Why Salt Melts Ice

On a cold morning, the road crew drives by and scatters white pellets across the icy pavement. An hour later, the ice is gone. What happened? The simple answer most people give is that salt is somehow "hot" or that it "burns" the ice away. Both are wrong. Salt is not warm, and nothing burns. To see what is really going on, you have to zoom in on the surface of the ice.

Here is the cartoon version of ice you probably already have: a frozen brick where every water molecule is locked in place. That picture is almost right, but it misses something important. Even at temperatures a little below freezing, the surface of ice is alive. Water molecules are constantly leaving the ice and joining the liquid film on top, and other molecules in that liquid are constantly latching back onto the ice. Both things happen at once. When the rate of melting equals the rate of freezing, the ice looks stable — but it is really a tie game between two opposing processes. Chemists call this a dynamic equilibrium.

At exactly 0°C in pure water, this tie game is balanced. Drop the temperature below 0°C and freezing wins, so the ice grows. Raise it above 0°C and melting wins, so the ice shrinks. That balance point is what we mean by the freezing point.

Now throw salt onto the wet film on top of the ice. Salt — sodium chloride — splits apart in water into two separate particles: a sodium ion and a chloride ion. These ions drift around in the liquid film and physically get in the way. Water molecules trying to latch back onto the ice surface keep bumping into ions instead. The freezing rate drops. Meanwhile, the melting rate does not change much, because the ions are floating in the liquid, not stuck in the ice. The tie game is no longer tied. Melting wins, and the ice shrinks.

To get the rates equal again, you have to cool things down further — slowing the water molecules so that freezing can catch up despite the obstacles. In other words, the freezing point has dropped. Salty water can stay liquid at temperatures well below 0°C. This drop in freezing temperature when something is dissolved in water is called freezing point depression.

Here is the surprising part. The size of the drop depends mostly on how many dissolved particles are floating around, not on what they are. A spoonful of sugar lowers the freezing point too, but less than the same spoonful of salt — because salt splits into two particles per unit and sugar stays as one. Properties that depend on the number of dissolved particles rather than their chemical identity are called colligative properties. Freezing point depression is one of them.

This is also why salt stops working when it gets cold enough. Around −18°C, even with salt blocking the way, freezing finally catches up to melting again. The road crew switches to something else. The salt has not failed; it has simply run out of room to tilt a game that the cold is now winning anyway.