This is topic Why is Water Heavier Than Air? in forum Books, Films, Food and Culture at Hatrack River Forum.


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Posted by Noemon (Member # 1115) on :
 
I expect that this is a stupid question, and that when somebody tells me I'll smack myself on the forhead and think either "oh, sure, that makes sense" or "heh--I can't believe I forgot that", but why would it be? I mean, it's made of just two things--oxygen, which is light enough that it doesn't settle to the ground at normal temperatures, and hydrogen, which is even lighter. So why, when you combine the two, do you end up with something heavier than either one of them?
 

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Posted by Bokonon (Member # 480) on :
 
http://www.usatoday.com/weather/wdensity.htm

Near the bottom. It has to do with densities.

-Bok
 

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Posted by mackillian (Member # 586) on :
 
Stupid humidity. --I--
 

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Posted by Happy Camper (Member # 5076) on :
 
You want the short answer or the long one? In brief. Okay, I didn't read that link so this may be covered there, but I'll do my best anyway. Water is a liquid, air is gas (well, yeah, I guess you knew that). It has mostly to do with the way the molecules interact with one another causing them to bunch up a lot tighter in water than air at the same temp. Water forms positive ends and negative ends (dipoles), similar to magnets, but electric. There's hydrogen bonding, which is (I think) the strongest intermolecular bond. Not present in air.

Ah, disjointed explanations, nothing like em. Sorry, I didn't consider the scope of the question before I decided to reply, hope this makes at least a little sense. I could probably explain it so it makes a lot more sense, but my abilities to teach generally depend on being able to show, and ask questions of the learner (teachee) (are either of those actual words??

[ August 07, 2003, 10:19 PM: Message edited by: Happy Camper ]
 

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Posted by Noemon (Member # 1115) on :
 
Thanks Bok!

[Edit--and Camper]

[ August 07, 2003, 10:21 PM: Message edited by: Noemon ]
 

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Posted by Happy Camper (Member # 5076) on :
 
Oh, my. I guess I should have paid a little more attention to the feel of the question. Methinks I was answering something you weren't really asking. Ah well, a little more [generally] useless info for ya at any rate.
 

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Posted by mackillian (Member # 586) on :
 
So if it reaches 100% humidity, does that mean you're swimming?
 

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Posted by Kayla (Member # 2403) on :
 
Gee, and I would think it would have to do with weight.

Hydrogen 1.0079

Oxygen 15.9994

I mean, oxygen is just (almost) 16. And water is like 18. Right?



Seriously, cold water is heavier than hot water.
 

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Posted by rivka (Member # 4859) on :
 
No, mack, it just FEELS like it. *shudders*

A compound has different properties than its constituent elements. Never mind water (a liquid) forming from two gasses -- how about good old table salt? A poisonous green gas (chlorine) plus a highly flammable metal --> compound you can't live without small amounts of. Neat, huh?

[ August 07, 2003, 10:47 PM: Message edited by: rivka ]
 

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Posted by Hobbes (Member # 433) on :
 
Hydrogen bonding is 10 times stronger than the next strongest form of inter-molecular bonding, but 10 time weaker than the weakest kind of intra-molecular bonding. In other words, the hydrogen bonding present in water that exists between multiple molecules is very strong. It's because the hydrogen atom in water only has one electron and it (the electron) resides much closer to the oxygen atom (well...quantum physics aside ) so the hydrogen atom (which is just a proton) can get much closer to the other molecule's negativly charged oxygen. It creates stronger and denser bonds.

Cold water is heavier than hot water... kind of. Everyone knows ice is lighter than water (which occurs because the crystal structure in ice seperates the molecules more than in water), so at 0 degrees, water (or ice depending) is actually lighter than warmer water. The temperature at which water (in any of the three basic states) is the heaviest is at about 4 degrees.

Hobbes

[ August 08, 2003, 01:38 AM: Message edited by: Hobbes ]
 

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Posted by The Wiggin (Member # 5020) on :
 
Now could we get it in english Hobbes. But serisly it kind makes since the way you explain it. I think. O well.
 

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Posted by Hobbes (Member # 433) on :
 
Sure Wiggin, I'd do it in Spanish but I can't say "intra-molecular" in Spanish.

Anyways, the basic thing is that the atoms in water are much closer together than in air. For the second point, the atoms in ice are set up in such a way as to make them farther apart than in normal water, which makes water denser.

Hobbes

[ August 07, 2003, 11:42 PM: Message edited by: Hobbes ]
 

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Posted by Chaeron (Member # 744) on :
 
I must concur with Hobbes. The hydrogen bond is unusually strong in water. The strong polar nature of the water molecules and the near absense of anything approaching an electron valence on the hydrogens make its inter-molecular bonding particularly strong. This is in sharp contrast even to other similar molecules, like hydrogen sulfide, which is a gas at room temperature despite sulphur being in the same group as oxygen with a greater molar mass.
 

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Posted by Hobbes (Member # 433) on :
 
Only three types of molecules exhibit hydrogen bonding. Those molecules are H3N, H2O (water), and HF. Water is the important one, because the other to don’t have as strong hydrogen bonding. H3N isn’t as strong because the Nitrogen doesn’t attract the Hydrogen’s’ electron as strongly as Oxygen, and it has to attract an extra electron as well (less power divided over more area). HF is a very, very powerful acid (it will seep through your skin and dissolve your bones if you get any one you! ), it doesn’t have the same kind of inter-molecular bonding because there is only one hydrogen per Fluorine, and though the Fluorine pulls the electron even farther away from the Hydrogen than Oxygen does, it can only form lines an such, non-crystal patterns. Which means they patterns that can sustain any kind of growth, and remain only on very small levels. If you want to know more about hydrogen bonding check out this site.

