Gases are frequently described as "colorless, odorless, and tasteless." I am wondering if this is redundant, since I can't think of many gases that have a detectable color to begin with, much less ones that have a color but no odor.

Are there any gases that are colorful and odorless at standard temperature and pressure? In particular, gases that impart enough of a color to white light that one could look into a room filled with the gas and detect its presence visually?

(In the limit I suppose one could argue that a standard atmosphere is "colorful" since over many miles one can see blue scattering, but the question here is whether it is ever useful or helpful to describe a gas as "colorless.")

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    $\begingroup$ $\ce{O2}$, $\ce{N2}$, noble gases, $\ce{SF6}$, lower hydrocarbons and freons... see, the list of odorless gases is pretty short, and it just so happens that all of them seem to be also colorless. $\endgroup$ Jan 29 '16 at 16:54
  • $\begingroup$ @IvanNeretin - interesting: I hadn't thought to ask the question that way. Now I'm trying to think of other odorless gases. $\ce{CO}$ and $\ce{CO2}$ famously make the list, but I can't think of others right now. $\endgroup$
    – feetwet
    Jan 29 '16 at 17:38
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    $\begingroup$ Yes, I forgot $\ce{CO}$. As for $\ce{CO2}$, to call it odorless would be a bit of understatement. $\endgroup$ Jan 29 '16 at 17:41
  • $\begingroup$ There are a few colorful gases but they also possess strong odors, such as the halogens. Iodine vapor in particular is a vivid purple. Even though it's "b.p." is listed as ~180 C, iodine is quite volatile at STP (just carefully sniff some crystals ;-). $\endgroup$ Jan 31 '16 at 2:51
  • $\begingroup$ $H_{2}$, $D_{2}$ $HD$ and other three isotopic forms with tritium : all are colorless and odorless ! $\endgroup$
    – ankit7540
    May 15 '16 at 7:21

There are plenty that are smelly and colourless, or are odourless and colourless but I would agree that there are none that are coloured and odourless. Surely it is just coincidental if any of these pairs occur because the chemistry involved in vision and smell are totally different. (Whatever the chemistry of smell is it does not involve photons!)
If you were less restrictive in your conditions then most gases will be 'coloured' in the sense that they have electronic absorption in the near ultra-violet and also infra-red at longer wavelengths. Perhaps some insects would see gases such as N$_2$O or O$_3$ as coloured. Also if you were to allow vapours then many compounds can be considered if heated, for example azulene (an isomer of naphthalene) is blue and has a small but measurable vapour pressure if warmed slightly. I don't think I want to smell any, however. The ideal of filling ones nose and lungs with aromatic or dye vapours that then condense is not an attractive one!

  • $\begingroup$ Azulene actually smells quite nice, kinda like crayons $\endgroup$
    – Ingolifs
    Feb 14 '19 at 8:14

I would say, there's a solid no, here.

Unless, y'know. You'd be willing to smell some fluorine, which, although, technically odorless, that's only because it tends to destroy the nerve receptors that actually handle smell. (Even this isn't technically true. Fluorine is smellable, to some extent, but only off of certain rocks.)

Something else that would be technically colorful, I suppose, would be iodine vapor. This is actually a very, very pretty violet color. However, this is neither nontoxic nor odorless. Nor tasteless. In fact, it's pretty toxic if you breathe it in.

Same goes for the rest of the halogens, really.

The problem with gases and color, is where color actually comes from and how you perceive it. Photons bounce off of whatever, the thing it bounced off of absorbs a few spectrums of light and reflects others, and you perceive the result of this process as color. (This is not true for light emitters. With light emitters, it emits the spectrum you perceive as color.) The issue is, there's not much for light to bounce OFF of, when it comes to gas. It's just so diffuse in the air, so there's not much for molecules to bounce off of.

Even when in liquid form, they're still really tiny molecules. Only oxygen is a faintly blue color. The halogens form fairly opaque liquids, but I have no idea why you'd want liquid halogens running around.

As for whether it's useful to describe a gas as colorless and odorless: Yes. In fact, it's extraordinarily useful for issuing warnings to anybody. Telling people that something that can potentially kill you, and then proceeding to tell them they can't see this thing at all, tends to invoke a heightened state of awareness for their own safety. Which is the goal, after all.

  • $\begingroup$ As @IvanNeretin suggested in the question comments: There may be very few odorless gases, and it may be that only two of them are potentially lethal. But the question was a little more abstract: Yes, it might require a large volume of any gas to reflect its color. So pick an arbitrarily large container -- e.g., a cubic mile if necessary -- when enumerating odorless gases that reflect visible light at STP. $\endgroup$
    – feetwet
    Jun 28 '16 at 16:29
  • $\begingroup$ Cubic mile? Even then, you're unlikely to get any gasses that truly produce any color. Our atmosphere produces a color because of the way the light scatters, and that's over 50 miles worth of air going straight up. But the few I suggested (i.e. The Halogen Gasses) are visibly colorful in an ampule full of it. As far as odorless gasses? I think you're outta luck. $\endgroup$ Jun 28 '16 at 17:18

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