Contrary to what everybody says, the ice on your driveway doesn’t melt, any more than sugar melts in coffee or tea.
People often confuse melting with dissolving. (“I don’t need an umbrella; I won’t melt in the rain.”) But melting, as you have already noted, requires heat. You can certainly melt ice or sugar by heating them, but that’s not what the salt does to the ice. The salt dissolves the ice.
People use the word melting for the salt-on-ice phenomenon only because they see ice disappearing and a liquid, salt water, remaining. And melt just happens to be the word that our ancestors invented for “ice go ‘way, water come.” Science teachers and textbooks, however, should know better than to fall into that linguistic trap.
Along with many others, you were probably taught in school that “salt lowers the freezing point of water” But that’s not literally true either. Throwing salt onto your driveway can’t possibly change the freezing point of water, the temperature at which good old aitch-two-oh is accustomed to freezing or melting.
That temperature, is the same for melting as for freezing, is 32 degrees Fahrenheit or 0 degrees Celsius. It always has been, and it always will be. What the textbooks and teachers should be saying is that salt water freezes at a lower temperature than pure water does. That’s quite a different statement.
On your driveway, the salt first makes salt water out of the ice, and then the resulting salt water remains unfrozen because its freezing point, not water’s freezing point, is indeed below the temperature of the air. A fine distinction, perhaps, but critical to understanding what’s going on.
First, how does the salt make salt water out of the ice? It happens that sodium and chlorine atoms (actually, sodium and chlorine ions, but we won’t quibble), which make up the sodium chloride, or salt, have a strong affinity for water molecules. Manufacturers have to add an anti-caking agent to keep table salt from gumming up in the shaker because of moisture absorbed from the air.
When a crystal of salt lands on an ice surface, the salt’s sodium and chlorine atoms some water molecules out of the surface. They then proceed to dissolve in that water to form a tiny puddle of salt water around the crystal. The puddle of salt water doesn’t freeze because its freezing point is lower than the temperate of the air.
The sodium and chlorine atoms that are now dissolved in the salt water keep nipping away at the ice surface like piranhas going after a meatball in a punch bowl. As the process continues, more ice continues to dissolve in the salt water, making more and more salt water.
Eventually, either all the ice runs out, or the puddle of salt water becomes so dilute that its freezing point is no longer below the air’s temperature, and it will freeze. But salt water freezes only into slush, rather than into hard ice. In either case, your ice-destroying mission has been accomplished.
