salt and snow
I was in a smowy country recently- and *surprise* it snowed. Which is all good and well. They put salt on the sidewalks to clear the snow- i get that. But why?
Bear with me: If salt lowers the freezing point of water, then surely it would lower the freezing point of snow. Surely this should result in the snow remaining on the sidewalk and not melting?
Bear with me: If salt lowers the freezing point of water, then surely it would lower the freezing point of snow. Surely this should result in the snow remaining on the sidewalk and not melting?
posted by Poly at
2:05 PM
6 Comments:
They use the salt to reduce the slipperiness. The salt etches into the ice and creates a rougher surface. If one does not 'salt' ones driveway and your neighbour slips and breaks her leg she can sue you with that wonderful European currency! They even used to salt highways so that the cars don’t slide around – don’t think they salt them any longer as the salt would damage the cars…
By
Gregor, at 12:39 AM
I have been in Aspen for 3 months and i still have no idea how this salt thing works. and there are parts of the pavement where the snow melts on impact no matter when... er... how?
By
Cat Murray, at 4:00 PM
Cat, my point exactly. It makes no sense. And the idea of salt creating a rougher surface makes sense but why does it melt?
I think we should just call it magic and leave it at that :)
By
Poly, at 2:02 PM
Think of one snowflake. We normally think of a snowflake as solid ice, and it mostly is, but it has some liquid water too. Some of the liquid water molecules are being captured by the ice (freezing), while some of the ice molecules are escaping into the liquid (melting).
At a given temperature, freezing and melting occur at about the same rate, so the snowflake doesn’t change much over time. This is called equilibrium.
When the highway department drops a salt crystal onto the snowflake, it dissolves in the liquid part. Because salt has a different size and shape than water, it doesn’t freeze into the ice. Instead, the salt gets in the way of some of the liquid water molecules, preventing them from being frozen into ice.
However, salt doesn’t keep molecules from leaving the ice and going into the liquid. In other words, salt speeds up the rate of melting without affecting the rate of freezing. Only if the temperature drops can freezing compete again. If the temperature drops a lot, freezing completely takes over and there is no liquid water to dissolve the salt and the whole process doesn’t work. That’s why when it’s really cold in Ithaca they just put down sand to improve tire traction.
But why salt? Shouldn’t the above reasoning apply for any solid that dissolves in water? The answer is yes! Sugar could be used too, but salt is cheaper. Properties that depend on the number of dissolved molecules, not their identity, are called colligative properties.
By
Anonymous, at 3:49 PM
Isn't your question confused? Lowering the freezing point means the ground temperature has to be colder for snow/water to freeze. Which explains why it doesn't remain frozen (at temperatures where it otherwise would) and instead melts in the water, which also hasn't, but otherwise would have, frozen.
By
REOsavVy, at 12:53 PM
agree with the last comment, read here for a good explanation: http://chemistry.about.com/cs/howthingswork/a/aa120703a.htm
By
Anonymous, at 6:34 AM
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