"Smoke" from conc. HCl or conc. H2SO4

[SL]

Almost every time when I open a bottle of conc. HCl or conc. H2SO4, I
see some some "smoke" coming out from the bottle. What is this "smoke"?
Is there an explanation for the appearance of this "smoke"? If anyone
knows what it is, please let me know. I am just curious. Thanks.

SL

[shitface]

This is HCl gas and SO3 gas respectively.

SL wrote in message ...

[Mike Wilson]

I believe the "smoke" you observe is from the vapors of HCl combining with
the moisture in the air to form tiny droplets of hydrochloric acid. We
rarely observe this with conc. H2SO4 since it has such a low volatility.

[Roman A Kresinski]

This reply is not reliable under standard conditions. HCl makes a mist
in the presence of ammonia fumes, but sulphuric acid only mists when very
hot, and then you have people choking and walking quickly for windows and
doors. SO3 is soluble in sulphuric acid under ambient conditions.

Most labs have ammonia bottles; in fact I have with my own eyes seen
laboratory technicians sedulously line up little handy common reagent
bottles in 15-bottle long racks, graded so as to keep the concentrated
alkalis and concentrated acids apart, left-to-right on each bench: the
result? They were stacked right next to each other on facing benches, the
conc. HCl bottle three inches away from the conc. NH3 bottle of the bench
opposite. They fumed in summer. The sulphuric one never did.

> This is HCl gas and SO3 gas respectively.
>
> SL wrote in message ...

[DanHarring]

I wonder if 'mist' is a more accurate descriptor, or 'fog' - although that term
is probably already in usage referring to mechanically-generated droplets (oil
from machines, pumps).
Since I like to comment on odors, let me say the the 'smell' of inorganic acids
is probably not olfaction, but rather the so-called 'common (or general)
chemical sense,' CCS, or irritancy. From my readings on the subject, I gather
they are sensed or perceived by separate cranial nerves. Many volatile
odorants have both aspects, aroma and irritancy (eg aldehydes, amines, fatty
acids). Pungency is a combination of the two, imho.
I wonder how the vapor pressure of these acids compares to the more typical
volatile organic compounds, is vapor pressure the mechanism causing acid fumes
to spread out so quickly?

>Mike Wilson wrote:
>I believe the "smoke" you observe is from the vapors of HCl combining with
>the moisture in the air to form tiny droplets of hydrochloric acid. We
>rarely observe this with conc. H2SO4 since it has such a low volatility.
>
>SL wrote:
>

[Brad Davidson]

Whether or not hydrochloric acid produces visible fumes or not under normal
atmospheric conditions depends upon the humidity and the strength of the acid.
Under very humid conditions, I've seen 31% hydrochloric acid produce visible
fumes and 35.5% will visibly fume even in fairly dry conditions. In both cases,
there is evolution of hydrogen chloride gas. Whether or not it's visible (as
opposed to smellable) is a function of it combining with the moisture in the air
to produce hydrochloric acid mist (vapor, fumes, whatever).

As for sulfuric, the only time I've seen it "fume" at ambient temps is when it
is greater than 100% (i.e oleum or straight sulfur trioxide).

Brad

[Eric Lucas]

No, it's not. Gases don't form visible smoke. The "smoke" you see is
droplets of atmospheric moisture that condense with the gaseous
HCl--basically a fog of aqueous HCl solution. And I've never seen conc.
H2SO4 smoke, although I suppose oleum (SO3 dissolved in H2SO4) might
outgas SO3 and create a fog of aqueous H2SO4 by absorbing and condensing
atmospheric water.

Eric Lucas

shitface wrote:
>
> This is HCl gas and SO3 gas respectively.
>
> SL wrote in message ...