[craigms]

I'm curious, why does NH3 sink in air when the molecular weight of NH3 is less than O2 and N2..?

[curiouscat]

I'm curious, why does NH3 sink in air when the molecular weight of NH3 is less than O2 and N2..?

Are you sure it sinks? Maybe it doesn't rise as fast giving the appearance that it sinks?

[craigms]

I'm not sure, whenever there is an ammonia alarm at work we have to go upstairs, they tell us ammonia sinks because it is heavier than air (which is isn't).

"Ammonia gas, while only about half as dense as air is still heavy enough so that it will sink to ground level instead of remaining suspended in the air."
This is the only info i could find on google, which still confuses me because if it's lighter shouldn't it rise?.

[Tittywahah]

Ammonia gas is indeed lighter than air, BUT, it is extremely hygroscopic.  So as it rises and absorbs moisture from the air, it sinks again. Of course, probably not in a classroom in the nevada desert.

[curiouscat]

Ammonia gas is indeed lighter than air, BUT, it is extremely hygroscopic.  So as it rises and absorbs moisture from the air, it sinks again. Of course, probably not in a classroom in the nevada desert.

What would it form though? An aerosol of Ammonium Hydroxide?

[craigms]

Ammonia gas is indeed lighter than air, BUT, it is extremely hygroscopic.  So as it rises and absorbs moisture from the air, it sinks again. Of course, probably not in a classroom in the nevada desert.

Thank you.

[Tittywahah]

I keep having to make apologies for myself - I am not a chemist, nor a chemistry student, I am self taught.  But my guess would be that what you say about Ammonium hydroxide seems logical, however I would be more inclined to simply call it liquified ammonia gas which will be alkali and corrosive and damaging to tissues, not the ones you sneeze with! The ammonia that is in liquid form is not ammonium hydroxide, now this is what I read many times months ago and there was a good reason for this, but please somebody else answer this since my knowledge is sparingly amateur over this.  However in the meantime I will try and look it up.

A quick searcgh just found these references, but I can not explain further, sorry.

It can be denoted by the symbols NH3(aq). Although the name ammonium hydroxide suggests a base with composition [NH4+][OH−], it is actually impossible to isolate samples of NH4OH, as these ions do not comprise a significant fraction of the total amount of ammonia except in extremely dilute solutions.[4] Wikpedia.

and

Ammonium hydroxide (NH4OH) is a common, though not entirely correct name for a solution of ammonia dissolved in water.
when dissolved in water ammonia reacts to a small degree with water to produce ammonium hydroxide (NH3 + H2O --> NH4OH). But this only forms in a small amount and most of the ammonia remains unreacted. This substance cannot be isolated as any attempt to separate it out will result in it reverting back to ammonia and water.  http://wiki.answers.com

So it appears that my original thinking was correct, that it is simply ammonia gas locked up with water molecules and then that sinks to the ground.

[curiouscat]

I would be more inclined to simply call it liquified ammonia gas which will be alkali and corrosive and damaging to tissues,

It cannot be liquid ammonia. NH3 has a Boiling Point of  -33.34 °C. I am assuming the original question referred to behavior around Room  Temp and atmospheric pressures.

Besides, if it were indeed pure liquid ammonia where did the water it has absorbed from the air gone? There's no H2O in pure liquid NH3. That's why I tend to be skeptical of the hygroscopicity explanation.

But I could be wrong. Since I can't come up with anything better to explain why Ammonia sinks. Maybe there is some other solution.

[Tittywahah]

I would be more inclined to simply call it liquified ammonia gas which will be alkali and corrosive and damaging to tissues,

It cannot be liquid ammonia. NH3 has a Boiling Point of  -33.34 °C. I am assuming the original question referred to behavior around Room  Temp and atmospheric pressures.

Besides, if it were indeed pure liquid ammonia where did the water it has absorbed from the air gone? There's no H2O in pure liquid NH3. That's why I tend to be skeptical of the hygroscopicity explanation.

But I could be wrong. Since I can't come up with anything better to explain why Ammonia sinks. Maybe there is some other solution.

I know this:  NH3 + H2O = NH4 + O2  The ammonia is converted to ammonium, the water is bound up I would imagine with the ammonia gas, it sucks the moisture away so to speak, and at this point I am at the edge of my knowledge, but the equation clearly demonstrates that hydrogen is lost to the ammonia gas, it becomes an ion I think.  Down at the electron and proton level I can not understand since I have only just begun to even grasp these concepts through videos and reading.  I know that there is an explanation, I just can not detail it, and I am sure there are a few giggles from the more professional readers right now.

[curiouscat]

I know this:  NH3 + H2O = NH4 + O2  The ammonia is converted to ammonium, the water is bound up I would imagine with the ammonia gas, it sucks the moisture away so to speak,

Nope. I don't think (IMHO) this is right at all, especially the O2 liberation and uncharged ammonium ion. But I'm no Chemistry expert either so I'll refrain from saying more. 

[Tittywahah]

I know this:  NH3 + H2O = NH4 + O2  The ammonia is converted to ammonium, the water is bound up I would imagine with the ammonia gas, it sucks the moisture away so to speak,

Nope. I don't think (IMHO) this is right at all, especially the O2 liberation and uncharged ammonium ion. But I'm no Chemistry expert either so I'll refrain from saying more. 

Yes well you know more chemistry than I do because you recognised it as wrong.  Anyway you forced me to dig deeper and I came up with this:  http://www.youtube.com/watch?v=a03s1brFae8  If you wait a minute it moves straight into the ammonia. Hope it helps.  I certainly understand things a bit better now.

Still at the end when she talks about the equilibrium shifting constantly and then says that most of the ammonia is still in a non-ionised form, that means I think that strictly speaking we still have a liquified gas.  Or as some would say, not ammonium hydroxide, but ammonia gas in water - so we are back to beginning about why the whole charade sinks to the floor as opposed to rising to the ceiling.