[yoyoman2]

I might be asking for alot here, but do you know of any Heavy, invisible and inert gases that are cheap?
I'm interested in making something "float" in mid air, with a container that is filled with the gas.

I dont have much knowledge in chemistry or properties different states of matter, so I dont know if it is even possible.

so what I'm asking for is that kind of gas, which I can float things on(small papers and stuff like that) at room temprature.

does such a gas exist? I only know of sulfur hexaflouride, but from what I found out, its rather expensive and I'm not sure I can float things on it, if it is in room tempreture.

the floating effect that I'm looking for is something like this: https://www.youtube.com/watch?v=u19QfJWI1oQ

thank you!

[mafagafo]

http://www.sigmaaldrich.com/catalog/product/aldrich/399108?lang=en

It is somewhat expensive.

You shouldn't expose people to it.

I AM NOT BEING SPONSORED BY S.A..

[billnotgatez]

Tungsten hexafluoride
http://en.wikipedia.org/wiki/Tungsten%28VI%29_fluoride

Tungsten(VI) fluoride, also known as tungsten hexafluoride, is the inorganic compound of tungsten and fluorine with the formula WF6. This corrosive, colorless compound is a gas under standard conditions, with a density of about 13 g/L (roughly 11 times heavier than air.[1][2][3]), WF6 is one of the heaviest known gases under standard conditions.

Well that is not very inert

[mafagafo]

Heavy and colorless. 2/3 done.

[Zyklonb]

Uranium hexafluoride is the densest known gas (in its gaseous phase, which is above 57 C at 1 ATM)
Of course, it's neither easy to get nor inert, just thought I'd throw that out there.
ONBr is dense and easy to make, not inert though .
Lots of Freon's are dense and quite inert, that may work.
The perfluorocarbons are low-boiling and have an extremely high vapor density. Simply heat perfluorohexane (C₆F₁₄) to a boil and you have a gas phase with a molar mass of 338g, around 14g per liter.
 Otherwise, try boiling some carbon tetrachloride and try to float your aluminum foil boat on these vapors.  (Assuming that's what your doing.)
 Trifluoroacetonitrile could be easily made via mixing P₂O₅ with trifluoroacetamide and dry distill it. A dense gas (95g/mol) will come over what could be easily condensed with liquid nitrogen to a white solid if needed.

 Trifluoroacetamide could be easily made from TFA and ammonia, and I think P₂O₅ is also not too to get.



 

[snorkack]

Uranium hexafluoride is the densest known gas (in its gaseous phase, which is above 57 C at 1 ATM)

I suspect not as dense as helium 3!

Lots of Freon's are dense and quite inert, that may work.
The perfluorocarbons are low-boiling and have an extremely high vapor density. Simply heat perfluorohexane (C₆F₁₄) to a boil and you have a gas phase with a molar mass of 338g, around 14g per liter.
 Otherwise, try boiling some carbon tetrachloride and try to float your aluminum foil boat on these vapors.  (Assuming that's what your doing.)

Then you must boil, again.
The heaviest perfluorocarbon gases are perfluorobutanes. Perfluoropentanes boil between 29,3 and 30,1 Celsius.

[Borek]

Uranium hexafluoride is the densest known gas (in its gaseous phase, which is above 57 C at 1 ATM)

I suspect not as dense as helium 3!

Huh?

[snorkack]

Uranium hexafluoride is the densest known gas (in its gaseous phase, which is above 57 C at 1 ATM)

I suspect not as dense as helium 3!

Huh?

The question was whether to compare densities of gases at standard pressure and temperature or whether gases at standard pressure and nonstandard temperature can be included in comparison.

If you count a dense gas at standard pressure but high temperature, like uranium hexafluoride, you should also count gases at standard pressure but low temperature.

Helium gas is very dense because it can be cooled to a very low temperature before it condenses.

[kriggy]

That is very bold statement IMO, give the fact that helium filled balloons fly in air which means it has to have lower density than air. And even if its density increases while you cool it (does it?) it is not relevant, because the qusetion asks for gases at room temperature

[Borek]

And even if its density increases while you cool it (does it?)

Yes, it does, that's what every gas does, you can easily check it just by playing with the ideal gas equation.

However, helium condenses at around 4K, so - even assuming ideal gas behavior - we won't get more than about 12 g/L. That's actually pretty close to the density of a liquid helium, which is most likely a good estimate of the highest density of the gas. But it is still less than the density of the gaseous UF₆ (13 g/L).

[snorkack]

And even if its density increases while you cool it (does it?)

Yes, it does, that's what every gas does, you can easily check it just by playing with the ideal gas equation.

However, helium condenses at around 4K, so - even assuming ideal gas behavior - we won't get more than about 12 g/L. That's actually pretty close to the density of a liquid helium, which is most likely a good estimate of the highest density of the gas. But it is still less than the density of the gaseous UF₆ (13 g/L).

Your number for liquid helium density is in wrong magnitude and the assumption of ideal behaviour is not a good one. Helium at atmospheric pressure is the least ideal gas because it has the lowest critical pressure, and helium 3 is far less ideal than 4. If Earth atmosphere were very slightly denser, helium 3 would be a permanent gas.
Helium 4 at 2,2 K and its then vapour pressure of 0,05 bar has density of 146 g/l. It is not much increased by further cooling.
Compressing helium 4 to 30 bar while cooling it to 1,8 K compresses helium 4 to 180 g/l while still a liquid.
Warming helium 4 under its vapour pressure to 4,2 K, where vapour pressure is 1 bar, allows it to expand to 125 g/l.
On further warming, the critical point of helium 4 is at 5,20 K, where pressure is 2,27 bar and density 69,5 g/l.
Expanding the gas, at 1 bar and 4,2 K the vapour density is 16,9 g/l. So more than 1/8 the density of liquid - no wonder considering the proximity of critical point.

So helium gas near its condensation point is very nonideal gas, and shrinks to a much bigger density than an ideal gas would.

[Borek]

Your number for liquid helium density is in wrong magnitude

My bad, I misread the table 

and the assumption of ideal behaviour is not a good one

I never assumed it is a good one, I just checked what number this assumption gives - and as I misread the table, it seemed like a surprisingly good one. My mistake.

Interesting piece of information, even if most likely useless for the OP. Thanks for that.

[snorkack]

Furthermore, it is fundamentally impossible for a gas to shrink ideally into liquid phase - because condensation itself is very nonideal behaviour.

A gas cooled at well below its critical pressure shrinks near ideally until boiling point, where it discontinuously shrinks to much larger density. A gas cooled near its critical pressure will deviate from ideal behaviour by being much denser than ideal gas near although above its boiling point. Something important to remember when handling any near critical or supercritical fluid. (There are also fluids that associate in gas phases even though they are far below critical pressure, like hydrogen fluoride or nitrogen dioxide.)

Going to fluorides:
Metal fluorides are not inert.
WF6 - molar mass 298, boiling point 17 celsius
UF6 - molar mass 349 to 352, boiling point 56 Celsius
NpF6 - molar mass 351, boiling point 55 Celsius
PuF6 -molar mass 352 to 358, boiling point 62 Celsius.

All perfluoroalkanes are fairly inert (are any lower perfluoroalkanes seriously sterically crowded?)
C4F10 - molar mass 238, boiling point -2...-1 Celsius. (Unlike alkanes whose boiling point is lowered a lor by branching, perfluoroalkanes have nearly equal boiling point regardless of branching)
C5F12 - molar mass 288, boiling point 29...30 Celsius
C6F14 - molar mass 338, boiling point 56 Celsius