do odours rise or fall

[fred]

Given that town gas is lighter than air so rises and propane is heavier than air so falls how does a bad odour perform ? Rise or fall or just permeate ?

[The Natural Philosopher]

well as you have deduced, either, depending...

Hydrogen sulphide is heavier than air.

As is Methanethiol..

But in gases diffusion is the order of the day isn't it? The air does not end as layers of nitrogen, oxygen, and carbon dioxide does it?

"In modern science, the first systematic experimental study of diffusion was performed by Thomas Graham. He studied diffusion in gases, and the main phenomenon was described by him in 1831?1833:

"...gases of different nature, when brought into contact, do not arrange themselves according to their density, the heaviest undermost, and the lighter uppermost, but they spontaneously diffuse, mutually and equally, through each other, and so remain in the intimate state of mixture for any length of time.?

Sez wiki...

[Martin Brown]

Diffusion dominates and super smelly molecules like mercaptans only takes a few to reach the nose for sensation overload.

Hydrogen and helium are light enough that they can slowly escape the atmosphere but everything else is pretty much bound and well mixed.

It isn't quite true as visits to volcanic caves like those in France where enough CO2 comes out of the ground to flood the deeper parts with CO2. ISTR Dog cave aka Grotto de Chein near Clermont-Ferrand being one such example - the gas onbly stays stratified because there are almost no drafts in much the same way as hot water floats on cold.

Those of a sensitive disposition might prefer not to know why it was called "dog cave". These days a candle performs the same function.

There have been nasty CO2 flood deaths caused by supersaturated deep water in volcanic lakes catastrophically releasing huge volumes of CO2 that initially rolls along the ground suffocating everything it in its path. Lake Nyos being the most infamous in recent times:

[Tim Streater]

But a big "lump" of gas will rise or sink as appropriate, but will be diffusing and mixing as it goes. Such as the CO2 instances referred to by MB.

And yes, Hydrogen and helium in the atmos are slowly lost, it may be because their molecular velocity is greater than the Earth's escape velocity. So as these diffuse around they slowly escape.

I don't know whether the Earth's gravity is strong enough to cause stratification in gases, but you can certainly separate gases with slight density differences using centrifuges - after all, that is how they separate U235 from U238, by spinning uranium hexafloride at

10,000rpm or whatever it is.

[Tim+]

Mostly diffusion I would think. "Town gas" doesn't exist any more though.

Tim

[whisky-dave]

I wonder where farts fit in here :)

[harry]

Yes it does. Just not used domestically.

[Brian-Gaff]

Well depends on what the odour is. If its tiny particles that are lightweight they go where the air goes, ie hot air rises and so do they. I don't think I've seen large enough particles to not do this, but that very much depends on what you are actually smelling. I believe that in most gasses for domestic use the smell is put in artificially so is probably meant to disperse to alert the people its actually there. Brian

[Tim+]

Go on, do tell. Where?

Tim

[Brian-Gaff]

Actually, in the main you are correct, but I clearly recall an experiment we did at school with a gas that started with the letters hex I think which was very heavy compared to air. If one carefully put it into, say, a fish tank, one could actually float a paper boat on its boundary layer with the air. It was however only just possible and did, in the end disperse. I doubt this experiment would be done these days as the gas is probably toxic, explosive or just hard to get nowadays without a bit of dosh. Brian

[Andy Burns]

SF6 sulphur hexafluoride, also has the opposite effect on the voice to Helium.

[Tim+]

How goes the search? Any news on town gas in the UK?

Tim

[harry]

It's a by-product of coke manufacture, needed to make iron/steel. Often used on site to drive gas turbine to make electricity. One hospital I worked at, we used gas from the nearby coking plant to raise steam to raise steam. Lots of problems due to the quality varying, impurities and unexpected plant breakdowns. Really nasty stuff. But cheap as it was a byproduct.

[Tim Streater]

Only nasty stuff if it's not purified. All gas, then produced by heating coal in the absence of air, had to be purified in this way to make it suitable for use before natural gas came along. The impurities condensed out and were an important feedstock source for the chemical industry. It's where aniline dyes came from.

[Jethro_uk]

Drifting (see what I did there ?) OT, the smell of cannabis (terpenes IIRC) is interesting. It is strong, but seems to rise rather than fall. Also it doesn't seem to cling to clothes (although *touching* the flowers will cling).