Salt on Ice - how does it melt? 9

[BRT549]

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?

 

[quark]

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,

 

[mb44]

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

 

[BRT549]

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?

 

[25362]

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.

 

[Rich2001]

This pdf, though about freezing food, has a lot of good information about the basics of freezing of solutions.

http://baen.tamu.edu/users/rmoreira/bsen474/freezing2/pdffilesfreez/FreProps.pdf

 

[BRT549]

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.

 

[Rich2001]

Walking on Ice increasure the pressure, decease the melting temperature.

See PT curve for Water

http://www.pinkmonkey.com/studyguides/subjects/chem/chap5/fig5_18.gif

 

[BRT549]

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.

 

[IRstuff]

Tires get hotter than, tho...

TTFN

 

[Rich2001]

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It is always good to make the email nazi's look bad.

 

[25362]

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.

 

[mb44]

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

 

[DonyWane]

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.

 

[25362]

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 ?

 

[Rich2001]

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.

 

[BRT549]

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.

 

[aviat]

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.htm

 

[pathfinder9909]

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.

 

[25362]

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.