It Snowed!

It snowed overnight where I live, and so now there's all kinds of melty white stuff on the ground.

We call this snow.

Before I get into the main part of this post let me first mention two brief facts about the relation of snow to salt. Perhaps throughout the season I'll come up with some more. For now there are just two:

1. When water vapor condenses to form clouds, it does so by clinging to condensation nuclei such as dust, ice crystals, and salt. Salt's all over the place, folks. Even in clouds (why do you think they're so white? (okay, that's not really why...)).
2. In movies, the familiar crunching sound of walking through snow is simulated by cornstarch, cat litter, or salt.

Anyway.  When it snows, roads are salted to make the snow melt.  But why does this happen?  The easy answer is to say that snow is evil and the holy salt melts it like Arcs melt Nazis.  What, after all, is the Ninth Circle of Hell covered in according to Virgil?  (Well, ice... but close enough.)

The sciency (and probably more credible) answer is that salt lowers the freezing point of water.  The lower the freezing point, the colder it has to be for water to freeze.

The reason this works is because the dissolved salt ions are an impurity.  As it gets colder, water molecules lose energy and slow down.  As they slow down, they come closer together and form hydrogen bonds with each other.  When salt is introduced, the water molecules cannot form the lattice structure characteristic of ice as easily, and so more heat needs to be removed from the system for the water to freeze.

In controlled environments, salt can decrease the freezing point of water to about -21 ºC (-6 ºF).  In practical use, such as on sidewalks or roads, salt lowers the freezing point of water to about -9 ºC (15 ºF).