Salt and Water

[Mag]

Now, correct me if Im wrong.

but I believe salt water freezes at a lower temperature than regular water (fresh water).

 

If this is true, I was wondering why.

[Mr Skeptic]

Basically, the salt ions interfere with the growth of water crystals (ice). Most things will decrease the freezing point and increase the boiling point of water, IIRC.

[Mag]

ah gotcha.

but would it work in practice?

 

For example, I talked to my friend who works Grounds here at the university.

and they use shovel-like tools to get ride of the ice, but they also have a propane torch (more like a flamethrower, lol).

And he said that he's melted the ice, only to have it freeze a few minutes later.

 

So I suggested he add salt to the water. But will it really work if its say 15º out? (that is to say, can the salt reduce the freezing temperature by 15º+?)

 

thanks

[Mr Skeptic]

The 0 F was originally defined as the coldest temperature he could get, based on salt water (this later turned out to be a bad idea). So you should be able to melt it with salt if it's above 0 F, but you would need more salt the colder it gets.

[MrMongoose]

Of course it works. When they grit the roads, thats mainly salt.

[Sisyphus]

The 0 F was originally defined as the coldest temperature he could get, based on salt water (this later turned out to be a bad idea). So you should be able to melt it with salt if it's above 0 F, but you would need more salt the colder it gets.

 

That is basically true, but it should be noted that the coldest you can actually get it to melt (that is, the eutectic point of water) is about -6 F.

 

So in answer to the OP questions: yes, it does work, and yes, there is a point at which it stops working, but you are only going to see that very rarely unless you live in some godforsaken northland.

[Mr.Chockuls]

you should try to put NH4NO3

[Mag]

you should try to put NH4NO3

 

um, yeah.

Why would I use that?

 

So in answer to the OP questions: yes, it does work, and yes, there is a point at which it stops working, but you are only going to see that very rarely unless you live in some godforsaken northland.

 

thanks

 

I wonder if the size of the salt will matter?

using a finer salt, or large crystals?

[DrP]

He may not even need to use the torch - just prinkle salt on the ice and it should start to melt. Salt water (IIRC) freezes at about 14 or 15 degrees below 0C.

[Severian]

They used to throw salt on the roads here to stop ice forming. But there were lots of environmental concerns about adding salt to the soil, so they stopped the practice.

[Jacques]

Here in Quebec we use calcium chloride and can melt donw to 0F.

There are other deicing salt. Take a look at this site for more info.

http://www.askthebuilder.com/251_Deicing_Salts_and_Concrete.shtml

 

Using fertiliser (NH4NO3) is not recomended because of damage it can cause on concrete and asphalte

[swansont]

The phenomenon is called freezing point depression (no, it's not a psychological malady)

 

Salts are particularly useful because being ionic you get multiple moles of solute for a mole of salt; it's the number of moles dissolved that tell you how much depression you get (it's a colligative property). Road salt is often calcium chloride; dissolving it is exothermic, so you get additional melting from the energy released.

 

http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/meltpt.html

[Bignose]

Here in Iowa they use a combination of both salt and sugar on the roads. They use calcium chlroide as the salt, and a sugar beet juice. It goes by the commercial name Geomelt. http://www.hawkeyereadymix.com/product_geomelt_55.html

 

I think that the nice thing about spraying a juice and not just a solid is that the liquid sticks to where you squirt it. So, instead of the salt bouncing everywhere as it falls out of the truck, the salt & sugar is going to stay right on the road and do its job instead of bouncing away.

 

There are many different approaches to this problem. When I was an undergraduate, the school would scatter ashes from the power plant on the sidewalks and roads on campus. It had some ice-melting properties, but I think its primary function was to increase traction.

[thedarkshade]

Now, correct me if Im wrong.

but I believe salt water freezes at a lower temperature than regular water (fresh water).

 

If this is true, I was wondering why.

Yes. I don't know if you have seen, but sometimes when temperatures are quite low, the municipalities order their workers to throw salt in the roads. And that is because salt decreases freezing point, and that is determined by cryoscopic coefficient!

[DrDNA]

Read....

