Eli5: Why does water that dries on glass leave “streaks” or grey markings on the glass?

Question

Eli5: Why does water that dries on glass leave “streaks” or grey markings on the glass?

In: 192

Answer 1

Tap water has sodium, magnesium, and sometimes potassium salts in it from either 1) dissolving minerals from the source or 2) added during the treatment process. This mineral content or ‘hardness’ can vary from municipality to municipality.

I worked as a QA chemist for a large municipality water system for a couple years and every month or so we’d get a call from a customer complaining about ‘white stuff’ in their water. It was always magnesium salts precipitating out of solution and perfectly harmless.