Exploding Science Fair Project?

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Did we do something stupid? You be the judge.
My 9-year old son is working on a project for his 4th grade science fair. The project involves showing how water expands when it freezes.
We were looking for something that would really show the power of freezing water. We went to the hardware store and bought two pieces of threaded pipe. We capped one end of each pipe with a threaded steel cap and then filled them with water. We then capped the other end with threaded caps. One pipe is brass and the other is steel.
We then placed them in the backyard to freeze.
What is going to happen? Will they burst explosively, split the pipe open or push the threaded caps off slowly?
Anybody want to venture a guess.---Marc
 
Science cataqlogs used to sell something called an "ice bomb" which was made of cast iron - which has a much lower tensile strength than steel. I suspect you are in danger of damaging whatever is nearby your project. /ubbthreads/images/graemlins/ooo.gif
 
Not sure if you will have enough volume of water to get the effect you want. When household pipes freeze and burst the pipe simply swells and then splits. You don't have to worry about an "explosive" project. /ubbthreads/images/graemlins/grin.gif

A sealed hard plastic item FULL of water will burst when frozen much easier than metal. I'm sure you know to stay away from glass so won't go there.

Good luck and have fun with the project.

dimwatt /ubbthreads/images/graemlins/au.gif
 
Threaded PVC fittings work much better!

The steel pipe, without quite a volume of water, may simply not do anything. Since freezing under pressure is less likely, it may not freeze as the thermal gradient rises and the pipe thread strength is sufficient to contain it.

You can see this more easily in a soda pop bottle since it is clear. Place 2 plastic unopened pop bottles in the freezer, checking frequently, after 1-2 hours or so and before they freeze (depends on how cold your freezer is), pull them out, and slowly open one. It will almost instantly effervesce and then the top 1/4-1/2 will freeze in slush as the accumulated pressure is released.

It makes great Coke slushies and is a great school demonstration.

Others here are better thermodynamic engineers and can explain it better than I, but I thought I'd get it rolling.

Bill
 
More than likely the pipe will split along a unseen seem.
If you can I would get a product called PEX. It can expand a great amount without bursting. After it thaws you can take a heatgun/hair dryer and heat it up till it is clear and it will return to original memory.

Just a thought.

mut
 
The pipe also won't burst because of its shape. The forces of the freezing water will not be distributed evenly and the pipe should burst down the side but not explosively.

For the experiment to work well you will need a couple of things.

The main one is a fixture in the shape of a sphere. Why? You want the forces to build up evenly across its shape so when the material fractures it fails all at once across its surface. That is really what causes the ice bomb to work. If you had a cast iron square box it wouldn't work. It would fail at a corner but the faces would not break. They might buckle but they won't break. The sphere is the secret to getting the forces right.

The second thing you want is distilled water. Why? You want as few impurities in the water as possible. You don't want to start crystal growth until the water is way below freezing. Ice crystals need something to help start them growing. A speck of dirt or even a physical shock. You want it to solidify almost instantly. This is similar to the effect described above when you open the super cooled soda (but for slightly different reasons). You want the pressure change to be sudden for maximum effect. The down side to this is that you'll have to get the object a lot colder than 32 deg for it to work. Maybe as low as 0. It will also be quite dangerous to be near because you don't know when it is going to go off. The common way is to put it in a bottle of liquid nitrogen or surround with dry ice. Its guaranteed to freeze and release the pressure quickly.

Oh.. If you have an empty 16-20oz soda bottle put a chunk of dry ice in it and screw on the cap. Be sure to do this in an open area and stay back from it once you tighten the cap!!! In about 5 minutes your patience will be rewarded by a sound that would compete with a 10ga shotgun blast.

Enjoy, be safe, and I don't recommend doing any of these things so don't sue me if they go horribly wrong. Not to mention how jumpy schools are you don't want to take anything there that could explode. You might be facing a felony.

Chris
 
the (low) compressibility of water helps reduce the danger. pressure vessels are sometimes tested to failure by filling with water and pressurizing. it's safer than testing to failure with air. the reason is that it takes less energy to raise the pressure on a volume of water, and so less is released when the vessel fails.

