Question

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.

Answer 1

The white powdery film is likely crystallized sodium chloride resulting from water evaporation and the salts reaching their solubility limit and depositing as solids in the form of crystals, illustrating the law of conservation of mass.

Step-by-step explanation:

The white powdery film that formed in the fume hood after the evaporation of waste solutions is likely due to the formation of sodium chloride crystals. During the evaporation process, the water component of the solution evaporates, but the salts do not. However, as the water evaporates, the concentration of salts increases until they exceed their solubility limit. At this point, the salts begin to crystallize and deposit on surfaces as the liquid solution becomes a solid phase. The transformation of these dissolved salts into solid form on evaporation is consistent with the law of conservation of mass, which you see when recovering sodium nitrate from the reaction between silver nitrate and sodium chloride.

It's important to recognize that even though silver chloride is described as "insoluble" in water, it actually does have a minuscule degree of solubility. This can be represented in an equilibrium expression for solubility, as seen with the equation AgCl(s) + H2O(l) ⇒ Ag+(aq) + Cl− (aq). However, in this case, the likely culprit for the white powdery film is the crystallized sodium chloride due to its much higher solubility compared to silver chloride or other potential salts in the mixture.