Science Questions

Why does water freeze from the top down?

Thu, 5th Sep 2013

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Pinchas Goldberg asked:

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?



Pinchas Goldberg



Laura - So, this is a really brilliant question. Itís all about how weird water is Icebergas a substance. Water, if you think about it, you know that ice floats. Thatís why your ice in your drink floats at the top. Itís 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 thanÖ

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. Itís all to do with the way that hydrogen bonds. So water bonds and it forms these things called hydrogen bonds. So, youíve 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, thatís when the cold water floats to the top and then eventually freezes.

Dave - So, itís sort of almost starting to freeze in little lumps and they're non-freezing. So, more of it is in the ice structure, so itís 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 itís 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, thatís something called super cooled water which you can see and itís 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, itíll freeze in amazing structures.

Pinchas - Is water unique?

Laura - I wouldnít like to say definitely unique because there can always Ė but itís 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 - Itís 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. Weíre just so used to water that we think ice floating is normal whereas actually, itís really quite strange.

Chris - Isnít there Dominic on some other planets? I mean, the people have said on Titan, Saturnís biggest moon, the pebbles that are made from the hydrocarbons that freeze on Titan would sink. They wouldnít float like the kind of ice that you get in the oceans here on earth.

Dominic - Thatís 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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As water starts to cool down it sinks because the molecules start to move in a way which reduces the disance between them due to their kinetic energy. As the water gets even more colder the atoms which make up the molecules start to bind together and that happens in a less dense configuration and that means the water becomes less dense and starts to rise. The coldest configuration means that water is in its least dense state. Pmb, Fri, 12th Jul 2013

Water is an exception to the usual rule that "heat rises".

Over a narrow temperature range around 0C, water bucks the general trend, meaning that ponds (and oceans) freeze from the top down, insulating the lower pond water from the atmosphere, and preserving life in the liquid water beneath the ice.
See: evan_au, Sat, 13th Jul 2013

If ice was to form sub-surface, it would float to the surface.

However, it is possible if you say put a freezer coil into a bucket of water, the ice would freeze on the surface of the coil.  Likewise, an ice cube may be able to skin over all the surfaces including the sides and bottom, then freeze the middle later. CliffordK, Sat, 13th Jul 2013

Pete's explanation seem pretty straight forward to me. And a density also represent a energy, doesn't it? So something 'dense' that you want to to transform into something else should cost you more energy, than doing it to something not that dense. And you can add that on earth you also have a atmosphere, with wind, transporting away heat from the topmost layer. yor_on, Sun, 14th Jul 2013

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