The Chemistry Beneath Your Boots

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Rock salt crystals

The New York area has broken free, for the most part, of the several feet of snow that fell on the region last weekend. And rock salt was our unsung hero in doing so. 

In order to understand how salt melted all that ice, first you have to understand a little bit about water.

Water molecules are composed of two hydrogen atoms and one oxygen atom, like so:

Water molecule simple gif animation

One end has more of a negative charge, while the other end has more of a positive charge. As a result, the molecules are drawn to line up. At room temperature, those molecules are moving too fast to stick together. But as water gets colder, these molecules slow down, and the magnetic attraction pulls them together into a nice orderly structure: a water crystal, or ice. 

On top of every patch of ice you see on the sidewalk lies a thin layer of water molecules that are constantly breaking off or joining the cold structure underneath. Adding salt knocks that exchange off balance.

Nathan Bellomy, a science educator at the American Museum of Natural History, explains it this way: Say you're a bricklayer, trying to turn a pile of bricks into a nice, neat wall. But then your assistant goes off the rails, and, every third brick, hands you a bagel or a baseball bat. That makes it harder to build a wall — or in this case, for water molecules to form ice.

"The ions that table salt breaks into, the sodium and chlorine, they get in the way of the molecules of water tugging themselves into a neat crystal lattice," he said. "They're just physically blocking."

That thin layer of water keeps growing deeper and deeper until the ice completely disappears. Voilá! You can see the pavement again.  

A Department of Sanitation depot full of salt and snow plows on the West Side Highway.


Hypothesis is written and produced by Alec Hamilton and edited by Matthew Schuerman. Sound design and engineering by Liora Noam-Kravitz. Original music by Josh Burnett.