MadSci Network: Chemistry

Re: Why does salt melt ice in the winter but freezes snow(slush) in the summer?

Date: Tue Nov 3 11:46:38 1998
Posted By: Tom Cull, Staff, Clinical Sciences MR Division, Picker International
Area of science: Chemistry
ID: 909354556.Ch

Hi Colleen

A small amount of impurity will typically cause water/ice to either boil at a higher temperature or freeze at a lower temperature. The most effective impurity is usually salt and for the specific case of melting winter ice -- a special form of a salt is used.

Let's look at the cold weather case first: The useful effect in this case is that the ice/water melting point (or solidification point I guess) will happen at a slightly lower temperature. So at around 26 to 32 degrees F, what would have been a rock hard block of ice will be a slushy mess.
Packed snow is very similar to ice and will react pretty much like a big chunk of ice. Loose snow is more like small unconnected pieces of ice and will melt only around the piece of salt. The surface area to volume ratio has a big effect on melting for a given mass of ice/snow. The center of big block of ice is insulated from the outside world by the ice around it, so the center of the block of ice (or packed snow) will stay colder for a longer time. I refer to this as the core temperature. Melting ice/snow requires getting the c ore temperature to be high enough that the core no longer can act as a heat sink.

Now let's consider the summer skiing case: Fake snow made from a machine is typically very powdery so that it can be easily spread over a wide range. Loosely packed snow melts pretty easily and the individual parts absorb into the ground. Salt can be used to prematurely melt snow so that it can be turned to ice in sheets. It is sort of a melt-refreeze process that hardens the snow. The underlying body of snow is much colder than the surface, so the surface is constantly melting and refree zing until the snow is firmly packed.

This cannot go on forever. Sooner or later, the overwhelming heat from the air/sun will win and the core temperature of the packed snow will rise. When this happens the ski season is probably over.


Tom "Office Chair Moguls Champ" Cull

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