What happens when water freezes?

26 July 2015


When water freezes it expands- what happens to the air in the water when this happens?


We put this question to Ewen Kellar.

Ewen - This is an interesting property of how gases interact with water because as water gets colder, it can dissolve more gas in it which is kind of the opposite to what you normally think when you're trying to dissolve sugar in water, where you've got to get warmer water to get more sugar in or more salt in. If we take a fizzy drink for example, where you've got carbon dioxide dissolved, when you actually have a litre bottle of such a drink, it actually contains about 6 grams of carbon dioxide which is about 3 litres if you were to let it out into the atmosphere.

Chris - That's about as many grams of sugar as there are dissolved in those fizzy drinks as well, isn't it?

Ewen - Yeah.

Chris - A slightly different concept, but...

Ewen - Yeah, absolutely. So basically, you can pack all that carbon dioxide into the water. Although, you generally need a bit of pressure to do it because that's why the carbon dioxide wants to keep coming out after when you undo the cap. But what happens is, when the water then starts to freeze, all the spaces, the cavities between the water molecules all start to disappear because the water molecules start orienting themselves, start to crystallise. And that basically starts kicking the gas out of the liquid and it generally starts to form tiny little gas bubbles and you'll see this in your ice tray. In a bottle of fizzy pop, if it's a plastic bottle, then there's no problem because what happens is, all that gas gets forced into that tiny little space at the top of drink. So, there's a very, very high pressure of gas sitting there when it's cold. But when it then warms up again, if you defrost your pop without taking the cap off then that will gradually all dissolve back in. However, beware don't try this with metal cans because the expansion of the drink will basically crackle in the side of the can and liquid will go absolutely everywhere and make an awful mess.

Chris - It would be a very strong alcoholic solution though, if it was a beer you did it with, wouldn't it?

Ewen - Yeah.

Chris - There'd be plenty of alcohol left behind because that wouldn't freeze, would it?

Ewen - Absolutely.

Ginny - I did that once by accident. I put some cans of coke in my fridge but then they got pushed right to the back of the fridge where it's really, really cold. I came back to the fridge a few days later and it was absolutely covered in sticky brown gunk and I reckon what happened is, because it was so cold, the coke can froze and burst.

Chris - You can get quite another interesting phenomenon with this - and we investigated this on the Naked Scientists a few years ago - which is that you can sometimes get a bottle of drink out of the fridge - especially if someone has been a bit over zealous with the thermostat and made the fridge a bit too cold - and it looks liquid. You pop the cap and it instantly starts to freeze and it does it from the neck downwards and then, before you know it, you're holding a completely frozen bottle of what was formally a fizzy drink. What happens, we think, when you do this is that you get the liquid being so-called 'super cooled', as in it's way below its own freezing temperature, but because you haven't disturbed it and the bottle is nice and shiny inside, there's nowhere for the first crystal of ice to begin to form. And then as soon as you pop the cap, you release some gas, some bubbles form at the surface and they act as a little disturbance in the shape of the liquid which gives you a crystal or the water touches the glass higher up and you then get a first crystal forming and then it starts to freeze all at once. It's pretty spooky because you go from having what you thought was going to be a nice cold beer to basically a bottle of ice, but there we are!

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