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This answer to Are precipitation and crystallization both analogous between chemistry and meteorology? discusses the history and use of the term "precipitation" in chemistry.

From what I remember in school I was introduced to precipitation in the context of reactions in liquid media and usually water.

In meteorology the term refers to a phenomenon in air where droplets or crystals form from some molecules mixed with others. I don't know if we can call the nitrogen/oxygen mix a solvent and the water vapor the solute, but some of the resulting droplet suspensions are indeed called aerosols.

But my question here is constrained to chemistry. Do we speak of precipitation in media other than typical liquids? Are there gaseous precipitations? Are there precipitations in supercritical situations? Do they happen in glasses or in (presumably pretty hot) solids?

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    $\begingroup$ I've cleared the comments on the question about whether the two questions should be merged into one. Personally, I think that they are better asked separately, as was done. I do recognise that's subjective and open to debate, but if anybody wants to have a lengthy discussion, please consider doing it in chat. $\endgroup$
    – orthocresol
    Nov 7, 2020 at 22:46

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As stated in my previous answer on precipitation, formation of precipitates in supercritical is a common issue. Ask those who do sub/supercritical fluid chromatography. Sometimes you inject a methanol soluble compound into a chromatography column and the mobile phase is supercritical carbon dioxide (often mixed with a small amount of methanol) and the compound precipitates on the head of the column. So there is no restriction on using the term precipitate in fluids other than liquids.

Wikipedia also talks about precipitation in solids as

In solids, precipitation occurs if the concentration of one solid is above the solubility limit in the host solid, due to e.g. rapid quenching or ion implantation, and the temperature is high enough that diffusion can lead to segregation into precipitates. Precipitation in solids is routinely used to synthesize nanoclusters

The issue of calling a mixture of gases as a solution or not, it purely a matter of semantics. There is no rule or law which restricts that the solvent and the solute must have (strong or a certain type of) interaction in order to be labelled as a solution. Even gases interact with other gas phase molecules. Microwave spectroscopists know that very well. Many authors use the term gaseous solutions. Even one IUPAC's terminology related document uses this term. Heavens don't asunder apart if a trivial gas mixture is termed as a solution. Keep in mind that if you look up IUPAC definition of solvent, it does not include the usage of solution for gases. Only solids and liquid solutions are defined. So it is better to follow IUPAC's nomenclature.

In short, keep the meteorological precipitation away from chemistry.

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In "chemistry", we generally have a good idea of what we mean by precipitation: something falls down out of a liquid and we filter it off. Then we extend the definition to meteorology for rain falling out of the sky. And although precipitation in solids has been mentioned, its importance has not. Well, maybe because much of the solid precipitation occurs in metallurgy - but, really, it all depends on the chemical interactions of individual atoms and largely needs to be explained with chemical insight!

For instance, precipitation hardening steels are a family of corrosion resistant alloys which can have three or four times as great yield and tensile strengths as the common types 304 and 316. They are used in the oil and gas, nuclear and aerospace industries. The alloys contain Cu, Mo, Al and/or Ti in addition to Fe, Cr and Ni. https://www.twi-global.com/technical-knowledge/job-knowledge/precipitation-hardening-stainless-steels-102
The first such alloy, 17-4 PH (17% Cr, 4% Ni), was developed by Armco Steel in the 1960's.

The chemistry involved can be considered to involve a solute in the solid Fe, Cr, Ni solvent, which doesn't flow much at all, but which allows migration of the solute, which is often C. Cool the "solution" slowly, and you get a precipitate of crystalline cementite $\ce{Fe3C}$; cool the metal rapidly (quench), and you get a frozen, more or less homogeneous alloy. Varying successive heat treatments can cause partial solution or partial precipitation, all of which can have enormous effects on mechanical properties, machinability, weldability and corrosion resistance.

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    $\begingroup$ Don’t forget precipitation of gas bubbles in nuclear reactor materials, just to add another twist. Much of OPs problem seems to come from the general vs technical uses of ‘precipitation’. $\endgroup$
    – Jon Custer
    Nov 7, 2020 at 16:07
  • $\begingroup$ Precipitation occurs in most metals. It is very important in : steels, nickel alloys , aluminum alloys . and - ringer- Be Cu. Precipitation hardening of low carbon ( weldable ) steels has revolutionized the pipelines ; using Cb , = Nb for younger engineers and Europeans . $\endgroup$
    – blacksmith37
    Nov 9, 2020 at 16:43
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This topic and the related one in your other question had 2 aspects: phenomenological and terminological ones.

Meteorological and chemical precipitation are different phenomena.

  • The former is the water phase change from gas to liquid or solid, that is (almost) independent on air presence, with vapour saturated pressure criteria, usually considered in context of falling on Earth surface.
  • The latter is based on limited solubility criteria of a compound in a liquid.

Solubility of a compound in liquids depends on this compound, the liquid and temperature.

The maximum compound concentration in a gaseous phase depends on this compound and temperature, but does not ( almost (*) ) depend on the kind of gas, its pressure or presence at all.

As gases do not dissolve compounds, but are just bystanders, we cannot speak about dissolving and precipitating from them in chemical sense.

Precipitation is applicable to solids with difficulty, as processes of dissolving and precipitating are mostly kinetically frozen.

Diffusion process in hot solids is definitely considerable, OTOH it is extremely hard to observe it. But it does happens and is observed in some circumstances, like steel or alloy processing.

(*) Almost: Saturated vapour pressure slightly depends on the overall pressure by a generic way. Compound phase equilibrium implies equality of compound chemical potentials in both phases. Liquid chemical potential slightly increase with total pressure, what is followed by slightly increased saturated vapour pressure.

This effect may be dramatic near liquid critical conditions, as seen for $\ce{N2O + O2}$ mixture Entonox, where the mixture is at the storage conditions gaseous, but pure $\ce{N2O}$ would condensate.

See also SE article and Wikipedia.

Other then this, gases do not affect the equilibrium vapour pressure, but may cause equilibrium disturbance by side reactions, turning vapour molecules into different compound or a compound adduct. The extreme example is putting liquid ammonia and hydrogen chloride at $\pu{-40 ^{\circ}C}$ side by side. Both vapours would never reach equilibrium due forming solid ammonium chloride.

Molecules moving by free flight in gases is a well known fact, which you can easily find and would not ask others to find it for you. For flying in vacuum, molecules need not other molecules flying nearby.

E.g. the mean free flight path of molecules in air is typically 70 nm, what is 2-3 orders more than molecule size.

Note that I was serving in the Czechoslovak army as an enlisted air force meteorologist in 1989-1990.

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