Sciencemadness Discussion Board

How much of water is empty space?

Wolfram - 23-2-2007 at 03:14

How big procentage of the water volume of water are consists of atoms and how big procentage is the empty space between the water molecules?

Levi - 23-2-2007 at 03:40

Well, 1 gram of water molecules take up about 1cm<sup>3</sup> and the approx molar mass of a water molecule is 18 g/mol. Therefore there are about warning: math ahead 1g*(mol/18g)*(6.022e23 molecules/mol) = 3.346e22 water molecules bouncing around in 1 cubic cm of space. If anyone happens to know the approx size of an O and H atom you can figure out how much of that cubic cm is actually water. I'm too lazy to look it up.

Furch - 23-2-2007 at 04:28

As I see it, it's a trick question... As I understand it, there is no matter in the sense that we think about it.

They say that the distance between a hydrogen atoms nucleus and its electron is proportionate to the distance between the earth and the moon, or the sun, I forget.

Point being, that there is more "nothing" than there is "something" in a mole of any substance, be it in solid, liquid or gaseous form.

woelen - 23-2-2007 at 04:57

An atom is 99.9999999999...% empty space. The nucleus is increadibly small compared to the size of the orbitals (whatever one would imagine with the orbitals).

So, the question of how much empty space exists in water is ambiguous. If you regard all space inside the said orbital radii as being part of atoms, then one could say that a few tens of percent of space is empty. If only space, occupied by the electrons and nuclei is regarded as being part of the atom, then the amount of empty space is almost 100%.

Space, really filled up with matter exists in neutron stars. Such matter is intensely dense, something like a few million tons per cm³.

Furch - 23-2-2007 at 05:30

Also, investigating the atom nucleus further, you will find subatomic particles (neutrons and protons, quarks etc.), and as far as I know there is nothing that proves or even suggests that these are made up of a "solid" substance. So there is a risk that indeed 100% of everything we know as solid is not. Rather some other variation of energy.

Wolfram - 23-2-2007 at 05:42

I dont know what you are talking about atoms are very very small tennis balls as I know it.;)

[Edited on 23-2-2007 by Wolfram]

[Edited on 23-2-2007 by Wolfram]

Levi - 23-2-2007 at 07:28

Quote:
Originally posted by Wolfram
I dont know what you are talking about atoms are very very small tennis balls as I know it.;)

I agree. Always have been and always will be.

Anyhow, a more practical question might be: "What is the distance between two adjacent water molecules at STP?" You can then specify that you want the measurement to be made from the center of mass of each molecule.
I think the answer is something like: 1/(3.346e22<sup>1/3</sup>;)cm but I'm not sure how hydrogen bonding affects the answer as that solution assumes the space between all adjacent molecules is the same. This is of course untrue regardless of H-bonding since the "temperature" of some molecules is much greater than that of others... Hmm--lots of stuff to think about with this question. Any particular reason you're asking?

[Edited on 23-2-2007 by Levi]

12AX7 - 23-2-2007 at 09:46

Ooh, I can do this!

Well, what's the molecular weight of water? About 18g/mol, or 18g per 6.03 x 10^23 molecules. 1g = 1ml H2O (at some temperature), so that's 6.03e23 / 18 molecules per ml.

Since hydrogen bonds (or other liquid bonds, if you want to talk about different condensed matter) are constantly reforming every picosecond, it's meaningless to talk about anything specific; only statistical averages apply, which are fine. Kind of a quantum mechanical blurryness, which there ought to be at this scale.

If I assume the density is even (it better be, at least on a macroscopic, averaged scale, and presumably of zero ionic strength or something), the volume per molecule should be the reciprocal, or 18/6.03e23 ml/molecule (obviously, mililiters are woefully large to express this volume ;) ). The dimension per side, then, is the cube root, or (converting ml to cm^3), 3.1 x 10^-8 cm or 0.31 nanometers per molecule. This sounds like a pretty reasonable intermolecular distance.

