Re: Why would clear rain drops turn white and leave a white residue?

Hello, Tom!

It is impossible for me to state with any certainty exactly what's going on, but I can give you 
some possibilities and suggest an experiment or two for you to do.

Rainwater is never entirely pure, because drops almost always precipitate on a non-water 
particle. Especially in urban areas, it also tends to contain chemicals and/or particulates to some 
degree.  So, if you happen to live downwind of a power station, you may have some acidic fly ash 
in your rainwater, and if you happen to live downwind of a cement factory, you may have basic 
cement dust in your rainwater, etc.  Similarly, when it's not raining, dust tends to accumulate on 
pretty much every surface, and thus the surface residues on your car door probaly aren't free of 
such stuff, either.

My best guess is that you had stuff on the car door that reacted with the rainwater.  If, for 
example, the surface of the door had something basic in it, acidic rainwater  might cause a 
reaction that could precipitate salts (CaSO4, for example, if the dust was cement dust and the 
rainwater was slightly acidic with sulfuric acid).  At this distance, via email, I have no way at all to 
know what the exact species are, of course.

But you may be able to get an idea of what's going on without too much expense.  If you have a 
scientific supply store nearby, get some pH paper at least, and they often have water testing kits.  
Or, go to a pet store and get some of the testing kit used by aquarists to check their water (they 
have them for pH and various chemicals such as nitrates).  Also possible is to find water testing 
kits that are used by people who have their own wells.  Or, you may be able to submit samples of 
water to either your city or county government (a service sometimes provided for the 
convenience of property owners using wells).  Lastly, but more expensive, testing can be done 
commercially...again, labs that test well water are probably most available. 

You can do two things now:  

1. In a known clean vessel (washed and rinsed in distilled water is best) you can collect some 
rainwater and see if it's acid, or basic, and (to the limits of what water testing facilities, or kits, 
are available to you) what's in it.

2.  Take some distilled water and use it to collect the residues on the door.  With a clean 
eyedropper, put drops on the door, let them sit a while, and then recollect them with the 
eyedropper. Then test the collected water.

Item #2 may in itself provide a somewhat more mundane answer.  That is, if the rainwater is 
actually pure, then all that may be happening is that the water dissolves something in the surface 
residues. This is very similar to the water spotting that is commonly seen with insufficiently 
rinsed glassware....residues of the soap are concentrated in the drops and, when they dry, leave 
behind a whitish spot.  So, if the distilled water drops turn white and then leave a white spot, we 
can be reasonably sure that the phenomenon you are seeing is a matter of what's in the surface 
residues, and the rainwater has little to do with it.

Good luck with your experiments!