Pressures involved with drinks in a freezer

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Offline kaukcz

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Pressures involved with drinks in a freezer
« on: 24/05/2008 03:58:14 »
We all know that when you put a sealed container with a good amount of liquid in the freezer is likely to "explode".  For the sake of argument lets talk about a full 2 liter of Cola.  Would it make the container less likely to explode if you squeezed out the air in the container before placing it in the freezer?  My thoughts are that if you did so, the container tries to return to its natural shape and that lowers the pressure in the container so it might help. Also since the container contains the same max volume in either scenario, and there is no air to take up space the liquid can expand further unimpeded.   However, the other thought is that the air provides a "cushion" because it is compressable where as the liquid is not.


Offline science_guy

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Pressures involved with drinks in a freezer
« Reply #1 on: 24/05/2008 04:58:00 »
most drinks have a large amound of water in them, which is the main agent causing the "explosion" of a frozen drink.

The Hydrogen bonds formed by water are at their strongest during the solid state, and they naturally form crystal structures, which take up more volume than the liquid, because of these forces.

so in a sealed can of soda, with no room for expansion, then the pressure inside the can will become to great to contain the formation of these structures, causing the can to "explode".

but assuming the same max volume, the only effect added air would have is increased pressure.  the concept of a "cushion" is just imaginary, since the air itself also adds to the effect of pressure.

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Offline Bored chemist

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Pressures involved with drinks in a freezer
« Reply #2 on: 24/05/2008 13:11:49 »
Actually the air does act as a cushion, but probably not enough to help much.
Lets ignore the facts that fizzy drinks have CO2 etc. in them and that air is soluble in water.
When the water gets cold enough it will freeze. This causes it to expand and, for any sensible pressure that you could get in a pop bottle, a freezer is cold enough that all the liquid will freeze if you give it time.
From that point of view you can imagine the ice and water as acting like a simple piston. 2 liters of water when it freezes will expand by about 10% ie about 200 ml. If there were a 400 ml airspace above it, this air would get squashed from 400 ml to 200 ml. Boyles' law says the pressure would double.
If there were 220ml of air then it would get squashed into 20 ml and the pressure would rise 11 fold. (of course the plastic would strech a bit so the pressure rise would be less than this)
If the bottle had 1 litre of air in it (A 3 litre bottle but only 2/3 full) the pressure would rise by just 1000/ (1000-200) ie just 25%

However, for any practical bottle of pop, there wil not be enough air to accomodate all the expansion of the water as it freezes. The rise in pressure will lower the freezing point of the water slightly so if the freezer werer only just below freezing the water wouldn't all freeze. But this effect is small- something like 1/100 of a degree for each atmosphere of pressure so a typical freezer at -18C (0F) is going to freeze water provided the pressure doesn't exceed something like 1800 atmospheres- that pressure simply isn't going to happen in a plastic bottle.

The other thing that can (and does) happen is that thebottle streches.
If it can expand by 10% or so without tearing then that's what will happen. If it isn't that elastic, the bottle will burst.
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