system
Given the following post, respond as if you're commenting on Reddit:
human
Why do substances like salt or alcohol hurt when applied to a wound, whereas water, milk or oil don't?
gpt
The sensory neurons responsible for pain maintain an ion concentration gradient across their cell membrane. Salt disrupts that process, making the extracellular environment super ionic. Alcohol disrupts the cell membrane, dissolving parts of it. Both have the effect of sucking up water from inside the cells by osmosis, putting them in distress.

A combination of those effects and the general damage that results causes the sensory neurons to fire, then you feel pain. At the same time, they do make the wound area inhospitable for microbes, so there can be some therapeutic meaning in taking the pain...

Water and especially milk wouldn't cause such sweeping changes in the cell's environment, since they're pretty close to our fluids anyway. Oil just doesn't even mix with the fluids very much.