Quote:

http://chemistry.about.com/cs/howthingswork/a/aa120703a.htm

"Colligative Properties and Freezing Point Depression

If you live in an area with a cold and icy winter, you have probably experienced salt on sidewalks and roads, used to melt the ice and snow and keep it from refreezing. Salt is also used to make homemade ice cream. In both cases, the salt works by lowering the melting or freezing point of water. The effect is termed 'freezing point depression'.

 

 

How Freezing Point Depression Works

 

When you add salt to water, you introduce dissolved foreign particles into the water. The freezing point of water becomes lower as more particles are added until the point where the salt stops dissolving. For a solution of table salt (sodium chloride, NaCl) in water, this temperature is -21°C (-6°F) under controlled lab conditions. In the real world, on a real sidewalk, sodium chloride can melt ice only down to about -9°C (15°F).

 

Colligative Properties

 

Freezing point depression is a colligative property of water.

 

A colligative property is one which depends on the number of particles in a substance. All liquid solvents with dissolved particles (solutes) demonstrate colligative properties. Other colligative properties include boiling point elevation, vapor pressure lowering, and osmotic pressure.

More Particles Mean More Melting Power

 

Sodium chloride isn't the only salt used for de-icing, nor is it necessarily the best choice. Sodium chloride dissolves into two types of particles: one sodium ion and one chloride ion per sodium chloride 'molecule'. A compound that yields more ions into a water solution would lower the freezing point of water more than salt. For example, calcium chloride (CaCl2) dissolves into three ions (one of calcium and two of chloride) and lowers the freezing point of water more than sodium chloride. Here are some other de-icing compounds:

 

Chemicals Used to Melt Ice

 

Name Formula Lowest Practical Temp Pros Cons

Ammonium sulfate (NH4)2SO4 -7°C

(20°F) Fertilizer Damages concrete

Calcium chloride CaCl2 -29°C

(-20°F) Melts ice faster than sodium chloride Attracts moisture, surfaces slippery below -18°C (0°F)

Calcium magnesium acetate (CMA) Calcium carbonate CaCO3, magnesium carbonate MgCO3, and acetic acid CH3COOH -9°C

(15°F) Safest for concrete & vegetation Works better to prevent re-icing than as ice remover

Magnesium chloride MgCl2 -15°C

(5°F) Melts ice faster than sodium chloride Attracts moisture

Potassium acetate CH3COOK -9°C

(15°F) Biodegradable Corrosive

Potassium chloride KCl -7°C

(20°F) Fertilizer Damages concrete

Sodium chloride (rock salt, halite) NaCl -9°C

(15°F) Keeps sidewalks dry Corrosive, damages concrete & vegetation

Urea NH2CONH2 -7°C

(20°F) Fertilizer Agricultural grade is corrosive "

 

http://chemistry.about.com/gi/dynamic/offsite.htm?zi=1/XJ&sdn=chemistry&cdn=education&tm=88&gps=170_25_1020_543&f=11&tt=2&bt=0&bts=0&zu=http%3A//www.howstuffworks.com/question58.htm

 

"Why do they use salt to melt ice on the road in the winter?

 

 

If you live in a place that has lots of snow and ice in the winter, then you have probably seen the highway department spreading salt on the road to melt the ice. You may have also used salt on ice when making home-made ice cream. Salt lowers the freezing/melting point of water, so in both cases the idea is to take advantage of the lower melting point.

 

Ice forms when the temperature of water reaches 32 degrees Fahrenheit (0 degrees Celsius). When you add salt, that temperature drops: A 10-percent salt solution freezes at 20 F (-6 C), and a 20-percent solution freezes at 2 F (-16 C). On a roadway, this means that if you sprinkle salt on the ice, you can melt it. The salt dissolves into the liquid water in the ice and lowers its freezing point.

 

If you ever watch salt melting ice, you can see the dissolving process happen -- the ice immediately around the grain of salt melts, and the melting spreads out from that point. If the temperature of the roadway is lower than 15 F or so, then the salt really won't have any effect -- the solid salt cannot get into the structure of the solid water to start the dissolving process. In that case, spreading sand over the top of the ice to provide traction is a better option.

 

When you are making ice cream, the temperature around the ice cream mixture needs to be lower than 32 F if you want the mixture to freeze. Salt mixed with ice creates a brine that has a temperature lower than 32 F. When you add salt to the ice water, you lower the melting temperature of the ice down to 0 F or so. The brine is so cold that it easily freezes the ice cream mixture. "

 

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