Quote:
flux does something chemically that allows the solder to really bond to the surface of the metal in a way that just heat alone won't.


According to Wikipedia, flux cleans the surfaces of oxidation, and aids wetting (reducing the surface tension of the liquid solder), which increases the contact area between the surfaces, and allows capillary action to draw the solder into the join more effectively. Doesn't say how it does it, though.
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-- roger