Re: Does rock salt melt ice better placed before or after the ice is formed?

I think you've already answered your own question, Christopher. You know that salt lowers the freezing point of water, so keeps ice from forming at 32F (0C). Read the following MadSci archive for information on how salt affects ice.

You also know that salt affects the freezing point of water better if it dissolved in the water first. The reason for this behavior is that the salt ions are keeping the water molecules from aligning properly to form an orderly crystalline ice structure. This other archive explains a bit about why salt is better at preventing ice formation.

So, if the salt is already dissolved in the water - i.e. if you add the rock salt BEFORE the water freezes - the salt will be more effective than if you add the salt ontop of the ice once it is formed. When you add salt after, you have to wait until the salt can dissolve and turn into ions before it will start to affect the H-O bonds and melt the ice. But you knew this information already...

So, I haven't told you anything you didn't already know but I hope this explanation has helped a little anyway. Thanks for your question!

Kieran