Salt on Ice - how does it melt?
Salt on Ice - how does it melt?
(OP)
I know about the depression of the melting point of a water/salt solution, but how does salt melt ice if they are both a solid. In other words, if I throw salt down on existing ice, there should be no reaction, eh?





RE: Salt on Ice - how does it melt?
It requires slow and regular motion for water molecules to go into ice. When we put salt on ice, the salt molecules obstruct the water molecular path resulting in random movement of water molecules. So less water gets freezed up.
Regards,
RE: Salt on Ice - how does it melt?
MB
RE: Salt on Ice - how does it melt?
RE: Salt on Ice - how does it melt?
As any regular book on General Chemistry would tell you, so long as the temperature is not too low, a few of the salt ions would attract water molecules from the surface of the ice crystals and gradually form liquid water solutions.
RE: Salt on Ice - how does it melt?
http://baen.tamu.edu/users/rmoreira/bsen474/freezing2/p...
RE: Salt on Ice - how does it melt?
RE: Salt on Ice - how does it melt?
See PT curve for Water http://www.pinkmonkey.com/studyguides/subjects/chem/cha...
RE: Salt on Ice - how does it melt?
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RE: Salt on Ice - how does it melt?
TTFN
RE: Salt on Ice - how does it melt?
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RE: Salt on Ice - how does it melt?
To BRT459. You are right. Salt addition forces the triple point to lower temperatures and pressures while keeping the equilibrium lines parallel to themselves. To the pressure exerted by heavy vehicles and skaters, it would probably be correct to add a heat effect resulting from friction of the wheels or skates on the icy road or rink.
RE: Salt on Ice - how does it melt?
RE: Salt on Ice - how does it melt?
Also, although it takes away a great teaching lesson from every P-Chem instructor, there is a lot of debate concerning the ice skating example. Although pressures are quite high when skating due to human weight and small blade thickness - a skate stil moves very well if you take it off of your foot and push it (at low pressures). I think that I read somewhere that the ability to skate on ice is more closely related to the thin layer of water that co-exists at the top of ice. The ice skating example is still a good calculation to do for P-chem and other thermo classes, though.
I think that the best way of looking at the roads is that high volume traffic (friction, emissions, etc) keeps the road warm. Also, some roads are equipped with insulating/thermal sinks under the roadways to aid in the prevention of quick freezes.
RE: Salt on Ice - how does it melt?
press., bar melting point, oC
1 0.00
10 -0.06
20 -0.14
30 -0.21
200 -1.52
500 -4.02
1,000 -8.80
2,000 -20.69
The pressure exerted by a male skater while rotating on the tip of one skate may be sufficient to reduce the melting point of ice by half a degree. I fully agree in that heat by friction and other sources as well as salt spraying, appear to be more effective in melting ice than pressure alone. Besides, rinks are kept well below zero degree C; thus, pressure alone couldn't be a factor in melting the ice surface, heat by friction or radiation from lamps, etc. could.
The topic of skate friction on "dry" or "wet" ice is still open for discussion, probably in another thread, right ?
RE: Salt on Ice - how does it melt?
RE: Salt on Ice - how does it melt?
Back to the road thawing faster discussion, I think that cars and trucks also tend to physically remove snow by blowing it off the road. Especially the dry powder snow found here in the Rockies (couldn't vouch for wet, soggy Eastern stuff blowing away so easily). Never heard of the heated roadway sub-bed before, but I suppose it would come in handy for multiple overpasses and interchanges.
RE: Salt on Ice - how does it melt?
Some road dept's add sand and gravel to the salt and it goes into these holes and provides traction. I think this is what BRT549 was referring to. In fact some people use salt to thaw frozen water pipes.
Some of the reasons given is that the vapor pressure of a solution is lower, (as colligative property) so it freezes and boils at a lower temperature. Also the system takes heat from environoment (endothermic) and I think that would cause a higher rate with increased traffic on the road due to more heat generated.
There are other substances that work better than salt for this but cause other problems or are more expensive. The following site lists them.
http://chemistry.about.com/cs/howthingswork/a/aa120703a...
RE: Salt on Ice - how does it melt?
