Salt on roads

[sorgoth]

I've been wondering for a while, and seem unable to find it, why putting salt on roads makes the snow melt at a lower temperature. Anyone here know?

 

[RichardR]

Google Search: salt water freeze "lower temperature"

Water exists in the liquid state between the temperatures of 0°C and 100°C. In the liquid state, molecules are provided with enough heat to move around, but not enough heat to break the relatively loose bonds between them. A liquid can therefore change shape to fit the dimensions of any container you may put it in. When the temperature is lowered to below 0°C, the molecules cease to move and they form a crystal structure, in which the molecules are held together by strong bonds. We call this crystalline structure ice.

Adding salt to water interferes with the formation of ice crystals. It therefore requires an even lower temperature to get water to freeze when there is salt mixed in. Just how much lower the temperature must be depends on how much salt has been added. The more salt you add, the lower the freezing point.

 

[Crossbow]

What RichardR said.

 

[JamesM]

RichardR's link, which mentions "heat" and "interference" of the crystal formation, implies that the effect is enthalpic, i.e. to do with interruption of intermolecular forces. My understanding of it is that one observes a lowering of the melting point even in ideal solutions, where the enthalpy of mixing is zero.

There is an entropic explanation. Consider pure water. Lots and lots of H20 molecules, as far as the eye can see. The melting/freezing point of water represents the tendency to go from a state of low entropy to higher entropy, which we can identify with an increase in disorder. As the solid will consist of H20 molecules locked into position (apart from a bit of vibrating), an increase in entropy will occur if the ice melts and forms a liquid, where there is some temporary short-term order, but in general, the water molecules are free to jiggle about almost as much as they please (albeit not as much as in the gas phase).

Now we add our salt molecules. In the liquid phase, not only can the water molecules jiggle about with gay abandon, the presence of the Na+ and Cl- ions adds an even greater amount of disorder to the solution. Of course, it also adds to the amount of disorder in the solid phase, but the increase in disorder is greater for the solution than the solid.

Hence, upon adding salt to pure water, we increase the tendency for the solid to become a liquid. This manifests itself in a lowering of the melting point. Once you have a bit of salt in contact with the snow, it will cause it to melt, which will dissolve the salt and spread the brine further, continuing the melting.

As to the question of why use salt in particular, I don't know. I assume it is to do with its cheapness, abundance, solubility in water, and also that it reduces the melting point by a useful amount.

 

[arcticpenguin]

This should be covered in any decent chemistry book under 'freezing point depression'.

'salt' can be used in the generic sense here, any of a variety of salts will work. NaCl is probably the cheapest, although extreme low temperatures or environmental considerations may indicate other substances.

 

[patnray]

Sodium chloride (NaCl) is most widely used because it is cheap. Calcium chloride (CaCL2) is also widely used. It works even better because each molecule supplies 3 ions. Both are corrosive and can damage vegetation in areas with insufficient drainage...