Why does water expand into ice?

[sunspot]

Here is an age old observation. If anyone has had a glass of ice water they will notice that the ice floats on top. This is due to water expanding when it becomes ice making it less dense than liquid water. The question is, why does this anomaly occur?

[Krz]

each water molecule is two hydrogen atoms bonded to one oxygen atom. because of how the atoms share electrons, a water molecule is slightly positively charged at the hydrogen atoms, and slightly negatively charged at the oxygen atom. The molecule's charged ends attract the oppositely charged ends of other water molecules. In liquid water, as molecules slip-slide past each other, bonds form, break, and reform. But by the time water has cooled to 4 C., the molecules energy has dropped enough that they are very near one another. So each H2O molecule forms more stable hydrogen bonds, with up to four other molecules.

[Nevermore]

In other words, ice is crystaline, thus, empty space.

[silkworm]

Water molecules are physically closer together as a liquid than a solid. Meaning that if you had a glass of water and a glass with the same volume of ice there would actually be more water molecules in the glass of water than the glass of ice. Liquid water is more dense than ice, that's why ice floats.

 

It's because the organization of the lattice structure of ice uses up more empty space than do a collection of water molecules.

 

Also, here's another well known tidbit. If you're at a freshwater lake that is frozen at the top, the water at the bottom of the lake will be 4'C because that's the temperature at which water (in any form) is its most dense.

[sunspot]

The answer is due to the unique nature of hydrogen bonds.

 

As the name "hydrogen bond" implies, one part of the bond involves a hydrogen atom. The hydrogen must be attached to a strongly electronegative heteroatom, such as oxygen, nitrogen or fluorine, which is called the hydrogen-bond donor. This electronegative element attracts the electron cloud from around the hydrogen nucleus and, by decentralizing the cloud, leaves the atom with a positive partial charge. Because of the small size of hydrogen relative to other atoms and molecules, the resulting charge, though only partial, nevertheless represents a large charge density. A hydrogen bond results when this strong positive charge density attracts a lone pair of electrons on another heteroatom, which becomes the hydrogen-bond acceptor.

 

The hydrogen bond is not like a simple attraction between point charges, however. It possesses some degree of orientational preference, and can be shown to have some of the characteristics of a covalent bond. This covalency tends to be more extreme when acceptors bind hydrogens from more electronegative donors

http://en.wikipedia.org/wiki/Hydrogen_bond

 

Here is what happens. When water molecules form, the oxygen takes the lion's share of the electron density in its two covalent bonds with hydrogen. The oxygen is stabilized by this extra electron density, while the hydrogen is given a potential. The oxygen is stabilized because it is able to complete its outer octet of 2P electrons. In liquid water, because the oxygen went through so much trouble to get this extra electron density away from hydrogen, it does not want to share it with any of hydrogen's cousins on other water molecules, or it would lose some of its octet stabilty. So the selfish oxygen will twist away making it hard for hydrogen to form linear bonds.

 

The partial covalent nature of hydrogen bonds implies that the strongest hydrogen bonds need to be linear, to line up the orbitals. Angle deviation away from linear will cause the hydrogen bond energy to drop off quickly. The twisting away of oxygen weakens the hydrogen bond requiring hydrogen come in closer to lower its residul potential using what is left, i.e., electrostatic attraction. This cause water to contact upon melting.

 

As we chill water to the freezing point, the oxygen can no longer twist away allowing the hydrogen bonds to develop their covalent character. This makes the hydrogen bonds linear. This allows the hydrogen to lower its potential at a further distance due to the linear nature of the bonds making them partially covalent, therefore defining higher bonding energy.

 

This unique molecular observation within nature demonstrates that although water defines a dipole, the hydrogen has more potential for the electrons of oxygen than the oxygen has potential for the positive charge of the hydrogen. If the potential was the same, both charges would attempt to lower their mutual potential. The cooling into ice would remove resistance allowing them to attract the closest and ice would contract, like all other polar solids. The opposite happens.

 

With a potential imbalance, the hydrogen really wants some and the oxygen plays hard to get, causing the hydrogen to have to move in closer. When they freeze the oxygen can not longer resist, allowing the hydrogen to use the optimize partial covalent hydrogen bonding distance. This will slightly destablize the octet stability of oxygen. As such, oxygen will share, but only at a longer distance. If we pump in some energy, the oxygen will again twist away, to regain its octet stability, causing the hydrogen to have to move in closer to lower potential. The result will be water contracting.

[insane_alien]

Sunspot: that was mostly superfluous twaddle.

 

in a water molecule, Oxygen is the most electronegative. this means that oxygen will have the bonding electrons around it more often than either of the two hydrogens. effectively this makes the oxygen behave as if it was slightly negatively charged and the hydrogens as if they were slightly positivel charged. This is called polarity.

Since opposites attract the oxygen in a passing water molecule will be attracted to the hydrogen(s) of the water molecule we were talking about.

This happens to be a relatively strong intermolecular bond and it makes the expanded crytaline substance that is ice. it also stops water boiling at -158*C or something like that.

