Frozen pipes and physics...a teaser

MeldrewsMate

MeldrewsMate

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A friend recently discovered his flat in the Pyrenees had frozen up, several radiators had burst, and many more pipe joints had pulled apart or pipes split longitudinally. This got me thinking...

We all know that water expands as it freezes (by about 10%). This is the reason that ice floats.

We are also told that cold water has less energy than hot water, and that we must remove energy from hot things to cool them.

So if we remove energy from the water, from where does it get the power to do the sort of destruction seen there? Try pulling a soldered pipe out of a joint; try pulling a 1" threaded iron fitting apart and you'll realise the power involved.

If the water, before it froze, had more energy then why didn't it also pull fittings apart?
 
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In physics terms, saying breaking a pipe is hard work is that it actually takes a large force. But that doesn't necessarily mean that it requires a lot of energy.

Energy used in a physical process like that is equal to the force times the distance over which the force is applied, breaking a pipe doesn't have to go far at all. The pipe barely moves at all before it cracks, so even though the force required is quite large, it only acts over a tiny distance, and therefore it barely takes any actual energy. What little energy is required can come from the water itself.

There is really really interesting stuff about water and how the energy of each pair of water molecules varies with the distance between them.
 
Swwils said:
In physics terms, saying breaking a pipe is hard work is that it actually takes a large force. But that doesn't necessarily mean that it requires a lot of energy.

Energy used in a physical process like that is equal to the force times the distance over which the force is applied, breaking a pipe doesn't have to go far at all. The pipe barely moves at all before it cracks, so even though the force required is quite large, it only acts over a tiny distance, and therefore it barely takes any actual energy. What little energy is required can come from the water itself.

There is really really interesting stuff about water and how the energy of each pair of water molecules varies with the distance between them.
Click to expand...

I think you will find copper pipe is quite ductile, and will expand prior to bursting.
 
It's still a very low energy interaction.

It's almost possible to crack a pipe without using any energy at all, but in reality there is stuff that happens that makes it use a tiny amount of energy.
 
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MeldrewsMate said:
We all know that water expands as it freezes (by about 10%). This is the reason that ice floats.
Click to expand...

Most things contract as you cool them, however water contracts until it reaches 4 degs C, then it starts to get less dense

so a lake for example wont start freezing until the min temperature of the lake is 4 deg.

nothing to do with the thread though....
 
If it were just about more energy making pope's (edit: too good a typo to edit out) explode then you'd have to use much tougher pipes for hot taps compared to cold. Which you don't.

What's actually happening as water freezes is that the water is forming into crystals, which sheds a lot of energy. Some of that energy is lost as heat (yes, even water at 0 degrees C has a lot of heat energy, zero degrees C is still 273 degrees Kelvin) and some goes to move the molecules into the less dense ice crystals, or to burst pipes.

Apparently it takes as much heat to get a lump of ice from 0 degrees to a puddle of water at 1 degree as it does to heat that same puddle to 81 degrees. Physics is confusing.
 
Notch7 said:
Most things contract as you cool them, however water contracts until it reaches 4 degs C, then it starts to get less dense

so a lake for example wont start freezing until the min temperature of the lake is 4 deg.

nothing to do with the thread though....
Click to expand...


The average temperature of the Sea is 4 degrees

nothing to do with the thread either.....
 
It also takes 538 times the amount of energy to boil water at 100°C than it does to raise water from 99°C to 100°C

Its thermodynamics, which is normally considered a physical chemistry discipline. (It can be a pretty dull subject TBH)
 
IT Minion said:
What's actually happening as water freezes is that the water is forming into crystals, which sheds a lot of energy. Some of that energy is lost as heat (yes, even water at 0 degrees C has a lot of heat energy, zero degrees C is still 273 degrees Kelvin) and some goes to move the molecules into the less dense ice crystals, or to burst pipes.
Click to expand...

Thank you for that most reasonable of explanations. It appears that the crystals of ice take up more space than the equivalent number of water molecules, despite having less energy within them. The damage is done, of course, when two separate areas freeze in separate areas of the pipework, trapping a volume of water between them. As the crystals grow they 'compress' the trapped water into a smaller and smaller volume, and the pressure goes through the roof very quickly - easily high enough to split a copper pipe, crack open plastic fittings, and push iron fittings apart.
The only thing I'd correct you on is the use of the 'degree Kelvin'. The Kelvin, in common with the Celcius, is the full desciption of that unit of temperature, as distinct from the degree Centigrade, and the degree Fahrenheit. I've tried telling the BBC not to use degree Celcius, but it falls on deaf ears!

