Salt melts ice essentially because adding salt lowers the freezing point of the water. How does this melt ice? Well, it doesn't, unless there is a little water available with the ice.
The good news is you don't need a pool of water to achieve the effect. Ice typically is coated with a thin film of liquid water, which is all it takes for the salt to work.
Pure water freezes at 32 F (0 C). Water with salt (or any other substance in it) will freeze at some lower temperature. Just how low this temperature will be depends on the de-icing agent. If you put salt on ice in a situation where the temperature will never get up to the new freezing point of the salt-water solution, you won't see any benefit. For example, tossing table salt (sodium chloride) onto ice when it's 0 F won't do anything more than coat the ice with a layer of salt. On the other hand, if you put the same salt on ice at 15 F, the salt will be able to prevent melting ice from re-freezing. Magnesium chloride works down to 5 F while calcium chloride works down to -20 F.
