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Everything, in theory, can have a smell, but that is not the whole point of this question.

My main query is, since things do smell, does that mean that everything is slowly evaporating (or, sublimating, I suppose)?

For example, if we perceive metal to have a scent, this must be because some of the molecules of metal are in the air or are at least affecting the gasses between the metal and our noses.

What is happening in terms of physics that causes almost anything to be perceivable by a (biological or otherwise) molecule receptor?

(There are a few similar questions closed as off-topic, but I would like to argue that this is a valid physics question.)

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    $\begingroup$ The answer to the metal question is here: Why can we smell copper? and here. I guess the standard haemogloubin explanation of the metallic smell of blood is false: Why does blood smell like copper? $\endgroup$ – Abhimanyu Pallavi Sudhir Nov 21 '19 at 16:20
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    $\begingroup$ Have a look at this video to know more about smell. Also not everything smells, like $ {CO_2}$. $\endgroup$ – user238497 Nov 21 '19 at 16:38
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    $\begingroup$ It is strictly speaking not evaporation, nor sublimation, but dissolution (or other surface reactions). There is not phase change taking place, but surface atoms being ripped free and mixing with the air molecules (unless you have an air-free vaccum, in which case actual surface evaporation might happen). $\endgroup$ – Steeven Nov 21 '19 at 16:42
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    $\begingroup$ Re, "Everything, in theory, can have a smell." To what theory do you refer? $\endgroup$ – Solomon Slow Nov 21 '19 at 18:26
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    $\begingroup$ Black holes would like to have a word with you. $\endgroup$ – AccidentalFourierTransform Nov 22 '19 at 23:15
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Not everything is evaporating. You raise the point that we can "smell" certain metals which, given how you smell most things would imply that the metal is evaporating somehow and entering your nose. You'd be right to think this is strange and in contradiction with the idea that metallic bonds tend to be strong and so unlikely to evaporate. This is a good answer explaining why we appear to be able to smell metals.

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    $\begingroup$ No, indeed everything is evaporating, solids, neutronium, even black holes; some things just evaporate relatively slowly. Whether we can actually smell single atoms, elementary particles or photons is a different matter. $\endgroup$ – Peter - Reinstate Monica Nov 22 '19 at 16:01
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    $\begingroup$ @Peter-ReinstateMonica surely if something has already evaporated it can't be evaporating? Gases for example. I'd go further and say that only liquids (and solids if we include sublimation which I think makes sense here) can evaporate, though extending this to exotic states of matter resembling solids/liquids is reasonable. $\endgroup$ – Chris H Nov 22 '19 at 16:52
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    $\begingroup$ @ChrisH Yeah, everybody dies. Whether the dead are exception or proof of that is, I guess, subject to debate ;-). $\endgroup$ – Peter - Reinstate Monica Nov 22 '19 at 17:01
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    $\begingroup$ @Charlie in the chemical sense, evaporation is "the process by which an element or compound transitions from its liquid state to its gaseous state", so solids do not evaporate either; they may sublimate, but that term is usually reserved for the few substances that perform this phase transition relatively quickly, like Iodine. I'm simply using "evaporation" in the sense of the OP, that is, become vapor. Black holes, obviously, are a special case; but black hole mass reduction through Hawking radiation is often called "evaporation". $\endgroup$ – Peter - Reinstate Monica Nov 23 '19 at 15:18
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    $\begingroup$ It'd be nice to quote from the answer to which you linked so that this answer can stand on its own somewhat. I like this "... it is not metal evaporation, but skin lipid peroxide reduction and decomposition by low valence metal ions that produces the odorants." in the linked answer as a description of why (the relevant) metals smell. $\endgroup$ – Kenny Evitt Nov 23 '19 at 23:58
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When a solid or liquid body is immersed in a gas (e.g. ordinary objects sitting in a room full of air) the thermodynamic equilibrium is a dynamic equilibrium in which there is a non-zero vapour pressure of the material of the object in question. That is to say, if you first replaced all the air by clean air, then afterwards the object would start to evaporate or sublimate until a vapour pressure of that material was built up again. This evaporation or sublimation will not go on for ever (unless the air is changed again); it will stop when there is a dynamic equilibrium between material moving between the two phases (solid/gas or liquid/gas).

