Since pure alcohol, for example pure Isopropyl alcohol and distilled water are supposed not to conduct electricity. What would happen if I dipped a turned-on PCB into it? . What would happen to a motherboard?
OUCH ! Your assumptions may be spot on, but OMG why would one try this?????????
There's something called 'leakage' - - current flow when it's not expected.
I was working in an aerospace lab and the the EE had me perform an experiment because I said "the resistivity of the salt water was too low and leakage would trigger the device". So we got a bucket of salt water and dropped the device into the bucket - - precisely 30 seconds later (the designed timeout), the bucket erupted and salt water spewed everywhere [yes, the device was an explosive link].
I wouldn't do it. Yeah in theory it should be ok, but you are going to have dust and other contaminants on the board, which then make your 'pure' substance no longer pure. If you really must submerse in something, I would do a power off complete rinse of everything with distilled and then go for it. Personally I'd use deionized rather than distilled but that isn't as easy to get a hold of for most people.
Or you could just use mineral oil like has been done many times before.