This past weekend everyone was out of salt and calcium chloride ice melting stuff, but one place was selling 10/10/10 (nitrogen, potash, phosphate: 10% of each) fertilizer as an ice melter.

I bought a couple bags of it and it did work fairly well.

I'm a science geek at heart, so am wondering 2 things:

1. Why would this fertilizer mix melt ice?

2. Is it as good as salt or calcium chloride?

Thanks!

I'm NOT a science geek but I do know that if you use other than a product specifically made for melting snow/ice and someone falls and is injured you have 100% liability in NY. I have no idea about anywhere else but would "guess" it's no different.

I have done several accidents involving (of all things!) kitty litter scattered on snow for either melting purposes or traction.

Someone fell, homeowner 100% liable.

In my state we're liable no matter WHAT you do.

The term is "leaving a hazardous condition". Sadly, in my state, this can come from just shoveling snow.

... but if you do nothing, then you're not liable since the condition was a result of nature.

Hello Rick:

I have absolutely NO idea... But, that never stopped me before.

Here's what I think... When fertilizer comes into contact with water, if begins fertilizing. Like ALL transference of energy, heat is a byproduct of that transference - ouila - melting ice.

Ok, hit me...

excon

I'm a dummy. I have no clue what "byproduct of transference" means. I'm hoping that someone will come along and tell me how it works and whether there is good reason to buy fertilizer cheaply rather than spending big bucks on calcium chloride to melt the ice that hits us hard sometimes.

Actually, any impurity that mixes with a substance would lower its melting point and raise it's boiling point.

Since fertilizer is made from exclusively ammonium products and nitrate products (both of which are very soluble in water, hence the very strict measures concerning their use)they act as very good impurities. And when they are in water/ice, they'll lower it's melting point.

Ice/water which melted at 0 C will now melt at say... -10 C (maybe exaggeration) so that at -5 C, the ice is actually water.

Now, is slat cheaper than fertilizer? If so, that is the reason why salt is preferred over fertilizers.

Any impurity will lower the melting point?

So would cat litter, or even just dirt work just as well as the fertilizer?

Technically, yes... then there is the question of the extent to which an impurity can reduce the melting point.

More details on this wiki article: Freezing-point depression - Wikipedia, the free encyclopedia

and this part in particular:

I personally never did this chapter in school, only was taught that impurities lower and raise the melting and boiling points respectively of substances.

A common practice for raising the boiling point is to put some salt in boiling water to boil eggs. The water will have a lower tendency to evaporate and the egg will boil faster.

EDIT to add: Oh, and if you want to compare, I'll need the I value of salt (sodium chloride), calcium chloride and cat litter and dirt... :o (But also the latter two's concentration)

It sounds like that is the bottom line.

So will 10/10/10 fertilizer lower the freezing point of water better than dirt?

... and how does that compare to calcium chloride or plain old salt?

I'm taking only the main variables:



K of water = 1.853 Kkg/mol
m = molality of solute (mol/kg of solvent)
i = number of particles per mol.

Let's take salt, sodium chloride.

m = 356g/L = 6.09 mol/kg
i = 2



That is -22.6 C is the maximum change in melting point salt can have on water (which is not attained as the solubility of salt will decrease, that is m decreases and that is non linear and difficult to estimate for me)

Calcium chloride:

m = 595g/L = 5.36 mol/kg
i = 3



(same issue as above)

Okay, after those values I need to find the solubilities of the 10/10/10 fertilizer (rather the exact chemicals present in it) in water at 0 C

Any SOLUBLE impurity will depress the melting point. Kitty litter will not melt ice but it will give you better traction, That's why I carry a bag in my trunk. Urea fertilizer (46-0-0) is an excellent ice melter and you can use it on your lawn in the summer.

Dang, you are blinding me with Science (Poetry in Motion).
Can you put your results into English that a dummy like me can understand?

Thanks for the addition, DrBob... but what I'm really asking is: Is there any reason to continue using Calcium Chloride or Salt as an ice melter IF fertilizer (which is much cheaper) is just as good.

In short: Is fertilizer AS GOOD as calcium chloride or salt as an ice melter?

From the answer I posted common salt is less effective than Calcium chloride. But then, I don't know what is the cost difference... if you are paying more to get just a little more temperature, what's the point?

Also, Calcium chloride might be more useful in colder conditions as it lowers further the melting point of the ice.

Now, if I get the ingredients of the fertiliser, I might calculate whether the fertilizer is better of not.

I'm sure it varies among manufacturers and mixes, but I think it's typical for 10-10-10 fertilizer to contain (NH4)2SO4 (ammonium sulfate to provide nitrogen and sulfur), (NH4)2HPO4 (diammonium phosphate to provide nitrogen and phosphorous), and K2SO4 (potassium sulfate to provide potassium).

