I can't seem to resolve how salt melts ice on a cold day.
Imagine starting with an equilibrated small bowl of ice in the kitchen freezer at -18C and a separate tablespoon of sodium chloride (also at -18C). The salt is then placed on the ice without ever leaving the freezer.
If you asked me for my naive prediction of what would happen, I would have said nothing. You have two solids well below their freezing points and there should be no liquid water that would be needed to solvate the Na+ Cl- ions.
Doing this experiment, I find a puddle of brine the next day. This shows there must've been some liquid water present to start solvating the ions, I believe this liquid water comes from the quasi-liquid phase at the surface of the ice (present at temperatures like -18C).
But some source seem to disagree, there is no mention of premelted ice surface here:
Energy is required to initiate the solution process and to continue it. The solution process, in the case of salt, will take place very slowly. A dry particle of salt placed on a dry surface will just sit there for a time until it can absorb enough thermal energy from the surrounding environment to a point where a liquid film is formed on the surface of the particle. This initial brine then triggers the solution of the rest of the salt. As the particle dissolves, it continues to absorb thermal energy from its surroundings. This type of absorption process is called an endothermic reaction.
This explanation also demands that some energy to initiate melting:
This earlier SA article says that the quasi liquid layer does indeed dissolve the salt:
Is the freezer experiment sufficient to show that there is a quasi liquid layer on ice at -18C? It is hard to reconcile with some of the above articles.