The latent heat of evapouration (water to steam) is about 2200 J/g, that to turn water into ice is much less, though still (from schoolboy memory) around 200 J/g. With the specific heat capacity of water at 4.2 J/g per C, the equivalent heat needed to turn ice into water at 0C would heat it from 0C to (200/4.2 = ) about 48 Celcius; not quite what you said, but impressive all the same (and based on 40 year old brain data).
 
MeldrewsMate said:
Thank you for that most reasonable of explanations. It appears that the crystals of ice take up more space than the equivalent number of water molecules, despite having less energy within them. The damage is done, of course, when two separate areas freeze in separate areas of the pipework, trapping a volume of water between them. As the crystals grow they 'compress' the trapped water into a smaller and smaller volume, and the pressure goes through the roof very quickly - easily high enough to split a copper pipe, crack open plastic fittings, and push iron fittings apart.
The only thing I'd correct you on is the use of the 'degree Kelvin'. The Kelvin, in common with the Celcius, is the full desciption of that unit of temperature, as distinct from the degree Centigrade, and the degree Fahrenheit. I've tried telling the BBC not to use degree Celcius, but it falls on deaf ears!

The latent heat of evapouration (water to steam) is about 2200 J/g, that to turn water into ice is much less, though still (from schoolboy memory) around 200 J/g. With the specific heat capacity of water at 4.2 J/g per C, the equivalent heat needed to turn ice into water at 0C would heat it from 0C to (200/4.2 = ) about 48 Celcius; not quite what you said, but impressive all the same (and based on 40 year old brain data).
Click to expand...

To add more detail the bursting usually takes place with the ice itself not trapped water.

As people touched on above, the energy of each pair of water molecules varies with the distance between them, approx. like this:
Schematic_of_the_Lennard-Jones_6-12_Potential.png

There is a distance where the energy is a minimum and that's the "natural" equilibrium distance between water molecules when there is no pressure. When the water is under pressure, the molecules get pushed together, so their actual distance will be a little closer than the minimum of the graph.

Bring in waters unique property that at its "natural" density at a constant pressure reaches a maximum at a certain temperature, around 4C, and that its frozen form ice is less dense than its liquid form.

If the water temperature is going to drop below 4C, the minimum shifts a little to the right, which means one of two things has to happen: either the water expands (if the intermolecular separation stays with the minimum of the graph), or its pressure rises (if the intermolecular separation creeps up the slope of the graph).

So put this in a pipe... As long as the pipe is OK, the water/ice cant really expand at all. So only thing to happen is pressure increase. The slope is steep! The force rises very quickly and so does the force on the pipe! At some point the force is so high it rips apart the bonds that hold the atoms/molecules in the pipe together and at that point the pipe cracks. They also almost always split they way they do because of hoop stress. Like a sausage.
 
Swwils said:
To add more detail the bursting usually takes place with the ice itself not trapped water.
Click to expand...

Thank you for that large graph; clearly size matters. Please elaborate on the meanings of the symbols, and the graph scales.
Your opening quote has me questioning some of your reasoning though, eg why do pipe freezing kits NOT burst the pipe if it's the ice that bursts the pipe?
 
MeldrewsMate said:
Why do pipe freezing kits NOT burst the pipe if it's the ice that bursts the pipe?
Click to expand...

Because you freeze the pipe over a very small length, and the pressure is released either side of the frozen area.

Burst pipes occur when long lengths/full systems are frozen.
 
I may get to see the resulting damage of all the science above. Got a 35 year old car that I drove into my garage in 2008 and left it there untouched until now. Just trailered it to a friend's house to put in his garage while I'm moving. When I originally put the car in garage it had antifreeze in coolant. Had a quick look under bonnet of car after taking it to friend's and the cooling system is empty. And there's a large water stain on the floor of my garage. :eek::eek::eek:

Haven't had time to investigate yet, but I'm fearing the worst. My own fault - should have drained it down, but never intended to keep it off road that long. Guessing that the antifreeze won't have remained effective for 13 years?
 
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