Message to Spencer Porter asking how salt melts ice -- followed by his reply.  Spencer has PhD in Chemistry, taught for a long while, then went to seminary, and thence back to chemistry.  He is a Mr. Science kind of guy.

From: MICHAEL ROULIER
To: PORTER-SPENCER
Date:  1/11/99 10:03am
Subject:  Ice Problem

Spencer,
       Mark and I have been wondering how (the mechanism) salt on the road causes ice to melt.   There are a number of wildly divergent and passionately held views in the Branch about what is taking place in an ice/water/salt system.
       When a vehicle drives over some salt the increased pressure on the ice would cause a bit of local melting (regelation) then some salt is dissolved, lowering the freezing point and more ice melts until the salt is dissolved and the resulting solution is diluted enough that its freezing point rises to ambient temperature - at which point the road ices up again.
       Similarly, if the salt crystal is slightly warmer than the ice, the salt crystal would give up some heat, melting a bit of ice, dissolving some salt and etc.
       What I don't understand is the most likely case where the salt crystal is at ambient temperature (below 32F) when it falls on the ice.  Somehow the ice gets melted.
      Please advise me if I am violating any of the Laws of Thermodynamics by asking this question.  I would not want to have any peccadillos in my past that would prevent me from running successfully for Congress or the Presidency.

--Mike

From: SPENCER PORTER
To: ROULIER-MICHAEL
Date:  1/11/99 1:41pm
Subject: Ice Problem -Reply

   No politician has ever violated the laws of thermodynamics.  All politicians violate all ten commandments all the time.  The trick seems to be being firmly against what you're doing.  A more useful analysis can be made using the seven deadly sins.  There are three for each party and one for those who don't vote.  It seems to me that the Republicans get pride, anger, and covetousness, while the Democrats have lust, envy, and gluttony.  This leaves sloth for those who don't vote.  Hardly anyone understands that these things are sins, and in fact the successful politicians of either party point to their party's sins as virtues.

   You've never told me which party you belong to, but you do seem to be too intelligent to be a Republican.  Anyway, if you want to run for Congress, your actions will be determined by which district you live in.  If you live in the first district, I suggest that you lean to the starboard and gaze firmly toward the stern.  If you live in the second district, try St. Jude.

  As for the chemistry, most of the things you mentioned probably play a role.  The basic phenomenon comes from the fact that solids are much more soluble in water than they are in ice.  Any added salt will dissolve in the liquid phase, if there is one, and its vapor pressure will fall below that of the solid which had been in equilibrium with the pure water at zero celsius.  Water molecules will then migrate across the phase boundary into the liquid.  Equilibrium may be reestablished at some lower temperature as the vapor pressure of the ice will fall faster with falling temperature than will that of the salt solution.

  If one were to place a perfect salt crystal on top of a perfect ice crystal, it is probably true that very little would happen, but even in this case I can't be sure.  The surface layers, an atom or two deep, are liquid like, and there would probably be some solution made.  This would start the process, and the salt solution would melt a bit more ice.  In the real world a bit of sunlight on the road's surface would probably suffice as sunlit ice has a layer of water on it.  

  To be sure pressure melts ice or else skis and skates would not work.  The amount of ice melted under most circumstances is small, and the water freezes as soon as the skater is past.  The solid-liquid line on the phase diagram of water has a positive slope and it is very steep.  I suspect that cars do more to mix the ice, water, and salt than anything else.  This can form some pockets of liquid with very high salt concentrations.

  In the eighteenth century Farenheit made salt-water-ice mixtures of varying concentrations, and the zero on his temperature scale represents the lowest temperature at which he was able to see liquid.

  The amount of freezing-point lowering is proportional to the concentration of particles, usually as molality, so salts do more than solids, like sugars, which are not ionic.  The more particles per formula unit, the better.  So calcium salts like the chloride are more effective than rock salt.  (If one tries salts of triply-charges cations like ferric or aluminum, the solubility of the salt is usually low.)  Calcium chloride does one other thing, and that comes from the fact that its solution process with water is exothermic.  

  You could think about these things as you trudge through the slush and sing Roy Roger's theme song.

  Thank you for the question.  It's not often that I get to do theology, politics, and chemistry all at the same time!