Recently, I posted a photo of the underside of a snail which was crawling vertically up a tank on our Science Communication Facebook page “Science It“.
When I shared the photo with my friends, there were several responses. One said he used to be crazy over snails during his Primary School days, and even kept a few as pets for a few weeks before releasing them back to the wild. One horrified me by saying that she liked to dissolve snails with salt during her childhood days. I was surprised because of two reasons; that snails can be dissolved with salt or salt water, and that people were actually enjoying doing that.
Someone then commented that the snails dissolve due to osmotic pressure. I quickly did a search on the term, and also discussed the Facebook conversation thread with people around me. This was what I learnt.
When salt is placed on a snail, or when the snail is placed in a highly concentrated salt solution, the salt concentration outside the snail becomes higher compared to the other side of the snail’s skin (a semi-permeable membrane).
Because of osmosis (the net movement of solvent molecules through a partially permeable membrane into a region of higher solute concentration, in order to equalise the solute concentrations on the two sides), the water from within the snail will move out of the snail through the skin towards the salt solution, causing the snail to shrivel up and die.
It can also be illustrated with a water tank with a membrane in the middle. When a lot of salt is added to one side of the tank, the water level in that side of the tank will rise due to the movement of water molecules across the membrane (osmosis).
So, when salt is sprinkled on a snail, it’s osmosis that kills it. Poor snail!