[sunspot]

The problem with that explanation is that ammonia has a dipole and also forms hydrogen bonds but does not expand on freezing nor does anything else natural, except the metal antimony. A simple charge dipole will get closer and closer as heat is taken away since there is less molecular movement resisting the attraction. This should continue upon freezing contacting further upon further cooling. Yet water pulls the charge away from each other during freezing so it can expand. This should be endothermic, if only charge attraction mattered, but it actually gives off energy even with the charge potential increasing. In other words, the charge dipole is worse off during frezing, yet the expanded water is more stable. It is not as simple as a charge dipole.

[Klaynos]

The problem with that explanation is that ammonia has a dipole and also forms hydrogen bonds but does not expand on freezing nor does anything else natural, except the metal antimony. A simple charge dipole will get closer and closer as heat is taken away since there is less molecular movement resisting the attraction. This should continue upon freezing contacting further upon further cooling. Yet water pulls the charge away from each other during freezing so it can expand. This should be endothermic, if only charge attraction mattered, but it actually gives off energy even with the charge potential increasing. In other words, the charge dipole is worse off during frezing, yet the expanded water is more stable. It is not as simple as a charge dipole.

 

 

Please for the love of all that is holy do some research before posting....

[Darkblade48]

The problem with that explanation is that ammonia has a dipole....

I read that and almost spit out my orange juice....

[sunspot]

A more detailed way to explain why water creates a lopsided potential has to do with orbitals. The oxygen atom ends in three 2P orbitals, which are orientated x,y,z, in space. It has four electrons in these orbitals and would like to get six to fill all six lobes. This will also complete the octet of electrons, i.e., 2S and 2P orbitals. As oxide or O-2, this occurs. Oxides of minerals are very stable and form crystals in various arrangements. These crystal arrangments will often spread the two negative charges of oxide over six or even more positive neighbors within a crystal lattice. All the P orbitals lobes share the extra charge.

 

In the case of water, the three 2P orbitals of oxygen are hybridized. Essentially the 2S and 2P orbitals of oxygen combine to make four sp3 orbitals. This is different than atomic oxygen which only has three 2P orbitals exposed to the world. Its 2S orbital remains buried underneath. The hybrid orbitals bring the 2S electrons of oxygen more to the surface and pulls the 2P orbitals down a little. This changes the combined orbital geometry into four tetrahedral orbitals.

 

When a hydrogen is bonded to oxygen, such as within water, the sharing with these sp3 hybrid orbitals gives the hydrogen's 1S electron more P-character. It sort of ionizes it slightly, delocalizing it toward the oxygen to fill the octet. This is stable for the oxygen because oxygen is trying to complete its sp3 hybrid orbitals. But it is unstable for the hydrogen because hydrogen's shared electrons is being ionized to be part of a P character hydrid orbital.

 

In other words, although hydrogen still has an electron, that electron is in an excited state relative to the needs of hydrogen, because it is holding the electron at a further distance in a partial P character orbital instead, of closer, like it would prefer, in an S orbital. With oxygen this does not pose the same ionization type problem, since this is where oxygen needs the electrons to be for stability. The net result is that hydrogen ends up with a higher potential than the oxygen.

[Cap'n Refsmmat]

Why did you ask the question when you know such a detailed answer?

[sunspot]

I have been trying to prove to everyone that hydrogen bonding hydrogen go into a hydrogen bond carrying the primary burden of potential. Nobody seems to see it or what to see it, so I have been presenting different angles until they can. This is an important understanding, because it allows one to model the cell in terms of the extra potential within hydrogen bonding hydrogen. This is also something that is being resisted.

[Nevermore]

The buzz-word box is full now.

[Cap'n Refsmmat]

Bingo!

[sunspot]

My problem is that I often know the answer before the solution. So I try to test the water with many solutions until I hit the right note. In my defense, I knew about the disproportionate potential twenty years ago. Science is just catching up. It seems like a lot of wasted time. The hydrogen bonding model of the cell should would have been fully functional by now.

 

What I think should happen is that the critics of science should have the same constraints as the creators of science. The creators of ideas have to play by the rules of science, but the critics can use irrational criticism. Maybe if some basic logic, conceptual inconsistency, existing data, etc., was a required part of scientific criticism, things would be much more constructive.

[Bluenoise]

My problem is that I often know the answer before the solution. So I try to test the water with many solutions until I hit the right note. In my defense' date=' I knew about the disproportionate potential twenty years ago. Science is just catching up. It seems like a lot of wasted time. The hydrogen bonding model of the cell should would have been fully functional by now.

 

What I think should happen is that the critics of science should have the same constraints as the creators of science. The creators of ideas have to play by the rules of science, but the critics can use irrational criticism. Maybe if some basic logic, conceptual inconsistency, existing data, etc., was a required part of scientific criticism, things would be much more constructive.[/quote']

 

But you don't offer any data yourself. Not that any is need since you're talking about a subject that's very well understood already....