Hobbes
 

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Posted by suntranafs (Member # 3318) on :
 
Ok... I've only had a couple college courses in chemistry, but it seems like you guys are over complicating the issue. If the question were: 'Why is water heavier than liquid oxygen and nitrogen?(which I kind of doubt it is) then I might be able to see where you're coming from.
As in a solid, though to a lesser degree, the molecules of a liquid have fairly strong intermolecular attractive forces that give it some structure. In a gas, such forces are miniscule, and theoretically balanced (or overbalanced) by reppeling forces. Hence, unlike a liquid or solid, the gas has no structure, and thereby tends to expand to fill the available space, and so can be very very light.

This also means that, unlike liquids and solids, gasses can be compressed (or cooled, thus decreasing their volume) to an extreme degree. Consequently, severely pressurized air can be plenty heavy.

[ August 10, 2003, 02:46 AM: Message edited by: suntranafs ]
 

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Posted by Raia (Member # 4700) on :
 
Umm... why not?
 

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Posted by Hobbes (Member # 433) on :
 
Actually, liquids and solids can be compressed.

Hobbes
 

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Posted by rivka (Member # 4859) on :
 
Actually, hydrogen bonding occurs in many complex organic molecules. It's one of the forces that helps fold proteins, and it is also the force that holds together the base-pairs that make the "rungs" in DNA "ladders."

Back to the subject of water. suntranafs, you are changing the question, but not really simplifying it. Because your answer then begs the question, "Why is water a liquid at room temperature, while hydrogen and oxygen are not?" Fortunately, that question has been more than adequately handled by HC, Hobbes, and Chaeron.
 

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Posted by Rahl22 (Member # 1376) on :
 
While I haven't studied fluids extensively, I was under the impression that liquids could NOT be compressed.
 

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Posted by Godric (Member # 4587) on :
 
Ack! Chemistry!

::runs away screaming in terror::
 

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Posted by rivka (Member # 4859) on :
 
Liquids are certainly compressible -- the CRC has pages on the compressibility of liquids at various pressures. Some solids can be compressed significantly as well.

However, only gasses can be compressed drastically and dramatically.
 

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Posted by Happy Camper (Member # 5076) on :
 
Woo! I got abbreviated.
It's the small victories and acknowledgements that make life worth living. Thanks rivka, you made my day.
 

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Posted by Hobbes (Member # 433) on :
 
Both true. Hydrogen bonding does show up a lot in organic molecules, but it is a result of an H2O within the organic molecule (or H3N or HF). Well, I guess HCL does exhibit a little hydrogen bonding too. I suppose my claim is more definite than I am.

Hobbes
 

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Posted by rivka (Member # 4859) on :
 
HC, I'm so glad that my laziness made you happy!
 

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Posted by Glenn Arnold (Member # 3192) on :
 
Umm, the specific density of liquid nitrogen is: 1.11.

This is slightly heavier than the specific density of liquid water, which is: 1.0.

The molecular weight of water is: 18.015
The molecular weight of Nitrogen is: 28.014

All the discussion of hydrogen bonding helps explain the discrepancy between the two ratios, but is basically irrelevant to the question at hand.

As far as whether water sinks in air: Bouyancy doesn't affect individual molecules, so in the atmosphere, gases are basically evenly distributed, despite their molecular weight.

The specific density of liquid air is: 1.026 for anyone that wanted to know, but since there is no such thing as "air molecules" (since air is a mixture) I used nitrogen because it makes up 78% of what air is.

Glenn Arnold (who spent nine years in the industrial gases business and happens to have those numbers at his fingertips)
 

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Posted by Glenn Arnold (Member # 3192) on :
 
rivka:

I submit that suntranafs is actually closer to answering the original question than the rest of you. Noemon made no reference to liquids or gases, only molecular weight. *Why* air is gaseous at room temperature and water is usually liquid at room temperature is merely complicating the issue.

By the way, steam (gaseous water) is lighter than air at the same temperature.
 

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Posted by rivka (Member # 4859) on :
 
Actually, Noemon said "heavier" -- and in context, that seems to have meant denser. While molar mass is relevant, the states of the various substances would be also, I think.
 

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Posted by Chaeron (Member # 744) on :
 
Glenn, it has already been established the gaseous H2O is lighter than air. That was in the first post. If no one bothered to read it, here is my full answer.

In terms of molar mass, water is lighter than O2 or N2. Because of the properties of gasses as described adequately by the ideal gas law. This states that if gas A has a lower molar mass than gas B, then at the same temperature and pressure, the gas with the lower molar mass will be the less dense. As a gas, water is lighter than air. This is why moist air is lighter than dry air. However, this doesn't answer the question of why water is heavier than air under normal conditions. Well, because it's a liquid, and therefore, denser than it is in its gaseous state. In fact, water is even denser than other liquids that have greater molar masses at room temperature, like octane (the primary constituent of gasoline). This is due to hydrogen bonding, as has already been described in more than enough detail.

[ August 10, 2003, 09:59 PM: Message edited by: Chaeron ]
 

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Posted by Chaeron (Member # 744) on :
 

quote:

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...in the atmosphere, gases are basically evenly distributed, despite their molecular weight.

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