave you with another ice tower video:

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[1] I cannot fathom why anybody prefers Fahrenheit, which starts with frozen brine at 0°F, the freezing point of water at 32°F and it places the boiling point of water 180° higher at 212°F. Because those are all such handy numbers. Human ‘blood-heat’ was supposed to be at 100°F but things got rearranged – so now it seems odd. Sheesh.