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Author Topic: Just why is ice brittle?  (Read 8866 times)

Offline Keef

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Just why is ice brittle?
« on: 12/01/2006 17:56:03 »
Why is ice brittle?

I have been trying to find the answer for some time to no avail.

I know that water uses the hydrogen bond.

That is where the two hydrogen atoms share their respective orbiting electron with the oxygen atom leaving a 'bare' postively charged proton/neutron nucleus combination. This in turn makes the hydrogen nucleus atrractive to the other oxygen in other molecules of water (polar valent bonding).

When we lower the temerature the water molecules line up to form a crystalline lattice thus water expands when frozen.

now it seems similar to ionic bonding. the orbiting electrons a 'tied up' as it were. But when you you try to cut a block of ice with a hammer and bolster it doesn't cleave like slate for instance.

 The positive and positive, negative and negative forces force line up and repel thus splitting the slate.

It seems to be more like co-valent bonding as in some ceramics. They are very hard and tend to shatter when struck in this manner.

This is further confusing as I thought slate was a ceramic?

so there must be both co-valent and ionic bonding in ceramics right?

Help!! it's driving me nuts!!

why is ice brittle?[?][?]


 

Offline rosy

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Re: Just why is ice brittle?
« Reply #1 on: 12/01/2006 18:25:46 »
Slate has stronger bonds in the horizontal than in the vertical direction, so it splits into horizontal sheets.
Ice has a more symmetrically 3D bonding pattern, like many salts, so breaks in more different directions.
 

Offline daveshorts

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Re: Just why is ice brittle?
« Reply #2 on: 15/01/2006 10:59:48 »
Imagine a big structure where everything is strongly bonded to everything else, and where nothing can move or flow (the bonding could be ionic, metallic or covalent) if you give the structure a load of energy by hitting it, the only way this energy can be absorbed is by breaking bonds. So the structure will break and the material is brittle.

Now imagine the same structure where it can flow slightly. This will take up a lot of energy so instead of breaking it will just deform a bit and carry on pretty much as it did before.

There are several different ways of achiving this ability:

Because the metallic bond is not directional and allways attractive a metal atom can move one place down the lattice and still be attracted.  this is quite easy to achieve in metals, so many metals are not very brittle, an can even be quite ductile (like copper). (in a ionic solid or ice, if it moves one place down the row you will get + next to + and it will be repelled so breaking)

Plastics can be tough because they have 2 strengths of bond the strong ones down the chains and weak ones between the chains, so they can absorb energy by moving the chains past each other slightly rather than breaking entirely. If the bonds between the chains are too strong they can also be brittle however.

As rosy says the fact that slate cleaves is just because it is made up of lots of sheets that are not stuck together very well. You could have a non-brittle substance that cleaved quite well - eg a pile of sheets of polythene.
 

Offline Soul Surfer

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Re: Just why is ice brittle?
« Reply #3 on: 18/01/2006 19:46:40 »
It all depends what you mean by brittle.  Do you mean it is relatively easy to break or do you mean that when pushed to the limit it breaks reather than flows like pitch or soft metals like lead and gold  (diamonds are brittle but very strong)

These properties depend on both the form and size of the crystals when the material solidifies

Very often solids form many small crystals (polycrystalline) and their srength depends on the shape and sizes of the crystals from which they are formed and How well the individual crystals stick together.  when you break them they crack rathe randomly around the crystal boundaries.  Single crystal materials are usually much stronger so one of the reasons for the strength of the ice is the size of the crystals.  When broken crystals will trnd to cleave along planes

Water as always is very anomalous.  Its liquid form is really a polymer and its first crystalline form is very loose as the fact that it expands on frezing shows.  There are many different crystalline forms of ice depending on the temperature and pressure when they are formed (not all of them float on water)

old ice at low temperature can recrystalise to become much stronger than fresly formed ice.

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« Last Edit: 18/01/2006 19:49:18 by Soul Surfer »
 

Offline Keef

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Re: Just why is ice brittle?
« Reply #4 on: 20/01/2006 14:56:41 »
Thank you all for the answers you have given, they are most helpfull. Okay lets clarify things a little.

The question was set by my lecturer in freshman engineering materials.

 Now to clarify I understand metallic, covalent, ionic, polar covalent, hydrogen bonding etc as stand alone explanations.

 I understand hardness testing as resistance to latice movement (dislocations etc flowing in metals and catastrophic failure in ceramics i.e very little plastic deformation = brittleness) Also how bonding energies that relate to these properties.

So what i'm really trying to understand is why these 'cystalline' structures form and if when we remove energy i.e cool the water do these bonds become stronger? is it an example of the polar covalent bonds lining up (I think so from previous study and post) and if so are the bond directional in nature (again I think so) but as they are polar covalent (similar to ionic in nature i.e electrostatic) (I'm guessing a little here)this why ice has a brittle characteristic little or no plastic deformation. so i release i was not correct in comparing the brittleness/hardness in ceramics to ice.

so what do you think is it due to the weakness of the bonding usied and how?

thanks in adva\nce

 

Offline daveshorts

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Re: Just why is ice brittle?
« Reply #5 on: 20/01/2006 21:53:07 »
The oxygen atoms are all slightly negative and the hydrogens are slightly positive. so if you cut the crystal in some cunning way it will look like

+ - + - + - + - +
- + - + - + - + -

if you try and deform this slightly it will resist but if you manage to deform it enough you will end up with the situation:

+ - + - + - + - +
+ - + - + - + - + ->

as you can imagine this will repell, causing the two lines of atoms to move apart. An Ionic ceramic will behave similarly.

As there are no other ways for the structure to deform ice is brittle.

In some ways the reason the structure is brittle is that the bonds are strong and can be repulsive.

Most tough things have some bonds that are allways attractive between 2 lines of atoms as you slide them along eg metallic bonds in metals and Van de Waals forces between the polymers in a plastic.
 

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Re: Just why is ice brittle?
« Reply #5 on: 20/01/2006 21:53:07 »

 

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