Why is it so painful to apply alcohol (or other topical treatments like merchurochrome) on open wounds, as opposed to an inactive solution like water?
Alcohol is a weak acid. One gets a chemical burn when it is applied on damaged skin.
Weak acid ? Alcohol is more of a dehydrating agent and disruptor of membranes. Here’s abrief review: Disinfectants
Membrane disruption is what acids do.
That may be, but the pKa of ethanol is about the same as that of water (15.9 vs 15.7). Neither are commonly considered to be “acids”. Nor are any of the other common alcohols, except phenol, particularly acidic.