Salt and pepper go together, but salt and slugs don’t pair so well. If you’ve ever battled the slimy gastropods in your garden (or just had a sadistic bent as a kid), you know that a few pinches of salt can kill them. But why? What is it about salt that causes the creepy crawlies to wither?
The answer, in a word, is osmosis: the process that occurs when a solution meets a permeable membrane. A solution is a homogenous mixture of two (or more) substances where one substance, the solute, is dissolved in the other, the solvent. When you have a solution on both sides of a permeable membrane, the solvent tends to pass through the membrane to whichever side has more solute so that concentration on both sides is evened out. That’s osmosis.
A slug has quite a bit of water inside of it, and the cells that make up its skin have highly permeable membranes. When you sprinkle salt on a slug, it mixes with the water in the mucus that the slug secretes to help it move around, creating a salt-water solution. That solution has a higher salt concentration than the inside of the slug, so osmosis occurs and water from the slug’s skin cells passes through the membranes to dilute the solution and even things out. If you use enough salt, the slug will lose so much water that it dehydrates, dies, and winds up looking pretty shriveled.
Humans can handle salt without the same thing happening because our skin isn’t as permeable as a slug’s. Put some salt in your eye, though, and you’ll get a small sense of what the slug is going through.