Put drops of water on glass or plastic sheet. After evaporation we see white soft dust there. What's that?
Minerals that had been suspended in the water. I'm not sure of the exact complex, but there is a heck of a lot of stuff in city water, and stuff in well water too. City water has treatment chemicals in addition to the minerals present where well water is often softened, which adds some sodium chloride (salt). Our city (and our water company, our city runs it's own water treatment) mails us a water report once a year that details the exact levels of minerals and chemicals in the water. I would implore you to write to your water company and ask them if they publish a report, and if not, suggest that they do.
all well water has a high calcium concentration, but it is also small amts of of trace elements. In my home town, people don't even need to drink milk, they get plenty of calcium in the water,, unfortunately it is the stuff that causes hard water, and you have to use greater amts of surfactant (soap) to do laundry.
the white soft dust that u see are impurities in the water which are dissolved in it and which resuface upon evaporation. ur water is not perfectly clean!
The concentration of what ever impurities were in the water to start with. Perhaps solid chlorine etc. The best way to see if no residue is formed is to used distilled water. It is water that you be free of almost all impurities.
If it was pure water, and there was no dust present in the room there should be no dust. But if it's tap water that is most likely lime deposits or deposits from the minerals in the water. Try the same experiment with distilled water (not bottled drinking water, because minerals are added).
