h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I suppose someone had asked this before, but I couldn´t find anything.
Imagine I take a glass bottle and pour water into it, which is 1 degree celcius. I then place this bottle in my freezer. The freezer removes heat/energy from the water, causing it to go below 0 degree celcius. The water expands as it freezes, and breaks the bottle. Now presumably breaking a glass bottle requires a certain amount of energy, what I don´t quite get is where exactly this energy comes from. Energy is presumably being removed from the system as it freezes, not added to it.

To piggyback on that post (and ask basically the same question), why does water expand when it freezes, rather than contract and become more dense? It seems counterintuitive.

Bomba's question is easiest to answer first.

Water in its liquid state is very unusual. It has a higher boiling point and melting point than other molecules of its size. The reason for this is hydrogen bonding, a state which arises from the combination of the dipolar O-H bond (the fact that the oxygen end of the bond carries a slightly negative charge and the hydrogen end a slightly positive charge), and the fact that hydrogen has no free electrons

The practical upshot is that hydrogen atoms are attracted to the oxygen atoms (it also works in N-H and F-H bonds) of neighbouring atoms, meaning water is actually semi-ordered. The hydrogen bonds pull molecules together for brief periods of time, before the molecules float off and briefly attach themselves elsewhere. All of this serves to elevate water's boiling boint from a theoretical -20degC to 100degC. And a good job too, otherwise we wouldn't be around on this planet and all the oceans would be vapour.

The hydrogen bonding does not operate in solid systems, and when water crystallises it does so in a fairly complex hexagonal lattice. Without the additional forces from the hydrogen bonding, then the molecules are actually further apart on average.

The answer to Xan's question, I suspect, is that the energy required to break the bottle was hitherto chemical potential energy - stored inside the liquid, similar to in a fuel. Only difference is that energy in a fuel is stored in covalent bonds, but in liquid water it's stored in hydrogen bonds.

B

As a postscript, I'm glad I've taught something today. Although, no doubt, Orcus or Al will come along and contradict me.

B

Is the snow affecting your school, Mu Beta?

I am at home today, yes.

B

Wouldn't some fo the energy released/used in breaking the bottle be the energy that went 'into ;' the water to freeze it... The err 'energy' that went into the 'system' to freeze the water, 'stored in the bonds of the water as its formed into ice'? which is then released err no that doens't make sense, because when the bottle brekas the energy of the bonds in the ice is sitll all there in the ice... isn't it? I'll get me coat

On the back of Mu's post my understanding is that the expansion of water/ice as it freezes is as stated. This page seems to explain things quite well - http://www.chem1.com/acad/sci/aboutwater.html . Basically ice has a more open and stable crystalline structure.

The energy to break the bottle comes from the water molecules rearranging themselves into the crystalline structure of ice and is mechanical. This property is one of the major component forces of erosion and is what destroys the roads in winter (ours here in North Wales are a mess and the council can't keep up!).

The glass of the bottle is brittle at low temperatures and without an escape the ice will exert enough force to break the bottle. If the bottle has no lid the ice will expand out of the top of the bottle.

Energy is taken out of the water to freeze it and as far as I know the energy to break the bottle comes from the resistance of the glass to the expansion of the ice. When the expansion energy of the ice exceeds the glass chemical bonds ability to resist the glass gives way catastrophically.

t.

Oh, B, you were doing so well up to this point...

>The hydrogen bonding does not operate in solid systems, and when water crystallises it does so in a fairly complex hexagonal lattice. Without the additional forces from the hydrogen bonding, then the molecules are actually further apart on average.<<

The hexagonal lattice of water is formed precisely *because* of the hydrogen bonding.

Otherwise - perfect

Most solids form by close packing of the molecules or atoms and are therefore more dense than in the liquid. Not the case for water as the hydrogen bonding is the dominant crystal packing force in this case - so water actually *expands* when it crystallises - as in that link above.

The OP is right that it required energy to break the bottle and that energy will actually detract and prevent the water from freezing in a real world situation. If you compress ice - it melts - there's a good experiment that we did in A-level physics where you get a big block of ice and hang two weights either side of it with a cheesewire connecting them. Watch as the cheesewire slowly melts its way through the ice block - but the ice refreezes above it, so after a while the cheesewire is actually embedded in the ice.

In your freezer, there is a driving force continually freezing the water and this overcomes the resistance of the bottle in the end.
The energy going into the system is that which heats the heat exchanger to cool the freezer. A slightly higher energy will be drawn at the point where the bottle needs to break to accomodate the expanding ice.

Good question by the way

Can I just add that when a solid crystallises it releases energy - this was of old called the 'latent heat of crystallisation' and is equal to the energy you need to put in to get the crystal to melt again, without its temperature changing.

In other words it's the energy that binds the crystal together. The crystal is at lower energy than a liquid state and so this heat is released upon crystallising.

So the latent heat is probably what you're after I guess.

Latent heat of evaporation is more easily visualised than latent heat o crystallisation. Latent heat is absorbed or released upon and change of state solid <--> liquid <--> gas.

Latent heat of evaporation is why we sweat. When we sweat the water evaporates and the energy required to turn a liquid into a solid is taken from our body heat. Thus this process cools us down.

Thanks, that sounds rather likely.

As water freezes, hydrogen bonds 'formalise'; that is, there are relatively few hydrogen bonds between molecules in liquid water. In ice, each water molecule can associate with 4 other molecules to form a diamond-like crystal lattice. (Remember, diamond is the hardest natural substance). These hydrogen bonds between water molecules are relatively long compared to the ordinary covalent bond between oxygen and hydrogen within the water molecule. Hence ice is less dense than water. Furthermore, when bonds form energy is released (exothermic), so I guess this is the energy used to crack a bottle.

Incidentally, Magnus Pyke's uncle (I think) worked for the MoD during WWII and designed an aircraft carrier made of ice/cardboard. Hence if this was holed by a shell, they could spray icy water over it and re-seal the hole. This was caused Project Habbakuk.

i only under stood 1/2 of all that, but isn't the bottle also having energy removed from it by the freezing process and when the bottle looses too much energy isnt the difference enough to brake the bottle?????

or am i a complete F**kwit

As the hydrogen bonds between water molecules 'formalise' in ice, the water molecules become more distantly spaced from each other. This causes an outward pressure which is the force that breaks the bottle.