It's not quite right to discuss the melting of salt water. What melts is the ice, and ice has almost no salt in it. It's true that the melting temperature is reduced if it's sitting in salt water instead of in plain water. So long as the water is kept stirred a bit, to keep its temperature uniform, the ice will melt more quickly in saltwater.
Why does having salt in the water encourage the ice to melt?
First, you should understand the basic rules of the game for problems like this. Nature tries to distribute energy around in whatever way allows its little parts to get to as many states as possible. The balance between freezing and melting involves two factors:
1. The frozen ice has lower energy than the liquid. This extra energy is released to run around in the environment, letting things in the neighborhood find many more different states than they otherwise could.
2. But the molecules lined up in the ice don't themselves have as many ways to move around as the ones sloshing around in the liquid.
What does salt do? The salt ions in the liquid get more states to run around in if more liquid is formed, and lose states if some of the liquid is lost to the ice. So having salt in the liquid favors forming the liquid state.
There's no 'chemical in' the salt. The salt itself is the chemical.
(published on 02/07/09)