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I just came to know concentrated HCl forms fumes because it dissolves in water vapors present in air. But why are these fumes white, HCl solution is transparent.

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    $\begingroup$ Water is transparent too; guess why fog is white? $\endgroup$
    – andselisk
    Jan 17, 2018 at 17:48
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    $\begingroup$ Tiny particles scatter light. $\endgroup$
    – gsurfer04
    Jan 17, 2018 at 17:51
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    $\begingroup$ All right so i get it now. Thanks! Water droplets scatter all wavelengths equally so they appear white. Am I right? $\endgroup$
    – SR810
    Jan 17, 2018 at 17:55
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    $\begingroup$ Also, any ammonia in the air (common in labs) reacts with HCl to make a powdery precipitate of NH4Cl. $\endgroup$ Jan 17, 2018 at 18:49
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    $\begingroup$ Same reason as chemistry.stackexchange.com/questions/26749/… and for all white things. $\endgroup$
    – Mithoron
    Jan 17, 2018 at 21:49

1 Answer 1

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Given that the fumes are scattering droplets, I would say that polar HCl acts as centre of nucleation for water vapour.

This is about the same way by which nitric acid containing small amount of nitrogen dioxide fumes white (in this case first the red-brownish oxide reacts with moisture to form the colorless acid, the latter in turn works as nucleus for water condensation and acidic droplets formation, ie white fume).

A more detailed or perhaps better answer should be under paywall:

Chang, O.-K. Why does concentrated HCl fume? J. Chem. Educ. 1985, 62 (5), 395. DOI: 10.1021/ed062p395.

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