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.