If rock salt lowers the temperature of the ice and keeps it from thawing when making ice cream, can it be used in a cooler to help keep the food colder longer? Also, if rock salt keeps the ice in an ice cream maker from thawing, how does it melt ice on the road?
It all has to do with the fact that rock salt lowers the freezing/melting point of ice. When you’re making ice cream, the rock salt doesn’t keep the ice from thawing. What it does is mix with the ice, melting it and lowering the freezing point of the liquid left behind. This brine has a temperature around 0 degrees Fahrenheit—cold enough to freeze ice cream. On roads, rock salt has a similar effect, melting the ice and lowering the temperature of the liquid left behind. But since the freezing point of the liquid is lower, not only has the salt gotten rid of the ice that formed previously, but much colder temperatures are now needed to turn the liquid that’s left back into ice.
By the way, we don’t recommend using rock salt in a cooler.