At What Temperature Does Rock Salt Stop Working?

Salt will “work,” i.e. it will melt ice, all the way down to its eutectic temperature of -6 ⁰F.  However, the “practical working temperature” of salt is generally considered to be higher than this.  In the highway deicing world the practical working temperature of salt is generally considered to be above 15 ⁰F or even 20 ⁰F.  There are two reasons for this.  

One is that the amount of ice that can be melted per pound of salt (or any other deicer) decreases with temperature.  At 30 ⁰F, 1 pound of salt will melt about 46 pounds of ice.  At 20 ⁰F, 1 pound of salt will only melt about 9 pounds of ice.  And at +1 ⁰F, 1 pound of salt will only melt about 4 pounds of ice.  So the colder it gets, the more salt is needed to provide a given amount of ice melting action. 

The second reason is that salt’s ice melting action slows as the temperature drops.  This would be less of an issue for applications where time is not critical (e.g. if one put salt on their snowy sidewalk and was not in a hurry to clear it), but in applications where removal of the snow is desired as soon as possible, such as highway deicing, the ice melting speed of rock salt becomes impractically slow at colder temperatures. 

Thus, while there is no standard industry definition of “lowest effective temperature” for highway deicers, 15-20 ⁰F is a commonly accepted rule of thumb.  The ice melting speed of rock salt can be improved  by blending it with a chloride salt brine to make a pre-wetted deicer, and this will provide some increase in effectiveness at temperatures below 20 ⁰F.