Tim

chemrox - 24-2-2007 at 22:20

all of it: matter is a thought

unionised - 25-2-2007 at 04:49

When liquid water freezes it expands by about 10% so, compared to the solid, about minus 10% of it must be space.

[Edited on 25-2-2007 by unionised]

vulture - 25-2-2007 at 07:13

Quote:

0.31 nanometers per molecule. This sounds like a pretty reasonable intermolecular distance.


Nope. An average hydrogen bond in water is about 1.97 Aengstrom which is 0,197 nm.

woelen - 25-2-2007 at 10:14

But the order of magnitude is indeed reasonable. The kind of calculations, done by 12AX7, only is very rough, but it is the kind of calculations, which gives insight in the order of magnitude of certain quantities and as such has great value.

Speaking of 'intermolecular distance' of course is somewhat ambiguous (but 12AX7 also said something like that), simply because of the irregular shape of the molecules, and also for the reasons stated by 12AX7.

12AX7 - 25-2-2007 at 10:33

Yeah, I can imagine that it'd be about half that considering the molecule is long and tends to orient with nearby dipoles -- that is, the hydrogen bond. On average, how many hydrogen bonds are there, anyway? They're constantly changing (except in ice). Also, is the hydrogen bond distance from H to nearby O (or O's lone pair orbitals) or center to center of the water?

My model assumes the molecules take up a cubic volume, which is obviously erroneous, but it's a lot easier than assuming spheres or three blobs of some geometry and working out the packing for them. Works out the same on the average, anyway, which is about all that matters in an entropy-rich liquid.

As a physicist, it is my business to make SWAGs and do the math in my head or on any nearby envelopes (OK, to do the scientific notation I used a calculator above). Chemists are just being anal (and I don't mean "analytic") when they want something like this to 9 decimal points. ;)

Tim

Mr. Wizard - 25-2-2007 at 10:52

Define "empty space", and the size space you are looking at.

[Edited on 25-2-2007 by Mr. Wizard]

Nerro - 25-2-2007 at 14:10

The space between the outer electrons and the nucleus is hardly empty space. It's littered with electrons that (together) need all that space. At any given moment it might be mostly empty space but that's not at all a realistic representation of an atom.

woelen - 26-2-2007 at 00:30

Well, if it is not empty space, then you could indeed say that the electrons are smeared out over a certain amount of space, but the reasoning is equivalent, then this smearing out can be regarded as a ultra-low density distribution of the electron over space. The electrons in their orbitals can be regarded as a spatial density distribution, both for charge and for mass.

You'd better say then that every point in space is 99.99999999...% empty space with a little bit of electron (and nucleus) in it. Fact remains that our normal matter (atoms, electrons) is increadibly low-density in terms of spatial occupation. Neutron stars demonstrate that matter can be condensed extremely, but the Pauli exclusion principle assures that this does not occur under normal conditions (electrons can't occupy same orbitals with same spin). Only, when they are destroyed by 'melting down' with the nucleus (as in the enormous pressure of a super nova explosion), this exclusion principle does not hold because the electrons are no more. The exclusion principle still holds, but at a lower level (the neutrons also cannot occupy the same 'orbitals'). When pressure further increases, then the neutrons also collapse, and there is no other form of matter, which by means of a Pauli-exclusion principle prevents further collapsing. From that point collapsing continues for ever, leading to formation of a black hole.

apidej - 26-2-2007 at 08:50

thank you ,very good

evil_lurker - 26-2-2007 at 18:07

How much of water is empty space?

I'm going to take the easy route on this one... 0%

One could take into consideration molecualar diameters and all that theoretical mathmatical gobbledygook but its all relative because if I dive into a swimming pool, I'm not diving into empty space, I'm diving into water.

12AX7 - 26-2-2007 at 19:09

On the average yes, but then again, nothing compared to neutronium, so there's always some absolute value important. Zero is always a good one.

Another question: how much of space is empty space? Counting mass-energy, of course.

Tim