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I dissolved a coin made of silver and copper and probably other metals in nitric acid. I neutralized the solution with some sodium hydroxide and then added salt(NaCl) to make Silver Chloride. A white percipitate formed and was filtered off. Some insoluable copper hydroxide was also mixed with the silver chloride. I washed the mixture with dilute hydrochloric acid to form soluable copper chloride, quite a lot of dilute acid was needed.

The waste solutions were left to evaporate in a fume hood so that they did not take up so much space. After a few weeks when i went to see if they had evaporated down i noticed a white powdery film that coated everything in the fumehood.

Why did this happen and what could this powder be?

I thought salts didn't evaporate with the solutions.

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It is probably NH4Cl(s), slowly covering everything nearby long term situated $\ce{HCl}$ solutions, especially if traces of ammonia occur in air. ( $\ce{NH3(g) + HCl(g) -> NH4Cl(s)}$ )

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    $\begingroup$ I tested this theory by scraping of some dust and doing a silver nitrate test on it which yielded a positive result for chloride ions. Then i added sodium hydroxide solution to the powder in a test tube and used a wet ph paper to test for ammonia gas which also gave a positive result. Turns out that the Acid-base waste jug's cap was loose and that was what probabily released ammonia. $\endgroup$
    – randomuser38924
    Commented May 17, 2023 at 11:36
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    $\begingroup$ It is quite common knowledge among chemists with some lab experience. $\endgroup$
    – Poutnik
    Commented May 17, 2023 at 11:47
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The salts don't evaporate, but fume hoods have air flow going through them to enable the removal of the toxic fumes through the hood. That air flow can pick up droplets of solution, which then evaporate in the space of the hood — rather than staying in your original container.

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