A famous puzzle is whether an ice cube melts to nothing more quickly in salt water or fresh water. The answer is fresh water, because the water melting off the ice cube sinks in the plain water and rises in the denser salt water. The sinking causes convection to play a large role, and the time difference is very large. See Ice melts slower in salt water? and many videos, using dye, that are easily found.
But what happens in the first second, before convection has an effect? In a "pure" situation where it is just salt water melting ice vs plain water melting ice, which melts the ice faster? In other words, which has the faster instantaneous rate of melting right at the start?
I and a friend have done several experiments.
In a 35-degree refrigerator, the overall melting story is reversed. This is expected because now the melt water floats on plain water (and on salt water), and so stays at the top. But why exactly does the salt water melt the cube a lot faster than the fresh water? This seems to indicate that in the pure situation, salt is faster.
At room temperature, with regular stirring of the water, the cubes melt at the same time. This is puzzling and seemingly contradictory to (1).
We have seen several possible explanations, but at this point we do not know the true story. There are likely several effects going on, but is there one dominant effect that drives the melting in the "pure" case?
Edited to respond to Sam Gerbil comment:
Classic case: Room temperature. The meltwater floats in saline; so saline is slower by a lot (loses).
In refrigerator: meltwater floats in both cases. But saline wins by a lot. Not clear why. Maybe because the meltwater rising rate is different because of different density. Or maybe saline melts ice faster because of freezing point depression (but I am not clear as to why exactly this affects overall energy transfer).
In refrigerator, with ice forced to bottom of container. My attempt failed. Jim just finished an experiment with nails frozen into the ice for weight. This worked very well and, again, salt is a big winner.
Stirring: Jim tried it at room temperature and states that the melt rates are the same. This is a bit of a mystery, as it contradicts the stirring experiment. Perhaps the stirring removes the refreezing effect.
One suggested answer: I've always assumed as the ice melts in saltwater the dissolved sodium and chlorine ions interfere with the strength of the H-bonds between water molecules and the process just becomes a runaway train. I love the idea of doing it in a refrigerator to eliminate the convection.
One difficulty is that convection is never entirely eliminated, so it is hard to be certain that the salt naturally melts the ice faster. But right now that is what it looks like. The two suggested solutions are: the refreezing that occurs in plain water slows the process down; the H-bonds weaken b/c of sodium and chlorine ions.