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Salt on Ice - how does it melt?
9

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?

Salt never melts ice but it prevents water from refreezing. You are overlooking the phenomenon that at the freezing point Ice and Water coexist. There will be equilibrium between water freezing into ice and ice melting into water at perfect freezing point of pure water. If you increase the temperature rate of melting increases and if you decrease the temperature freezing of water takes place.

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?

I thought it can be described simply as the salt lowers the freezing temperature of the water, hence it is no longer cold enogh to freeze, therefore it melts

MB

RE: Salt on Ice - how does it melt?

(OP)
If I throw salt onto a frozen walkway, the solid salt particles will slowly create pockets of melt.  But since the temperature is way below freezing, was there some heat of reaction to get some ice melted into the saltwater solution in the first place?

RE: Salt on Ice - how does it melt?

3
To BRT549: there is a reaction.
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?

(OP)
25362:  So if people are walking on the salt-strewn ice, they are accelerating this ionic migration by crushing the ice and increasing contact and surface area.  The walkways should melt faster.

RE: Salt on Ice - how does it melt?

(OP)
The pressure effect is separate from the salt effect.  Isn't the pressure effect used by ice skaters?  I think that's why roads melt faster than the surroundings - pressure of the tires (especially semis) accelerates melting rate.

Rich2001:  not sure I want to click on a site named "pink monkey".  Liable to set off the email nazi's at work.

RE: Salt on Ice - how does it melt?

Tires get hotter than, tho...

TTFN

RE: Salt on Ice - how does it melt?



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RE: Salt on Ice - how does it melt?

To Rich2001. Have a look at the water P-T phase diagram. The almost vertical equilibrium line dividing ice from liquid water slopes towards lower temperatures as the pressure rises. Meaning that at a given temperature the ice melts (enters the liquid domain) with increasing pressure. And that is exactly what you said. To avoid confusion: if you change the wording of your statement from a "decrease in melting temperature" to a "decrease in freezing temperature", all becomes clearer.

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?

Vehicle engines give off a lot of heat as well, this probably has a greater effect, I think its many kW's, maybe even hundred of kW's

RE: Salt on Ice - how does it melt?

I haven't ran through the calculations - but I don't think I have to.  Because of the size of a human's feet, I think that the induced pressure would have very little effect on the freezing point of the solution.  With roads, cars do weigh a bit more and the tread is a bit smaller than feet, but I still think we are talking small pressures.  

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?

To DonyWane, no need of calculating the pressure effect. It is well documented. See some examples of what the CRC Handbook of Chemistry and Physics has to say on ice:

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?

Ice skate blades are not flat, but have a hollow. (Hollow - The curve that runs lengthwise down the centre of the skating surface of the blade.) There roe the contact area is quite small ascompared to a flat blade.

RE: Salt on Ice - how does it melt?

(OP)
This thread has taken an interesting twist with the skate question.  It was originally posted to help me answer a 15 year-old's question that came up in his high school science class, whereby the teacher presented a heat-of-reaction answer rather than the freezing point depression I was familiar with.  I have armed the aforementioned teenager with plenty of theoretical ammo this morning.

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?

2
If you take some tap water and put it in a freezer and let it freeze solid and then add some salt to the top of the ice and return to freezer, the salt will melt into the ice.  The ice melts nearly straight down and creates a small hole, the depth of it depends on temperature and salt concentration. Eventually the solute becomes diluted and it quits melting and turns to slush.

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?

The 1 thing people fail to think about is that there is energy from the sun, either in the UVA or UVB range if there are clouds as well as in the infrared range. Even if it is 25 degrees farenheit there is solar energy helping melt at least the top film of water on the ice and that is all that it takes to start dissolving the ice.

RE: Salt on Ice - how does it melt?

The latest I heard about ice melting on a deeply frozen rink by even the smallest -light weight- child skating around is that the water molecules on the surface are rotating at incredibly high speeds, a fact that enables the formation of a semi-liquid film that the slightest of forces can release as liquid water.

RE: Salt on Ice - how does it melt?

3
There's a chemical potential (i.e. a driving force) for salt to dissolve in water, since there is a higher concentration of sodium and chloride in the salt crystals than in the water.  By the same process, salt attracts water to its surface (i.e. it deliquesces).  These two processes get the dissolution started, resulting in a film of brine at the ice/salt interface.  From there on in, it's a simultaneous heat and mass transfer process looking for an equilibrium.  

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?

I was taught that salt dissolving into water was exothermic.  Is this right??  

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?

No. It was not exothermic. (some of the papers I saw on this topic say that the exothermic reaction is the first misconception people think of)

Regards,

Eng-Tips.com : Solving your problems before you get them.

RE: Salt on Ice - how does it melt?

Chalk up one more in the category of what the professors told me that I now know to be bogus.

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?

To rmw, while I don't exactly know what they taught you about heats of solution of chlorides in water at 25oC, let's clarify that of the common chlorides, sodium, potassium, ammonium and silver are endothermic (cool on solution), while lithium, magnesium, calcium and aluminium are exothermic. Talking of sodium compounds, most of them dissolve exothermically, for example: bromide, iodide, hydroxide, carbonate and sulfate; on the other hand, fluoride and nitrate are endothermic.

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?

How about when salt molecule touch ice, it depress the ice molecule causing it to give up heat as it lowers it temperature. The heat is then absorbed by the solid ice and caused it to melt. The melt water mixed with the salt solution and is then depress to causing it to give up more heat as it lower it temperature.  The process then repeats.   

RE: Salt on Ice - how does it melt?

Thanks, 25362.

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 just wondering this exact thing.  I realize that salt added to water lowers the freezing point.  I was under the impression that when salt mixed with water it was an exo reaction.  I remember a high school chemistry experiment where one chemical caused exo and one endo.  I'm wondering now if they are both salts?  

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?

49078 - Common (table) salt is sodium chloride.  See 25362's post of 18 Feb.

RE: Salt on Ice - how does it melt?

So road salt is nacl?

RE: Salt on Ice - how does it melt?

(OP)
Thanks for reviving an old forum topic.  Got some great answers, and another good question or two.

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?

Overwhelmingly the most popular road salt is NaCl, cheap because it's mined in enormous quantities and the run-of-the-mine material is good enough for this use.  Other ice melting materials tend to have limited uses (for sidewalks etc.) due to cost.

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