# How wood is less dense than water?

1. Oct 13, 2011

### AakashPandita

Wood is less dense than water.
This means that particles of water are more close together than particles of wood.
And so the kinetic energy of water particles should be less than the kinetic energy of wood particles.
But it is not so.
Why?

2. Oct 13, 2011

### xts

I'd revert your question: why do you think it should be so?

BTW: Actually the first of your doubts is correct - small molecules of water are closer to each other that large molecules of cellulosis in wood, separated by large gaps.

3. Oct 13, 2011

### AakashPandita

I do not know that KE part.

4. Oct 13, 2011

### phinds

Most wood has lots of hollow spaces in it at the macro level. Water does not.

Your statement "Wood is less dense than water" is absolutely incorrect. It WOULD be correct to say "some wood is less dense than water", and it would even be correct to say "most wood is less dense than water" but I can name you dozens of woods that are NOT less dense than water.

EDIT: and if you are willing to consider wood from a tree that has been just now cut down, I can name even more dozens of woods that are more dense than water.

5. Oct 13, 2011

### uby

Be careful with how you define density. If you take a block of metal and put a big hole in the middle, has its density changed? (Yes and no, depending on whether you are talking about the bulk density or the skeletal density). Wood has lots of "holes" in it filled with air, which has practically no density relative to condensed phases.

Perhaps molar volume is a more instructive value than density for your understanding. Take the theoretical density (g/cm^3) and divide by molecular weight (g/mol) to get the molar volume (mol/cm^3).

6. Oct 13, 2011

### phinds

The OP was comparing wood to water and it seems totally reasonable to me to say that if something sinks in water, it is more dense that water. Why is this not a perfectly reasonable way to look at it?

I was commenting ONLY on his statement about density. I believe your comment is more intended to address the issue of kinectic energy, yes?

7. Oct 13, 2011

### Staff: Mentor

I don't see how kinetic energy applies to this. It is perfectly possible for a hot fluid to have something much colder than it floating on the surface.

8. Oct 13, 2011

### uby

I was only commenting in reference to the original poster's conception about the relationship between density and bond length.

9. Oct 13, 2011

### nasu

Actually the second statement does not follow from the first one in general. Only if the two materials were made from the same kind of particles.
Heavy particles that are farther apart may result in higher density than lighter particles which are closer together.

As an example, the nearest neighbor distance (minimum distance between two atoms) in lead is 3.5 Angstrom and in aluminum is 2.9 Angstrom. The atoms of aluminum in a piece of aluminum are closer than the atoms of lead in a piece of lead. However the density of lead is about 4 times larger than that of aluminum. Of course the light aluminum atoms have smaller radius so they can come closer to each other and still the result is a density lower than that of lead.
If you think about the packing fraction rather than the distance between particles, then again there is no simple relationship with density.

I don't know where did you get your idea about the part about the kinetic energy. Are you thinking about thermal expansion, maybe?

Last edited: Oct 13, 2011
10. Oct 17, 2011

### AakashPandita

okay....i understood wood is less dense because it has gaps at the macro-level.

11. Oct 17, 2011

### Staff: Mentor

The reason is complicated and not solely due to "gaps". Wood is made primarily of water, cellulose, hemicellulose, and lignin. ALL of these are made up of oxygen and hydrogen and all but water have carbon as well. The molecular structure of these will mainly determine it's density, but carbon itself has an atomic weight of 6 while oxygen has an atomic weight of 8, making carbon lighter than oxygen. So for equal sizes, any molecules with carbon would be less dense than those with only oxygen. So if these are less dense than water, then they will contribute to wood's ability to float.

12. Oct 17, 2011

### nasu

I think you sub-estimate the role of gaps in this case. The theory regarding the relative mass of carbon and oxygen atoms is nice but probably not quite right.
Some of the compounds that you mention (cellulose, lignin) are denser than water even though they have plenty of carbon.
And anyway, wood that was subject to pressure high enough to eliminate most of the air will sink in water.

13. Oct 17, 2011

### Staff: Mentor

I didn't mean to downplay the role of gaps, I just wanted to add that the density also depends on what the object is actually made up of. Gaps and air are most definitely a large factor.

14. Oct 17, 2011

### Danger

You guys know a lot more about this than me, but I do feel obliged to mention one problem that I have with the original question. This is basically just an extrapolation of Phinds' first post. Everyone is assuming that the scenario is in reference to liquid water (at STP?). To me, though, that is something that should be specified in order to obtain relevant opinions. After all, the solid, gaseous, and plasma phases all have vastly different densities and structural integrity than does the typical liquid one. Any wood, and almost anything else, will float on a sufficient thickness of ice even though it has low density, but balsa and maybe cork are the only ones that I can think of which might do so on steam. (I've never seen a pure steam environment, though, so I don't even know if that's possible.)

15. Oct 17, 2011

### Staff: Mentor

Sure Danger, but unless stated otherwise I'm going to assume water to mean liquid water at STP.

16. Oct 17, 2011

### Danger

As do I; I just thought that it might be worth mentioning. Assumptions can get you into trouble.

17. Oct 17, 2011

### sophiecentaur

That sort of argument is OK when discussing the relative densities of gases but, once the molecules start packing together in a condensed form (especially with gaps) I think the conclusions are not certain. (Take the anomalous expansion of water, for instance).

18. Oct 17, 2011

### Staff: Mentor

Who ever said the conclusions were certain?

19. Oct 18, 2011

### sophiecentaur

Don't we aim in that direction?

20. Oct 18, 2011

thanks.