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Are there gases that are not transparent at room temperature (i.e. at temperature below the point where the substance starts to radiate visible light due to heating)?

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  • $\begingroup$ Also pressure may be important. $\endgroup$ – Mithoron Jun 9 '15 at 13:15
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    $\begingroup$ I think, thick enough iodine or bromine fumes could be non-transparent albeit at sli~ghtly elevated temperatures. $\endgroup$ – Jan Jun 9 '15 at 13:31
  • $\begingroup$ @Jan what about mercury? $\endgroup$ – Anixx Jun 9 '15 at 13:39
  • $\begingroup$ Gaseous mercury is colourless. $\endgroup$ – gsurfer04 Jun 9 '15 at 21:25
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    $\begingroup$ Radiating light doesn't make gases opaque - the substance of the sun is transparent (i.e. some of the photons we see originate from below the surface, and pass through the layers above them) en.wikipedia.org/wiki/Limb_darkening $\endgroup$ – Random832 Jun 9 '15 at 21:59
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First, a little bit of background. Transparency is not an absolute property of a material. Every substance is opaque, so long as light has to pass through enough of it, and opacity also changes according to ambient conditions. Some substances, such as most metals, are opaque even in $100\ \mathrm{nm}$ thin films, while many gasses will let a small amount of light through mostly unperturbed even after several kilometres. For example, here is a measure of how far photons of different energies can travel on average in the pure elements before interacting once with their atoms (the graph is actually for high energy x-rays rather than visible wavelengths, but they're all photons nonetheless). It makes intuitive sense that a gas should let more light through, as the atoms in it are much more spaced than in a solid or liquid.

The two general processes that work to make materials non-transparent are photon absorption and photon scattering (reflection being included as an example of scattering). When we say a material is "transparent" colloquially, what we generally mean is that the material, at the thickness observed, does not scatter much light and thus allows a coherent image to pass through the material (basically, you can see through it to the other side without a lot of distortion, that is, the object is "see-through" or clear). Interestingly, this means that a clear material need not be colourless, as correctly pointed out by Geoff, because photons of a given colour may be absorbed while letting most other wavelengths pass through without scattering.

Gasses in general are highly transparent because they are both highly colourless (absorb little light) and very clear (scatter little light) unless you're looking through several kilometres of gas, as I mentioned previously. However, there are examples of highly clear but coloured gasses, as it is possible for them to absorb a significant amount of photons of a certain energy due to low energy electronic transitions, while allowing the rest of the visible spectrum to pass through unimpeded. While some gasses are coloured and others are not, I believe all pure gasses must be very clear (small amounts of gas will scatter almost no light), and they can only scatter a fair amount of light across a short distance in conditions where they are probably not best described as gasses anymore (plasmas, supercritical fluids, etc).

Perhaps the most classic example of a coloured gas is nitrogen dioxide, $\ce{NO2}$, a strongly brown-red noxious gas which is easily formed by decomposition of nitric acid, among several other ways. The unpaired electron in its structure is somewhat uncommon, and it can be easily excited to a higher electronic state upon absorption of photons in the blue region of the visible spectrum. Since blue light is strongly absorbed even in relatively thin samples (a few centimetres of $\ce{NO2}$ gas), the rest of the white light passes through with essentially no scattering, forming a perfect image of the other side but with a strong red tinge.

Even more interestingly, $\ce{NO2}$ reacts with itself at low temperatures or high pressures and dimerizes to form dinitrogen tetroxide, $\ce{N2O4}$, a colourless solid/liquid/gas (depending on conditions). In other words, the two substances are in a reversible equilibrium:

$$\ce{2 NO2 (g) <=> N2O4 (g) + energy}$$

Coincidentally, this equilibrium is characterized by an equilibrium constant which is close to 1 at ambient conditions, and this constant can easily be changed by varying temperature or pressure to higher or lower values. This means that under relatively easily achieved conditions, it is possible to study the reversible change of a coloured gas into a colourless one!

enter image description here (Source)

Though I focused on $\ce{NO2}$ because it is a remarkable case, there are several other examples of clear but coloured gasses. As Geoff mentioned, the halogens all form coloured gasses, though bromine is a volatile liquid and iodine is a solid at room temperature, so a small amount of heating is necessary. Ozone, $\ce{O3}$, is somewhat blue (source), though its colour is best observed in the liquid phase. The same goes for oxygen gas, $\ce{O2}$, which is slightly blue, though it is not the cause for the blue sky. There are probably a few more examples out there.

