Why does water freeze from the top down?

05 September 2013

Question

We all know that heat rises. So if I heat a kettle of water, the hotter water rises to the top, and the colder water will fall towards the element, get heated & rise. How is it possible that when I freeze water, it solidifies from the top down? If the hotter water is near the top, why does it solidify first?

Yours
Pinchas Goldberg

Answer

Laura - So, this is a really brilliant question. It's all about how weird water is as a substance. Water, if you think about it, you know that ice floats. That's why your ice in your drink floats at the top. It's why we can ice skate on rivers although make sure the ice is fairly solid first.

Chris - Good job, you can ice skate on rivers rather than.

Laura - Rather than having to go down to the bottom. But I think probably, the wild life and the fish at the bottom of the lakes would quite like that as well. It's all to do with the way that hydrogen bonds. So water bonds and it forms these things called hydrogen bonds. So, you've got the oxygen in the middle of the water and then these two hydrogens coming off of it. And so, you're right. That as you heat water up, it becomes less dense and it floats. But then at about 4 degrees, there's this wonderful sort of turning point and actually, you're cooling it down, it gets denser and denser and then these hydrogen bonds come into play and it starts to make this structure, making it less dense. So, that's when the cold water floats to the top and then eventually freezes.

Dave - So, it's sort of almost starting to freeze in little lumps and they're non-freezing. So, more of it is in the ice structure, so it's bigger than it should be, so it starts to expand as it gets colder which is really, really strange.

Laura - Yeah, absolutely. The thing about temperature, you have to remember it's an average. So, things are going on at a different temperature.

Chris - There was an amazing episode of a David Attenborough programme where they were looking at life in the Antarctic and they show this very, very cold water coming down from the surface and then hitting the floor of the ocean, and then it just comes down and freezes in like a stalactite going down, hitting the ocean floor and then spreading as sheet of frozen water along the bottom. Because once it had enough ice there to start the freezing process, that then kick started the formation of a whole crystal and that whole nucleation process kicked in.

Laura - So, that's something called super cooled water which you can see and it's really cool. If you get water and leave it to stand, it can get colder than the freezing point without actually freezing. If you then pour out, it'll freeze in amazing structures.

Pinchas - Is water unique?

Laura - I wouldn't like to say definitely unique because there can always - but it's one of those really, really weird substances. You get different types of ice. So, in different temperatures and pressures, ice can form different crystals and there's all sorts of odd things that it can do.

Ginny - It's certainly very unusual amongst the kind of everyday things you'd see. So, if you had some oil and you froze that, oil ice would actually sink in oil. Now, we all think that would be really weird, but actually, solid should be denser than liquid so they should sink. We're just so used to water that we think ice floating is normal whereas actually, it's really quite strange.

Chris - Isn't there Dominic on some other planets? I mean, the people have said on Titan, Saturn's biggest moon, the pebbles that are made from the hydrocarbons that freeze on Titan would sink. They wouldn't float like the kind of ice that you get in the oceans here on earth.

Dominic - That's absolutely true. These moons of planets like Saturn, they have not water oceans, but oceans made of hydrocarbons, these materials like petrol and they behave in a way that most materials do. Their solid forms are more dense and they sink as compared to ice that floats.

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