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A comment under the Space SE question Is there really precipitation on Mars? Does frost count? brings up the use of the term "precipitation" in chemistry and we wonder if precipitation in weather and in chemistry are in fact analogous insofar as they both start as small particles suspended in the 3D medium (atmosphere or solution) and only fall to the ground or the bottom of the container if/when sufficiently large to make headway against Brownian or other motion.

In chemistry, on Earth in gravity, if a material from solution conformally, over all surfaces vertical and horizontal (e.g. the sides as well as the bottom of a container) is that still sometimes considered precipitation? Or is that crystallization from solution, and analogous to frost formation but not snow or rain?

Images of frost formation: http://www.its.caltech.edu/~atomic/snowcrystals/frost/frost.htm

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  • $\begingroup$ It would seem that the accepted opinion is that Mars had surface water in the past. So precipitation could have taken place. $\endgroup$
    – MaxW
    Nov 7 '20 at 2:34
  • $\begingroup$ @MaxW thanks but let's keep that conversation on the other post (where it is most welcomed!) and not here. This is strictly a question of terminology. $\endgroup$
    – uhoh
    Nov 7 '20 at 2:36
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In chemistry, on Earth in gravity, if a material from solution conformally, over all surfaces vertical and horizontal (e.g. the sides as well as the bottom of a container) is that still sometimes considered precipitation? Or is that crystallization from solution, and analogous to frost formation but not snow or rain?

Precipitation, as typically used an understood in chemistry, is really a very very old terminology (~ several hundred years). It refers to the formation of a solid by adding or mixing chemical reagents or even by cooling. There is no such requirement that the precipitate has to settle to the bottom. For example, colloids may not settle for ages. Precipitation involves crystallization, but not all precipitates are crystals (organic substances can be precipitated out by adding suitable solvents to a solution).

Now there are special ways to generate precipitates in solution. There was an experiment which we taught in classical analysis. The experiment goes like this. Add urea to an iron salt. No precipitate forms and there are no seeds to begin with. Let it boil slowly. With the passage of time, the entire solution becomes basic and it would uniformly produce precipitate iron hydroxide. Since it was a in situ chemical decomposition of urea to produce iron hydroxides in solution, this process occurs very uniformly in the solution i.e., there is no localization where two reagent were being mixed. This yielded uniformly sized particles. Such a process is called homogeneous precipitation in solution. Contrast this situation where we would add NaOH solution to iron salts. It will be impossible to avoid localized mixing.

Meterologists use the term precipitation as well, but as per the unabridged Oxford Dictionary (can't provide a link as it is by subscription), the full word is atmospheric precipitation.

In case you are interested, the etymology is

Etymology: < classical Latin praecipitāt-, past participial stem (see -ate suffix3) of praecipitāre to throw or cause to fall headlong, to ruin, destroy, to fall headlong, to suffer ruin, come to grief, to hasten, to rush, in post-classical Latin also to cause to be deposited as a solid from a liquid solution (a1490, 1652 in British sources) < praecipit- , praeceps (adjective) headlong, sheer (see precipe n.). Compare Middle French, French precipiter , précipiter precipit v.

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  • $\begingroup$ Thank you for this informative answer! If my take-home summary of it were the following? "In Chemistry the term "precipitation" is quite broad and inclusive, but when used in meteorology we can't necessarily draw any specific analogy." $\endgroup$
    – uhoh
    Nov 7 '20 at 3:22
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    $\begingroup$ Yes, correct. In chemistry, it is a wide and broad term. $\endgroup$
    – M. Farooq
    Nov 7 '20 at 3:26
  • $\begingroup$ This is fun, so I've just asked In chemistry do we speak of precipitation in media other than "normal" liquids? $\endgroup$
    – uhoh
    Nov 7 '20 at 3:31
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    $\begingroup$ See precipitation and crystallization in chemistry context, and precipitation in meteorological context. $\endgroup$
    – Poutnik
    Nov 7 '20 at 14:16

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