In article . com, Doc wrote:
When soldering copper pipe, what does the flux do to cause the solder
to get drawn into the joint? Something to do with capillary action?

The main thing I see is preventing oxidation of the metal, even reacting
with oxygen in the oxide to remove some of any existing oxide for some
metals, including copper and lead/tin, whichever of those in "ordinary"
solder tends to oxidize when such solder is molten.
Some metals are reactive enough for their oxides to be unable to be
reduced by flux, with aluminum being a prime example.

- Don Klipstein )