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Q & A: freezing 1/2 a bottle of water

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Most recent answer: 04/22/2015
is there a way to just freeze 1/2 a unopened bottle of water, soda or juice without freezing the whole bottle. Thanks
- scott taylor (age 52)
hilton head sc usa

Certainly. You can put the bottle in the freezer and check periodically until it's about 1/2 frozen. Of course you need to be sure that you've got some bottle (not glass!) that has enough flexibility that it won't break when the water freezes and expands. 

You may want a way to get partial freezing that doesn't require frequent checking. If you have certain types of bottled water (and maybe this works for some types of soda and juice) you can cool them to say -10°C and the liquid won't freeze. It's called "supercooled" and we have some extensive discussions of this effect. () Then if you remove the bottle and shake it a bit to get freezing started, it will partly freeze and then stop because that much freezing warms it up to 0°C. About the coldest you can get the supercooling to happen at is around -20°C, but that only leaves the water about 1/4 frozen after shaking.

For the colas and juices there's another way. The sugars and other solutes lower the freezing point. () As some of the water freezes that drives the solutes out of the ice, where they don't fit,  into the remaining liquid. That lowers the freezing point of the liquid more. So by very carefully setting the temperature of the freezer, you can tune it to the point where half the cola freezes and the other half remains concentrated liquid cola. In prinicple you could do this even with really tiny amounts of solutes, but then you'd have to tune the temperature just a very tiny bit below 0°C. With a more concentrated sugary solution you don't need such fine tuning, maybe somewhere around -2° C will be about right. You still might need to tap the bottle at times to get the freezing started, since supercooling would get in the way of keeping track of exactly what temperature you need in the long run.

Mike W.

(published on 04/18/2015)

Follow-Up #1: bottom half of bottle frozen

I was looking more for a way to freeze 1/2 the bottle. Not as a slushy, but the bottom of the unopened bottle frozen solid and the top 1/2 liquid so it is drinkable, and the bottom keeps the top cool as it melts
- Anonymous

In that case, what you might want to do is put the bottle in the freezer upside-down. Since ice is less dense than liquid water, it goes to the top.

For our answers involving colas and juices, this should give ice at the bottom (once you turn the bottle rightside-up, and concentrated cola at the top. For the slush formed after starting freezing from a supercooled liquid, you could sort of centrifuge the bottle swinging it by hand to get mostly ice at the bottom. Here's something to think about, and to test: which end of the bottle should you hold as you swing it around, if you want the ice to concentrate at the bottom?

Mike W.

(published on 04/22/2015)

Follow-up on this answer.