question about liquid water

[ejloomis]

Hi Randy,

This question isn't directly reef-related, so if you choose not to answer it I understand.

I know that ice frequently contains ionic defects, such as H3O+ + OH-. Is the same true of liquid water? Is there any such thing as a "pure" sample of liquid water that is just H20 (and larger than a dimer or just a few molecules?).
In other words, would it be correct to say that any given sample greater than a few molecules is going to be a complex compound of H and O molecules, not all of them H2O, or is pure H20 possible in larger, even macroscopic quantities?

Strange question, I know, but I've been arguing with someone about whether water is "just" H20.

Thanks,
Eric

[Randy Holmes-Farley]

I'm happy to try to answer any interesing question, as this one is.

In pure water there is approximately 0.0001 mmole/L of H+ and an equal amount of OH-. This comes from the autodissociation of water:

H2O <---> H+ + OH-

There is also extensive hydrogen bonding between water molecules. Hydrogen bonds are reasonably strong, but form and break apart very fast.

There is no molecule larger than H2O that hangs around for macroscopic times (say, greater than a millisecond). The large hydrogen bond networks have important impacts on the properties of water (like the unusually high boiling point), but are not discrete entities.

[tatuvaaj]

There are different isotopes of both hydrogen and oxygen in natural water (number in front of oxygen are isotopes).

H2 16O
H2 18O
H2 17O
HD 16O
HD 18O
HD 17O
D2 16O
D2 18O
D2 17O

Where D=deuterium, 2H

It has been fascinating to learn that water isn't same even if it is 100% pure H2O. Actually there is a standard isotopic composition called VSMOW (Vienna Standard Mean Ocean Water).

These isotopes have different properties than "ordinary water", for example D2O has a freezing point at 3.82 C

OK, this might not be the most important fact to remember when your fishes have ich :)

[Randy Holmes-Farley]

Good point, Tatu.

Maybe raising the D2O content of the tank is a cure for ich!

[Gamera]

I wouldn't soak your fish in deuterated water for ich. While the levels required for killing someone by replacing water for 'hot' water are ridiculously high, you can kill someone by offsetting the chemical kinetics via kinetic isotope effects.

I have the numbers somewhere...

[Randy Holmes-Farley]

I meant it as sarcasm.....

[ejloomis]

Great -- thanks for the info.