This allows us to smell most things, as you say. I don't know if there are other issues though. I guess that smell involves some complex chemistry, such that the presence of some molecules is detected indirectly through their effect on others, etc.

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Well. "Everything smells" breaks just at consideration of methane. Simple molecule $CH_4$ does not smell. Water $H_2O$ does not smell either. Most gases, in fact, do not smell. "Smelling" tends to be achievable only when reactivity of molecule is high enough. Consider ozone $O_3$, it smells because it is unstable $2O_3 \rightarrow 3O_2$ (the exact reactions are rather complicated but you can refer to corresponding literature about ozone). Chlorine also smells starting from very small concentrations, same with phosphorus.

If we consider organic compounds, mostly we smell what our organs are trained to smell, biologically in the way of evolution. We are good at detecting etheres, but we fail at simple yet multiatomic molecule of naphtalin.

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    $\begingroup$ Reactivity, +1. "Gold does not have a smell. Interestingly many other metals do, but it’s not the pure metals that have the smell, but in fact the oxides, sulphides and other chemical compounds with the metal. Iron compounds are a good case in point, where for example there is a similarity between the smell of blood and that of damp rusty metal." – quora.com $\endgroup$ – Mazura Nov 22 '19 at 22:53
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Smell depends on certain volatile molecules which are detected by our noses. Of course, there isn't an unlimited supply of those molecules, so there will indeed come a time when the smell ceases.

That said, as you can see from the link in Charlie's answer, what we actually smell is not necessarily what we think it is. Copper smells a certain way, yet what we detect isn't copper. Therefore it's not correct to say copper is evaporating. It's reacting and changing its chemistry, and therefore there's less copper left, but it's not evaporating.

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    $\begingroup$ @Mazura: Presumably "there's less free copper left" is what was meant. $\endgroup$ – Sean Nov 23 '19 at 22:16
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This is one of those extreme questions where the answer is "yes, everything evaporates" and "no, you shouldn't act as though everything is evaporating."

In chemistry class, we typically talk about reversable and irreversable chemical equations. Reversable ones achieve some equillibrium concentrations, while irreversable ones end up with all of one compound or another. It's short, succinct, and wrong. But it's good enough to be really effective in virtually all cases!

In reality, all chemical transformations we find are reversible, existing in some equilibrium or seeking an equilibrium. So yes, there is an equilibrium concentration of metal atoms in a solid with metal atoms in gassious form.

However, that equilibrium can be so lop-sided that we can't even measure it. When this happens, we call it irreversable because there's really no reason to think otherwise.

In the theoretical side of things, this is known as entropy increasing. The entropy of a gas is higher than that of a solid, so there is a very slow shift towards things evaporating. If you start talking on timescales of 100,000,000,000,000 years, this effect is very powerful. In fact, wait long enough and we have reason to believe protons, themselves, may evaporate! However, on human timescales, it's not so useful. Practically speaking the rate that metals evaporate into the air is so extraordinarily slow that we can basically ignore it.

The most famous example of this thinking that I know of is from the high vacuum physics world. High vacuums are the really nasty vacuums that make the vacuum of space look weak and tepid. In that world, there's a phrase, "everything outgasses." Everything you put in a high vacuum chamber evaporates. Many things that we think of as a solid or a liquid evaporate/sublimate fast enough to actually mess with your vacuum. At these extreme vacuums, there aren't very many atoms bouncing around in the first place, so just a little evaporation makes a big deal. A single fingerprint can outgas for weeks, preventing you from pulling the high vacuum you seek. Thus those who work with vacuum chambers are very strict about cleaning surfaces to remove such artifacts.

Of course, some things just don't have a smell. In high vacuums, we see lots of hardware made of stainless steel. The chemical bonds in stainless steel are very strong so it evaporates very slowly. It evaporates slow enough that we can't really detect it, so we treat it as though it doesn't evaporate.

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Technically, everything IS evaporating, it's just a question if our olfactory abilities are advanced enough to detect it. But the metal smell coming off metal is actually the result of a reductive chemical reaction between skin lipid peroxides and the metal itself. These usually produce an array of molecules that do evaporate readily (unlike metal), and give each metal a distinctive smell, but the main ingredient of this smell is Oct-1-en-3-one.

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