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How/why does salting a race course work?

post #1 of 21
Thread Starter 

I've skied for several years in a charity ski race, and one year they had to salt the course 'cause the weather was very warm (probably close to 50).  It really did work, the course was much firmer than everything else on the mountain.  However, I haven't been able to find any explanation of *why* salting a race course works.

 

At the highest level, it would seem that the salt is causing some of the snow to melt and refreeze, resulting in a firmer, icier surface.  However this doesn't make sense that this could happen when the temp is above freezing.  Salt would definitely causing the top layer of snow to start to melt, but now you have a salt solution that freezes at a *lower* temperature than plain water.  What's the mechanism that causing this water to refreeze when the ambient temp is that warm?

 

I'm obviously missing something here, but can't figure out what it is.

post #2 of 21

A solution of salt in water does have a lower freezing point than pure water.

However, it takes heat to form this solution.

This heat is called the heat of solution.

 

the heat required to form a solution comes from adjacent snow crystals so, even tho the salt solution has a lower freeze point, the snow adjacent to the solution cools to supply this heat.

 

Then , if the saltwater can drain away, you are left with a snow mass that is colder than it was before the application of salt.

post #3 of 21
Thread Starter 

Ah, ok.  That makes sense.  Thanks!

post #4 of 21

Here's a link to a very informative website, you can plug in almost any question and get it answered, quite amazing really.

 

www.google.com/search

post #5 of 21
Thread Starter 

You know, I appreciate a nice JFGI as much as the next guy, but good luck finding the answer.  I tried... maybe your google skills are better than mine.  Nowhere could I find an explanation like dakine's on what the actual mechanism is that causes it to firm up a race course when ambient temps are above freezing.

 

So thanks anyway, but you can go now.

post #6 of 21
post #7 of 21
post #8 of 21
Thread Starter 

 

Quote:

 

Cool.  Thanks for finding that for me!  Your search skills are definitely superior to mine.

 

Quote:

 

Yeah, all that said was this:

"When a race course is salted well, it causes the salt crystals to melt and refreeze which subsequently causes them to bond tightly together."

 

I was trying to figure out why it would actually refreeze if the solution has a lower freezing point, but now I understand that's not exactly what's going on based on the above 2 explanations.  I noticed something when rereading that article to see if I'd just missed the explanation:

"The different types of chemicals cost different amounts of money. Sodium chloride, the most common chemical used, costs $5 per 50 lbs. bag. The next most common one, calcium chloride, costs $16.99 per bag."

 

Anyone know if that's true that calcium chloride is used?  Calcium chloride has an exothermic heat of solution which means the mechanism explained so far can't occur.  Given wikipedia's track record for accuracy, I could believe that the answer is simply that calcium chloride isn't actually used, but I obviously don't know.

 

None of this really matters of course, just the geek/3 year old in me that always to know *why*.

post #9 of 21

I thought the second one down explained it quite well.

post #10 of 21

At least in WC they use whole bunch of stuff. These guys are quite good with deciding when to use particular thing. They use artificial fertilizers, salt and whole bunch of other stuff depending on current snow, temperature and humidty conditions.

post #11 of 21

 

Quote:
Originally Posted by karpiel View Post

 

Here's a link to a very informative website, you can plug in almost any question and get it answered, quite amazing really.

 

www.google.com/search

post #12 of 21

I'm too lazy to look up the previous links, but here it is in KISS form.

 

When you stick some ice in your cooler it makes the inside of the cooler cold as the ice melts.

It takes heat to melt ice, heat to increase the temperature of water and heat to boil water.

It takes only takes about 1 cal to heat 1 gram of water by 1 degree C, but it takes 80 calories to melt that same gram of ice to make water. 

Melting all that snow by adding the salt sucks the heat right out of the mixture and lowers it's temperature. 

 

The salty water drains away leaving the much colder hard ice.

post #13 of 21

 

Quote:
Originally Posted by Ghost View Post

... 

It takes only takes about 1 cal to heat 1 gram of water by 1 degree C

...

 

Replace "about" with "exactly".

post #14 of 21

Here's my theory of how it works.

 

Basically, on a hot day, you have the air temperature---let's say it's 38 degrees.  Then you have slush on the surface, which must be 32 degrees.  And then you have snow below the slush, that is 28 degrees.  Normally, what happens is that heat from the 32 degree slush is transfered to the 28 degree snow, and melts it, generating more slush.

