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Name: Maria
Status:  student
Age: 20s
Location: N/A
Country: N/A
Date: 6/4/2004


Question:
I understand why pipes burst when they are frozen. I am trying to present an experiement to the class about this. Is there any experiments that you can recommend that would demonstrated why they burst? Something simple that I may do in the classroom.


Replies:
You don't say how old your students are, but anything that shows the expansion of water when frozen should do it. I am assuming you don't actually want things to explode in your freezer, but just to show the concept. Perhaps you could fill a couple of plastic water bottles and freeze them (I think milk containers work--it has to be somewhat flexible to expand rather than explode). Have a couple of non-frozen water bottles for volume comparison. For that matter, just freezing water in plastic cups and comparing to the same volume of liquid water would do it, too, since the ice volume will expand upwards in the cups, and students should be able to measure height differences.

Good luck,

Pat Rowe


Hello Maria-

1/2-inch plastic (PVC) pipe is cheap. You can afford to fill 1 foot with water with one end un-capped, and 1 foot with caps on both ends. The caps might not even need glue, just the normal friction of shoving them on. I'm sure the capped pipe segment will burst; maybe the un-capped pipe will burst too. The latter depends on whether the water sticks to the plastic as it freezes on the ends first, and on whether the water, as it freezes, expands in all directions independently, like a rigid solid, or merely expands in volume like a liquid. In that case the liquid may escape out the open end and the pipe won't need to break.

Be a little careful; plastic pipe gets brittle when cold and cracks abruptly instead of stretching, and then the shards are light but sharp. On the other hand, the expansion occurs very slowly, so it probably won't fling pieces. Until I found out how it behaves, I'd put it in two layers of plastic baggy, or a few layers of paper towel wrapping, or a larger plastic pipe, or something. A plastic bag will also contain any not-yet-frozen water which tries to spill out thru cracks.

I guess you need to allow freezing at a normal (slow) freezer rate; dunking in LN2 will make the plastic so brittle that it may crack even before the water inside starts freezing.

Normal anti-freeezes (ethylene glycol) would soften and start dissolving plastic (PVC) pipe, but isopropyl rubbing alcohol (70%) won't have that problem and will suppress freezing.

As for _why_ water expands as it freezes, I can't really think how to "demonstrate" that, other than playing with molecule- and crystal-lattice toys. Water molecules in an ice crystal must all be linked together in a regular latticework, where each oxygen in the middle of the molecule has two hydrogens pointing at it from the ends of two other water molecules. This is because of "Hydrogen Bonding". It's a rather spaced-out arrangement, shown in the text sketch below. On the other hand, in liquid water some of the molecules can, for some percentage of the time, be in close-packed positions (say, like nested boomergangs, or other) instead of those hydrogen-bonding positions, so the average density is a little higher.
..............
.       O - .
.H     /     .
. \   H      .
.  O         .
. /   H      .
.H     \     .
.       O - .
..............

Plastic disposable cups or 8-oz. PETE drinking-water bottles might make adequate demos, too. If your plastic cup is filled to the rim and covered with plastic film like "Saran Wrap", any expansion upon freezing will be evident by bulging or rising or cracking. The waters bottles with cap will probably just look a bit bloated.

If you can freeze water sealed in a copper pipe or refrigerator tube; the copper is soft enough to be interestingly stretched rather than broken.. Try taking a 3-10 foot length of 3/8" copper refrigerator tube, and coil it into a 2-4" wide helix so it fits into your freezer or LN2 dunk tank. Dunk it in deep water and get all the air out, then fold back one end sharply with a pliers, so the water can't drip out there. When you freeze this, the water will stretch it a little, and when it thaws the water can settle to the bottom of the newly wider tube, ready to stretch it more when it freezes again. After 10 cycles the bottom end of the tube should be stretched to a noticeably fatter diameter.

Make an identical piece that never gets this treatment, for visual comparison. To be sure the top end of the helix freezes first to seal the water in, surround the bottom half of the helix with plastic or paper to slow the influx of cold there.

Alternatively, if you practice adjusting the cooling distribution, you may be able to get an "ice worm" to extrude out of the open end. Especially if you first oil or wax the inside of the copper tube, for at least 1 foot near the open end.

I wonder if soap in the water helps it slip. If it works you can say: "This "ice worm" is the extra volume which the pipe must accomodate or else break open." Water only expands about 8% when it freezes, so a perfect ice worm from a 36 inch tube can only be about 3 inches long. It's likely to turn out somewhat shorter than that.

Jim Swenson



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