why and how does rock salt melt ice?

Answer 1

Interesting question.
Salt changes the property of the ice, making the ice molecules interact with the salt. the sodium and chloride ions interact with the water molecules disrupting the crystal lattice that is formed by the ice crystals. once the salt has been incorporated into the water, it forms a new solution and decreases the freezing temperature of the solution. this is why salt water freezes at a much lower temperature than pure water. (the same is true for boiling water, if you add salt, the boiling temperature increases for the solution, one reason you add salt to boiling water to cook pasta).
for more information check out:
http://www.chemguide.co.uk/physical/phaseeqia/raoultnonvol.html
http://antoine.frostburg.edu/chem/senese/101/solutions/faq/why-salt-melts-ice.shtml

Answer 2

Paul M's answer is correct and complete.

I wonder, though...if you placed, on a piece of ice that had no liquid water on it, rock salt, while it was in a completely dark room at a temperature below freezing, would the rock salt still function in the same way? It seems to me that the salt has do dissolve in a liquid for it to work to lower the melting point of the ice.

Answer 3

it lowers the melting point of ice