The expression “rubbing salt into one’s wounds” means making a bad, or painful, situation even worse than it had been.
What is it about salt that would make a wound hurt more? And other substances that have the same effect, e.g. alcohol?
Both of the substances you mention have a strong dehydrating effect- to hazard a guess.
Apart from any kind of chemical reaction that would occur (I don’t know the mechanics of this but I know it hurts), there’s also the fact that salt is coarse and grainy. Rubbing sand or gravel in a wound would hurt too. Introducing any foreign substance to a wound can be painful. I’ve had cuts that were incredibly sensitive to things as mild as water.
Ok, let’s forget the “rubbing” part. If you just sprinkle salt onto a wound, wouldn’t it be more painful than sprinkling sand, for example? And doesn’t alcohol hurt more than water?
I think the pH differential acounts for most of it. Blood is slightly acidic and salt is much more alkaline.
Rubbing salt into a wound was a useful technique to reduce infections in the pre-germ theory, pre-antibiotics era.
They didn’t know just how it worked, but it did have some effect. The dehydrating effect of the salt, and the change in the pH of the wound area would kill a lot of the bacteria in the wound, thus decreasing the chance of an infection.
Also, even in ancient times, salt was a fairly pure, hygienic product – a lot of effort went into attempting to remove any impurities from salt (to get the highest price). The salt was probably cleaner than the rags used to bind up the wound.
Of course, it hurt.
But that wouldn’t necessarily have been a big disadvantage, if it convinced the patient that he had received effective medical treatment. The placebo effect was one of the more effective tools of ancient doctors. (Even today, I have relatives who are convinced that the sweet cherry-flavored cough drops aren’t as effective as others, though they all have the exact same active ingredients. Or that orange-flavored children’s aspirin isn’t as effective as plain children’s aspirin.)
Salt hurts like many other substances, because it contacts a live nerve. It wouldn’t hurt if a nerve wasn’t physicaly touching the substance. A stable molecule like pure water won’t hurt. The farther from a PH of 7 a substance is, the more reactive it is. The nerve is chemicaly reacted apon by the salt, and it sends it’s impulse to the brain to signal it’s being damaged. Chemestry Acids and Bases.
Actually blood has a pH of about 7.4, just a little more basic than neutral water. Also, table salt NaCl is not appreciably basic or acidic. It is the product of a reaction between certain acids and bases, for instance NaOH and HCl. There are other salts that are basic or acidic, though. Ammonium salts are acidic for example.
Where did you get either of these ideas? They’re both wrong.
The pH of blood is tightly controlled between 7.35 and 7.45, which is slightly alkaline. If your blood was actually acidic, you’d be dead.
Table salt (NaCl) is neither acidic nor basic (alkaline). Pure NaCl dissolved in pure water will produce a pH of exactly 7.
None of this acid-base chemistry has anything to do with NaCl (other than the fact that it is a product of NaOH and HCl). The only thing that the solid NaCl will do is dissociate into hydrated ions when it is dissolved in water. No further chemical reaction takes place, and the nerve is not being reacted upon by the salt. The sodium ions and the chloride ions are as stable as the liquid water you mentioned.
P.S. Also, what quelquechose said.
Sorry, that’s what I get for spouting off about things I think I know. I stand corrected.
Yes and yes. For real fun, try lemon juice.
I knew when I woke up this morning I’d scrambled my 30 year old chemestry lessons. 
I was coming to this thread to confess the error. It seemed right last night.