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    $\begingroup$ I like your answer! After reading your first paragraph it occurred to me that we take it for granted that the concept of transparency always applies to visible light. It seems obvious in retrospect that all substances should block light over some wavelengths, even if it is not obvious that they would block light over all wavelengths. $\endgroup$ – thomij Jun 9 '15 at 17:45
  • $\begingroup$ You mentioned metals being opaque in films 100nm thick. Would a film of, say, iron actually be transparent if the film were only a few atoms thick? $\endgroup$ – Brian Gordon Jun 10 '15 at 1:32
  • $\begingroup$ @BrianGordon Yes, a layer of iron only a few tens of atoms thick will be transparent. As part of my work I regularly deposit very thin metal films by evaporation, often on the order of 10-100 nm thick, to form conductive contacts for circuits. I have not personally worked with iron, but I can tell you that I have deposited transparent silver films with thicknesses of 10-20 nm, which is around 100 atoms thick. This is often exploited to make transparent electrical contacts. Similar gold films are also both clear and colourless. $\endgroup$ – Nicolau Saker Neto Jun 10 '15 at 1:45
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I'd separate transparent and colorless.

Most gases are transparent or very nearly so because the concentration is low and absorptions are often weak.

Chlorine, though is yellow-green, and has a noticeable color (from Wikipedia) chlorine ampule

Other halogens such as bromine and iodine do have observable colors as vapor, although as mentioned in the comments, you often need slightly elevated temperature for significant vapor to form.

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    $\begingroup$ Are there gases with metallic or non-metallic luster? Will supercritical mercury have it? $\endgroup$ – Anixx Jun 9 '15 at 14:01
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    $\begingroup$ It might, but I've never seen an image of supercritical mercury. It's electrically conductive, but you need high pressure and temperature. $\endgroup$ – Geoff Hutchison Jun 9 '15 at 14:11
  • $\begingroup$ Under high enough pressure will chlorine become non-transparent or will become liquid/solid earlier? $\endgroup$ – Anixx Jun 9 '15 at 14:47
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    $\begingroup$ I've never even contemplated the existence of supercritical mercury until Geoff's comment. Cool stuff! $\endgroup$ – Curt F. Jun 9 '15 at 16:34
  • $\begingroup$ @CurtF. Neither did I until asked. But I figured a little searching might be worthwhile. :-) $\endgroup$ – Geoff Hutchison Jun 9 '15 at 19:28
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The answer to your question is yes, there are non-transparent gases, however, it depends upon the wavelength at which you are observing and how much gas you are looking through. At some wavelengths the gas is opaque at others transparent. The amount of light absorbed depends on its concentration, the path-length through which the light passes, and how strongly a molecule absorbs the light. It is quite easy to have conditions that absorbs 99.9% of all light at a given wavelength.

If you are observing in the infra red then simple gases such as oxygen or nitrogen have no transitions which absorb photons as they have no dipole which can 'capture' (in a vibrational transition or rotational) the energy of a photon. (Technically weak transitions can occur by other perturbations but for the present discussion we ignore these details).

Oxygen has low lying electronic transitions, which is when an electron is promoted from the ground state to an excited state, but these are in the near infra red and cannot be observed by eye. NO$_2$ also has visible electronic transitions as it is brown and so do the halogen gases and vapours. Mercury vapour also absorbs in the visible and this can be seen as shadows of vapour from the liquid when illuminated by light also from a mercury lamp. The transition is so intense that the vapour becomes opaque.

At shorter wavelengths such as the ultraviolet, oxygen, nitrogen and water vapour and most all other vapours and gases become opaque at relatively low pressures and path-lengths. i.e. if our eyes were sensitive to uv light only the air would look black.

At very short wavelengths, x-rays are absorbed by a molecules atoms (not by the molecule itself) but they are also elastically scattered (no energy imparted to atom as in diffraction) or inelastically scattered (some energy imparted).

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