I'm too lazy to figure out the proportions which would result in a yield of 10% N, 10% P, and 10% K by weight.

Like many other behaviors we observe, this phenomenon is a manifestation of the struggle between maximum entropy and minimum enthalpy in the universe.

Entropy is a measure of the "randomness" of things. Stuff (especially on a molecular scale) doesn't like to be sorted and filed. If you take a cup of red Kool-aid and a cup of blue Kool-aid and place them together in the same pitcher, they will almost instantly mix to form purple Kool-aid. It doesn't matter how careful you are; once the two liquids are allowed to "mingle", they'll do so until the entire pitcher is filled with a homogeneous mixture of the two. Likewise, if you cook bacon in a frying pan at one end of your kitchen, the mouth-watering scents that emanate from the pan are not content to just hover nearby. They mix with the air molecules and spread quickly throughout your house. Processes like these are known by names like osmosis or diffusion, but really they're just a result of the universe attempting to achieve maximum entropy.

Enthalpy, the competing principle in this case, is a measure of the amount of internal energy something has, and the universe is always trying to minimize it. If you pick a rock up off the ground, you've added gravitational potential energy to it. If you let go of it, it will do whatever it can to decrease that potential energy; in this case it will simply fall back to the ground.

Now consider the struggle between entropy and enthalpy when it comes to the formation of ice. When water is in liquid form, its entropy is relatively high because all those molecules are bouncing around randomly. When it freezes, however, its molecules form a well-ordered crystalline structure, tightly bound in place. Now the entropy is low, and the universe doesn't like that.

Contrast that to the enthalpy: With liquid water, the molecules are dancing around from place to place with relatively high energy (and hence, high enthalpy, much to the chagrin of the universe). When they freeze, however, their energy is much, much lower, so their enthalpy is lower, and the universe is happy.

Notice that those two things are in opposition. Apparently it's impossible to make the universe happy. (Insert wife joke here).

If you have a glass of lukewarm water, the water contains enough energy to push the entropy/enthalpy balance in favor of the former. Despite the strong desire for the lowest possible energy state, the water molecules are energetic enough to let the forces of entropy prevail. As you cool the glass down, however, the advantage for the entropy gets less and less until you reach 0 degrees C, at which the balance of power flips, and enthalpy wins. The molecules no longer have enough of their own energy to maintain their randomness, and the water freezes into ice. (On a side note, every time a molecule finally gives up its freedom and freezes, it gives off the last of its remaining energy to its neighbors. This is called the "heat of fusion". In so doing, it warms everybody else up a little bit, postponing the freezing process, and ensuring that the overall temperature will never drop below 0C until every last molecule has frozen.)

When you add a solute to the water (such as a salt, be it sodium chloride, calcium chloride, or any of constituents of 10-10-10 fertilizer -- yes, those are ALL different salts), you're effectively increasing the randomness of the mixture. Now, instead of just water molecules bouncing around randomly, you've also got various electrolytic ions bouncing around as well. The universe likes this! It tips the balance of power more in favor of entropy. It means that the temperature has to be even colder before enthalpy can win. So it has the effect of lowering the freezing point of the solution! How much it lowers the freezing point depends on the type of solute and its concentration, and that's what Unknown008 was offering to calculate.

Does that make it clear as mud?

I think we are dealing with the sort of topic where science helps explain what is happening but there is a lot of more practical considerations that come to bear on the real world situation. I have used urea with good results for several years. Then the feed-and-grain store in the next town got all yuppie and stopped selling it. I bought it in 50 lb bags. I think it cost about 80 cents per pound. In my local supermarket salt and calcium chloride come in 10 or maybe 25 lb bags. They probably cost 60 cents per pound and come in neat, easy to pour containers. There is not much economy of scale. Commercial fertilizer comes in big (size) bags but they don't seem weigh all that much - I'll bet they have fillers to bulk it up - more for your money and easier to spread. Slow release fertilizers have urea-formaldehyde resins to remain active for longer times. Polymers are exactly what you DON'T want to melt ice because the science wants lots of particles - ions, molecules, etc. Any water soluble material will melt ice - you could use sugar or pour liquor on your driveway, kitty litter and sand are for traction only.
NaCl is cheap: dig it out of the ground, screen it for size and spread it. That's why the Highway Dept uses it. CaCl2 is a industrial by-product, works better than salt but is a little more expensive. That's why it is less frequently used. I saw urea used in an article on runway deicing in Alaska - it's strong point is it's noncorrosive nature. You know what salt does to your car, BAD for airplanes.
In the case under discussion, they're out of salt and so you use fertiliizer as a fall back product. Probably too expensive for everyday (or every storm) use but it will melt ice, and that is what you need to do NOW!