I predict no apparent damage. /ubbthreads/images/graemlins/smile.gif
 
Water though is one of the only materials that swells when it freezes. You will get a very high pressure and a violent explosion of the containment vessel when that happens (assuming the correct shape and materials of construction).

I've seen it in person. Its amazing what a little water can do... You wouldn't expect a little water to freeze and cause a cast iron ball to explode but it can. The glass flask the liquid nitrogen was in was history.

Chris
 
I have to agree with watt4. Pressure vessels require periodic re-certification. This is done by hydrostatic testing, which is done by filling the vessel with water and increasing the pressure to the specified level. Water's compressibility, compared to gasses, is so low that very little energy is stored and when failures occur, very little energy is released. Pressure vessels are typically constructed from ductile materials. Cast iron is typically very brittle- think WW2 "pineapple" grenade, and could produce some low velocity fragments. The risks associated with your experiment are IMO negligable.

Larry
 
I stated in my first email that a pipe won't explode due to its shape. I went on to say how you would create an explosion and why.

The reason why hydrostatic testing is so safe is not only because of the compressibility of the water (or lack there of) but because the pressure source has a very low volume. This limits the expansion potential to the volume of the pressure source (in most cases a small positive displacement pump). Any slight failure of the item under test and the fluid pressure is released. No big boom. Just a mild sharp pop.

The experiment with ice is not the same. The pressures available are WAY higher than you'd expect. The pressures are in the 1000s of BAR/sqin (BAR not PSI) for freezing water at say -20 deg. This is the from the best of my memory.

Also the expansion ratio for water to ice is about 10% increase as it freezes.

My point? The pipe wont do much as I said before because of its shape. A sphere of the correct material with high purity water and a low enough temperature will make a good sized pop.
 
Place your bets. /ubbthreads/images/graemlins/popcorn.gif /ubbthreads/images/graemlins/popcorn.gif

mut
 
My bet is if they freeze they will split and the caps stay on undamaged. No explosion.

I wonder how cold its supposed to get there tonight and if the pipes are on the ground or not. They might not freeze at all. If the temp gets down into the low to mid 20s and the pipes are not laying on the ground there should be something to see in the morning. /ubbthreads/images/graemlins/smile.gif

Chris
 
Ice bombs are really cool to watch (safely of course) but will only explode with force if they are rapidly cooled (like dropping them in liquid nitrogen. As for what is going to happen with the pipe, you are all very safe. Depending on the specific either nothing, 2, or 3 will happen. Thankfully number on will not unless it starts raining liquid nitrogen (and then the pipe is the least of your problems.) /ubbthreads/images/graemlins/grin.gif

-Mike

EDIT: My bet goes for the ice bending the brass pipe (if it gets cold enough) but not bursting it.
 
Xrunner, /ubbthreads/images/graemlins/thumbsup.gif The bending will be due to concentricity issues with the extrusion process. I will also predict it will bulge and not split! (Of course, with Chinese pipe... /ubbthreads/images/graemlins/icon3.gif

Larry
 
Pretty hard to do a good prediction as we don't know exactly how cold it will get in your backyard. I will stick with they will burst at the seem.(bulge first then split)

mut
 
I wouldn't bet on either of the pipes breaking. Most steels, and all brass is pretty maleable, in other words it can be stretched by the forces. It is castings that break. They are made of large crystals are very hard,and cannot move relative to each other. Materials the bend before they fracture are unlikely to break if you freeze them. Ceramics, glass, and castings will break before they bend much, so are more likely to crack when the water freezes. The only things that will break are those that will fail if you strain then by more than say a part in a thousand. FOrgings bend, and at the end of the day, pipe making process for steel, brass and copper are effectively a forging, not a casting
 
I still predict minimal to no swelling of the pipe with no bursting or splitting. Had too many water pipes freeze in the MidWest! /ubbthreads/images/graemlins/yellowlaugh.gif
 
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