 

When you put salt into the slush, it lowers the temperature of the slush (because salt lowers the freezing temp).  Now the temperature of the slush is 28 degrees.  Since the temperature of the slush now matches the temperature of the snow underneath it, there will be no more melting of the snow underneath it.  This allows you to have a firm layer of 28 degree snow underneath a thin layer of 28 degree slush. And the slush will no longer melt the snow beneath it--leading to a stable situation to ski on.  As long as you have a layer of salty slush on top, the snow beneath it CANNOT possibly melt.

 

This is just my theory--but it seems to makes sense.

post #15 of 21


 

Quote:
Originally Posted by mrzinwin View Post

When you put salt into the slush, it lowers the temperature of the slush (because salt lowers the freezing temp). 


 

I believe salt lowers the freezing point of the water (slush), not the temperature of the it?

post #16 of 21

 

Quote:
Originally Posted by NE1 View Post

 


 


 

I believe salt lowers the freezing point of the water (slush), not the temperature of the it?

 

Yes, but try taking a pot of water, add ice cubes, and then heat it up on the stove.  Put a thermometer in the pot.  Although the stove is heating the water, you will notice that the temperature does not change.  The temperature of the water will remain at 32 degrees until all the ice melts.  Now, if you add some salt, the temperature will decrease--to, let's say 28 degrees until all the ice melts.

 

The reason for this is because the heat from the stove is being used to melt the ice--and so whenever you have a mixture of ice and water (as in slush), the temperature of that mixture will always remain constant at the freezing point of the water.  If you add salt, you lower the freezing point, and so you will lower the temperature of that slush.

post #17 of 21
Thread Starter 

 

Quote:
Originally Posted by mrzinwin View Post

 

If you add salt, you lower the freezing point, and so you will lower the temperature of that slush.

 

Actually, the reason the temp drops is because of the endothermic heat of solution of the salt as mentioned above.  It requires some heat to dissolve the salt in the water, and that heat comes from the surrounding snowpack.  Lowering the freezing point doesn't mean the temperature will drop, it just means that the solution will no longer freeze at 32 degrees.

post #18 of 21

Although the process is sometimes called "salting," what we do in our area doesn't involve salt. (More accurately, not a salt like NaCl or CaCl2 that melts ice.)

 

We use a 46-0-0 urea fertilizer.  The fertilizer works best when there is a high water content, i.e. enough to form a nitrogen/water solution.  Chemists would know that dissolving nitrogen compounds in water reduces the water temperature (negative heat of solution).  The result on our race course is that the snow freezes firmly again, even when the air temperature is up as high as 10C or 50F. 

post #19 of 21

 

Quote:
Originally Posted by jaobrien6 View Post

 

 

 

Actually, the reason the temp drops is because of the endothermic heat of solution of the salt as mentioned above.  It requires some heat to dissolve the salt in the water, and that heat comes from the surrounding snowpack.  Lowering the freezing point doesn't mean the temperature will drop, it just means that the solution will no longer freeze at 32 degrees.

 

That's just not correct.  I know because in high school, we did this exact science experiment.  We took a beaker filled with water and ice and heated it.  The temperature remained steady at 32.  Then, we added salt.  The temperature fell to 29. 

 

This has nothing to do with the heat of solution, which is irrelevant because there is a stove, heating the beaker.  It has everything to do with freezing point depression.  Any mixture of ice and water will always carry a temperature of the freezing point of water (because any water/ice mixture is at an equilibrium of melting/freezing).  If you decrease the freezing point, the temperature of the ice/water mixture will continue to hold at the freezing point.  It's just that now--the freezing point is lower.

post #20 of 21

Good point on the fertilizer Mogul Muncher. I've experienced this first hand (so to speak) when mixing fertilizer up in a drum of water in summer and getting VERY cold hands. This is the same fertilizer I used to firm up a slushy race course during the winter. Not sure on the effects of the application of fertilizers to the snowpack in winter with the consequent melting and run-off in the summer. Any issues with that?

post #21 of 21

The urea will break down into nitrates and nitrites, with the possible contamination of groundwater by nitrates (above drinking water standards).  Might be a problem for nearby drinking water wells.  It depends on how much is used (it's not like you're de-icing thousands of planes a day at the airport).


Edited by Ghost - 3/15/2009 at 02:33 pm
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