Are there any opaque gasses?

I remember that a question that I remember someone raising in high school chemistry was whether there were any opaque gasses. Most people know that there are clear solids (quartz, diamond, water), clear liquids (water, ethanol), clear gasses (nitrogen, oxygen, carbon dioxide), and that there are opaque solids (calcite, iron, malachite), and opaque liquids (mercury, most any metal when melted), but there don’t seem to be any opaque gasses.

Are there any opaque gasses? It doesn’t matter whether or not the substance is found as an opaque gas in nature or even can persist in a gaseous form in temperatures and pressures found on Earth, as long as it has either been observed in a laboratory while in a gaseous state and its opacity verified or the scientific establishment is convinced that it can exist as an opaque gas.

Note that the following don’t count:

  1. Aerosols, such as smoke
  2. Colored gasses such as chlorine that you can still see through.
  3. Gasses in a dark room, box, or cave
  4. Your mom

It all comes down to what your standard of opacity is. Everything has some opacity, such that if you look through enough thickness of it, it’ll be opaque. The usual way of describing opacity is in terms of something called optical depth, which is thickness of material you need to cut down the light by some fraction (typically 1/e, where e is 2.71828…). So, what sorts of optical depth are you looking for? A millimeter, a meter, a kilometer, a light-year?

Just something that crossed my mind, but technically the air is opaque, in the sense that in the daytime the sky appears blue because of the way it filters light?

Being colored and being opaque are the same thing, depending on choice of wavelength and distance (Chronos’s point). Bromine gas is brown in color, I think, and so you couldn’t see very far through it. I bet you couldn’t see the sun, if you replaced the atmosphere with the same mass of bromine or chlorine. I also speculate you couldn’t see a purely blue light in a room whose atmosphere was bromine at atmospheric pressure, though I’m not very confident the room wouldn’t have to be larger than most rooms in houses, and also don’t know how hot it would have to be to achieve atmospheric pressure in bromine gas. Liklier, you could fill a room with chlorine gas at atmospheric temperature and pressure, and not see a pure red light across it. Well, in the seconds before your eyes melted. These gasses are unpleasant. Do these count as opaque?

In thin enough layers, even metals are transparent. Distance is really everything, here.

Iodine sublimates to a violet colored gas at room temperatures, and may be getting close to hat you are after:

As noted, it’s a matter of degree.

How thin is that?

I bet if you compressed these partially opaque metals, it would increase their opacity. But gases being relatively thin, it’s still going to take a bit of thickness to be completely opaque.

Does Chlorine fit the bill?

Slight nitpick - calcite can bequite transparent.

The Apollo astronauts wore helmets whose visors had a layer of gold 50 nm thick on them, giving them nice tinting against the sun.

Chlorine was specifically excluded as not sufficiently opaque.

Bromine gas is more opaque than chlorine gas.
Nitrogen dioxide is also pretty dark.

I seem to remember some coloured metal containg gases that were pretty dark colored, it could have been a chromium carbonyl

I thnk that one problem with gases is that they dont scatter like many solids - it is the scattering of light that makes silica sand or calcite opaque, not any absorption of light as such. Gases lack the scattering boundaries of solids and so have to rely on light absorption, which for anything small enough to be vaporized is not likely to have a high coefficient of absorption. The other problem is that very few molecular compounds absorb over thewhole vivisble wavelength, so like bromine will always let some colours through.

Potentially one could make even air opaque if one could get a highly turbulent flow of heated gas mixing with cold air, the light would be bent and scattered at the many temperature boundaries

maybe something like this though Im not totally convinced the effect is due to turbulent air in this example

Water vapour.

Except that gaseous H2O is transparent. It only appears white when it is a mix of gaseous water and very small liquid drops.

Really? I didn’t know that.

This really is the heart of it. Gases are, by their very nature, far less dense than liquids or solids. The mean density of gas at standard temperature and pressure is three orders of magnitude (or more) less dense than liquids or solids, which means that gases would have to be 1000 times as thick a layer (or more) than liquids or solids to provide the same absorption or scattering. Add to this that atoms or molecules in a gas can’t act cooperatively, forming absorption bands as you have in solid materials, and you have even less opacity from gases relative to solids.

Smoke, as firemen know, can be extremely opaque, but it’s really only opaque because it contains a lot of suspended solid matter. I think that’s really the case with an awful lot of opaque liquids as well.

True - superheated steam leaks from e.g. a ruptured steam locomotive boiler would be well-nigh invisible - until it cooked you like a Maine lobster.:mad:

I’m not mad, that’s just the best smiley…

Would the light absorption of a given gas scale linearly with the gas’s density? In other words, while a 30 foot layer of bromine gas would be effectively opaque at one atmosphere, perhaps a 1 foot layer would be opaque at 30 atmospheres.