If CO2 is so heavy, how come it doesn't sink and suffocate us? A postscript.

Still they keep coming, to a posting I did nearly 5 years ago.

Posted on this site, Jan 12, 2010

It wasn't as if  CO2 and its behaviour in a gravitational field was a burning issue at the time. In fact that posting was a side-issue from my then preoccupation with the climate change/global warming debate (which I was glad to withdraw from, given the brow-beating denialist tendency on MSM forums).

It was provoked as I recall by a barmy comment to the effect that CO2 had been made "heavy" by a Benign Presence (Gaia? Guardian angel in attendance?)  so as to stay at lower altitudes, feeding our plants, not reaching higher altitudes where it might cause mischief!!!

Here's a snapshot from my sitemeter, showing that 11 of the last 20 visits have been to that posting  (13 is a more typical average).

The red tags are mine, pointing to the CO2 posting

  Quite why it still gets all the attention I haven't a clue. Maybe it's because  I'm a Londoner it's currently the second  listing one sees if entering (CO2 heavy) into Google. Sometimes it's the first he says in a rare moment of modesty-bypass.

To be honest, I've been somewhat embarrassed that a instant tutorial should now be seen as the first or second stop on a simple non-technical web search. So much so that I gave it a spring clean this last September, adding bits here and there to strengthen the case (as I'm only 99.9% certain about my conviction that  normal g forces - from the Earth's own pull - are insufficient to cause unmixing of CO2 and stratification, ONCE THE GAS has diffused and mixed with the nitrogen and oxygen of air).

With that as preamble, folk should perhaps understand why I'm back again, still fine-tuning, still whittling down that stubborn 0.1% of doubt.

What I wish to describe now is a thought experiment. (Yes, I  know it should or could have been a real one, but if thought experiments were good enough for Albert Einstein then they are good enough for me - that's my story and I'm sticking to it).

It's a development of the 'teaching aid' in the original posting, which began with a brief look at the behaviour of gases trapped inside balloons, where they do indeed show their heavier or lighter-than-air characteristics. That's before the gases have escaped from their balloons, then diffused and mixed with air and lost their ability to "sink", settle out, stratify, call it what you wish.


Last night I had a brainwave. Why not keep the gases trapped inside their balloons, and allow them to mix by diffusion (which may take a few minutes, possibly a lot of minutes for totally even and homogeneous mixing, but mix they will, such is the nature of gaseous diffusion).

 How might the behaviour of the balloons compare before and after mixing? Let's do that thought experiment.

Before opening the valve: the two attached balloons may ascend, descend, or stay put, depending on the relative size of the two balloons, and the average density of the two gases compared to that of the surrounding air. If the average density is less, the system ascends etc. (And it won't matter a jot whether the gases are separate or pre-mixed  or post-mixed for that to be true - important for what follows).

But one thing's for certain. The two balloons will remain oriented with respect to each other, as in the diagram, with the blue helium balloon on top. That's because it always experiences more upthrust than the red balloon, displacing a greater volume of air for a given weight of enclosed gas. If one attempts to alter the stacking geometry, the system will self-correct when released.


Now let's picture what happens if the valve is opened, or the fusible wax plug is melted, allowing the two gases to mix. One could allow mixing by diffusion only, which means a lot of waiting. Alternatively one can speed up mixing by inverting the balloons as shown below.

xx

The lighter gas helium, now underneath, will ascend; the heavier CO2 will descend, and being a two-way countercurrent system there will be faster mixing than if it were by diffusion alone. A series of inversion, re-righting, re-inversion etc should result in a homogeneous distribution of gases between the two balloons.

What happens when one releases the two (still attached) after mixing?


Answer:

If the original system ascended, so will the new one.

If the original system descended, so will the new one.

If the original system was perfectly balanced, neither ascending nor descending, so will the new one. 

But there will be a difference. Both balloons are now equally buoyant (or non-buoyant, depending on the proportions of the two gases). So there will be no tendency for one balloon to be above the other. In other words, the two balloons can adopt any configuration through 360 degrees (with a slight tendency maybe for the smaller balloon to 'lead the way'  if rising or sinking, due to aerodynamic differences).

So the two attached balloons might go up, go down, or stay put, looking like this:

or like this:

or any angles of rotation in between.

One thing's for certain. Restoring the original configuration to blue on top, red underneath will not cause reversion to the original self-correcting orientation, since that would require that the gases unmix, with CO2 going back into the red balloon, helium going back on top.That as we've seen. simply does NOT happen at normal values of g. Which is where we came in...

I'll be tacking  the essentials of this posting onto the original one in the next day or two as a postscript.