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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6Water is transparent too; guess why fog is white? – andselisk Jan 17 '18 at 17:48
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something related to refraction maybe. – SR810 Jan 17 '18 at 17:50
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4Tiny particles scatter light. – gsurfer04 Jan 17 '18 at 17:51
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2All right so i get it now. Thanks! Water droplets scatter all wavelengths equally so they appear white. Am I right? – SR810 Jan 17 '18 at 17:55
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2Also, any ammonia in the air (common in labs) reacts with HCl to make a powdery precipitate of NH4Cl. – DrMoishe Pippik Jan 17 '18 at 18:49
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HCl is a colorless gas; it forms tiny droplets with moisture in air , as noted -fog. I have read that it( HCl ) is used for "skywriting". – blacksmith37 Jan 17 '18 at 20:45
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1Same reason as https://chemistry.stackexchange.com/questions/26749/why-is-snow-white?rq=1 and for all white things. – Mithoron Jan 17 '18 at 21:49
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1I used to work in a chemical factory in the engineering dept. where we had a diazo (blueprint machine) which developed the prints using ammonia gas. On the days when the HCl truck delivered we had a dense fog in there. – A. J. deLange Jan 27 '18 at 16:49
1 Answers
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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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