RE: Salt on Ice - how does it melt?
RE: Salt on Ice - how does it melt?
As the salt dissolves, the resulting brine has a depressed freezing temperature relative to pure water. The "heat of solution" has to come from somewhere- in an insulated container, the resulting mixture temperature drops to compensate for the heat of solution, such that 33 parts of salt added to 100 parts of ice will produce a brine/slush mixture at ~ -21 C. (that's how my grandfather's generation made their ice cream, before mechanical refrigeration made it easier). In an open system, the heat of solution will be withdrawn from the surroundings and the equilibrium will be shifted away from salt/ice and toward brine until one or the other is gone.
Salt is useless below temperatures which will freeze the resulting brine mixtures. The colder you get, the more salt you need- and beyond ~ -20 C it becomes practically pointless to apply salt. At those temperatures, we generally switch to sand as a cheaper and lower environmental impact option to give more traction on ice.
On a busy road, you have hundreds of vehicle tires doing work on the snow, and vigorously admixing the snow/slush with any salt present. It's the heat generated by friction, plus the mobility of the brine/slush to flow and via spray, that accelerates removal of the snow from the road. Hence it gets all over every surface of your car, and also into streams and soils nearby....
RE: Salt on Ice - how does it melt?
Moltenmetal, great post above. I will have to read it several more times to digest it, though.
RE: Salt on Ice - how does it melt?
Regards,
Eng-Tips.com : Solving your problems before you get them.
RE: Salt on Ice - how does it melt?
The real hard part for me back then, working my way through college as a full time student (jr) engineer, was sitting in class and biting my tongue when they taught things that I knew to be incorrect because of my industrial experience, albeit limited at that time. At least I was sufficiently intellegent to recognize that I was a participant in an autocracy, even though I live in a democracy. Not in the classroom, however.
Having said that, I acknowlege that the education they gave me is what has brought me to where I am now, and I am grateful for that.
RE: Salt on Ice - how does it melt?
Salts' dissolution is considered a two-step process. At first, salts ionize through an endothermic process (aka lattice enthalpy), then the ions solvate (hydrate, in water) by an exothermic reaction. Thus the net enthalpy of solution of a salt is the sum of both enthalpies:
NaCl(s)=> Na+ + Cl- +787 kJ/mol
Na+ + Cl-+ water=> Na+(aq) + Cl-(aq) -784 kJ/mol
-----------------------------------------------------------------------
NaCl(s)=>Na+(aq) + Cl-(aq) +3 kJ/mol
Therein lies the explanation of why some salts warm on solution and others cool.
RE: Salt on Ice - how does it melt?
RE: Salt on Ice - how does it melt?
I'm thinking that they did not teach us ME's much about heats of solution, and that it was probably a ME professor that made the statement, not a chemistry prof, and I don't remember taking any Chem E classes at all. Anyway, I have spent my entire professional life wondering if it was in fact, exothermic, so as to provide enough heat to "melt" road ice, then how did it make ice cream in the old freezer. Mystery solved. I can sleep at nights, now. I love this forum.
RE: Salt on Ice - how does it melt?
I was under the impression as well that calcium chloride is the most common salt. This could be wrong though. I also read that when CaCl2 mixes with water it was exothermic.
Now, assuming that some salts are endo and some are exo which seems to agree with this forum, and possibly my chemistry lab from high school. Am i correct to say that an exo reaction melts the ice forming the salt solution. And that for an endo reaction the salt absorbs energy from the surroundings (the sun, etc) until it dissolves again forming the salt solution?
Both of these result in a salt solution where the freezing point is lowered?
RE: Salt on Ice - how does it melt?
RE: Salt on Ice - how does it melt?
RE: Salt on Ice - how does it melt?
Sometimes CaCl2 is used as road salt. I used to work at a magnesium facility that sent a waste stream of HCl to an adjoining plant that made calcium chloride. Seems to me they sold about 60% of it to the Utah highway dept. I think it has less of an environmental impact than sodium chloride, and might also rust metals slower. Likely the most common road salt in an area is based on availability and freight costs.
RE: Salt on Ice - how does it melt?