Grit and snow

[Col]

How does salt i.e. the stuff gritters throw out melt snow?

[greenday]

A very general answer is that impurities always lower melting points. If you think of the salt as an impurity in snow or ice, it will reduce the temperature needed to melt the snow or ice.

[Clanad]

When salt is added to an icy surface, it will seek heat from the air and pavement, and seek humidity from the air, snow, and ice to dissolve. Once dissolved, it becomes a salt brine solution. The salt brine solution melts the surrounding snow and ice. The melting spreads out from that point until the ice is melted or the salt is all dissolved.
 Salt acts as a freezing point depressant. This means that when salt is added to water, the freezing point of the resulting salt brine becomes considerably lower than that of pure water. This salt/water solution freezes at -21.1�C. As you can see, the freezing temperature of Sodium Chloride Brine is much lower than the well-known freezing temperature of water at 0�C. This is why salt is called a freezing point depressant; it lowers the freezing temperature of water...

[pinus]

Using the Fahrenheit scale, water freezes at 32 degrees, brine (salt water) at zero degrees. 

When he was working on his thermometer, Fahrenheit couldn't (reliably) get any colder than frozen brine, so he set that as his zero.  Having chosen 180 degrees for the difference between the freezing and boiling points of pure water, the freezing point of water became 32 degrees.

Why 180 degrees?  probably on account of the number of factors - 2, 3, 4, 5, 6, 9, 10, 12

 

All right - that's more information than you need!  :-)

[NetSquirrel]

http://tinyurl.com/5mvkl ...Deja vu, Clanad?

As for how salt melts ice, this has to do with the way salt and water interact with each other on the molecular level.

Water (H2O) and salt (NaCl) are both known as polar molecules.  That is to say, the electrons bound to a polar molecule are not shared equally between all the atoms forming that molecule.  In a water molecule, there are two hydrogen atoms and a single oxygen atom.  The oxygen atom exerts a large "pull" on the six orbiting electrons, while the two hydrogen atoms each exert small pulls.  The result is that the electrons orbit closer to the oxygen side of the molecule than the hydrogen side.  In salt (NaCl) it is the chlorine atom that pulls the electrons closest.

The side of the molecule that "wins" the electron tug-of-war will gain a slightly negative charge (because electrons are negatively charged).  The opposite side of the molecule will gain a small positive charge.  These two sides are referred to as "electronegative" and "electropositive" respectively (at least, that is what I call them).

When salt is added to liquid water, the electropositive (hydrogen) ends of the water molecules find themselves strongly attracted to the electronegative (chlorine) ends of the salt molecules.  At the same time, the electronegative (oxygen) ends of the water molecules find themselves attracted to the electropositive (sodium) ends of the salt molecules.

[Continued...sorry]

[NetSquirrel]

The result is that the NaCl molecules are pulled apart and become surrounded by water molecules.  When a NaCl molecule divides in this way, two ions are formed:  Na+ and Cl-.  (This is understood best by examining a periodic table - sodium is in group 1 and chlorine is in group 7.)  See this excellent diagram somebody has made: http://tinyurl.com/59b55 (the red dots are oxygen molecules, beige are hydrogen, blue are sodium and green are chlorine).

Why have I mentioned liquid water?  Look at the diagram on the right, which shows salt fully dissolved in water (in reality, not all molecules would be surrounding the dissolved ions - many would be drifting freely as in diagram a).  If you wanted to freeze the water now that it has salt in it, what would happen?  Normally, at 0�C water molecules are moving slowly enough to line up positive-to-negative and lock in place, forming solid ice.  (They have to spread apart a little to achieve this.)  When dissolved salt is present, it becomes harder for those water molecules occupied with surrounding the salt ions to join with their neighbouring water molecules and form ice.  A lower temperature is required; say, -5�C.  Add more salt, and an even lower temperature is required.  Dump a load of salt directly onto some ice, and the resulting "pull" of the NaCl on the frozen water may be strong enough to force the water out of its neat, lined-up shape and back into a liquid.  More salt will immediately dissolve into the liquid water, and then more ice will be melted, and so on.  The process will cease to melt any more ice when the water-salt solution becomes too diluted.

[Gef]

They all contain an element of truth but it would take me ages to point out the inaccuacies in the answers of clanad, pinus and NetSquirrel. Well done for trying though.

[NetSquirrel]

Thank you for being merciful, Gef!

[NetSquirrel]

Incidentally, if there are any major holes in my answer you'd care to fill in...? Just the most blatant erros would be fine - I don't want to give Col an incorrect understanding of the topic (or indeed, myself).

[bernardo]

pinus - when I was at school, I was told that Herr Fahrenheit intended his scale to be a centi-grade scale with a range of 100 degrees.  0 degrees was, as you say, the lowest temperature he could get with ice and salt, but the other fixed point (100 degrees) was based on the temperature of the human body.  i have also read theories that he wanted to divide his scale into 12 degrees, but multiplied it up to 96 when he realised that the scale was greater than he expected.  Both figures are reasonably close to the actual temperature of 98.6 degrees.  Presumably it was only later that the Fahrenheit scale was fixed at 32 and 212 degrees.

[Gef]

Sorry NetSquirrel. Yours was a very good answer except for the fact that sodium chloride is an ionic compound and does not exist as molecules. Solid salt exists as a giant crystal lattice (crystals) of oppositely charged sodium and chloride ions. Sorry if that sounds condecending.

[NetSquirrel]

On the contrary, Gef.  I really dislike it when people stay silent when they know, with just a few words, they can help someone out.  You are correct about the lattice structure, of course, and quite an important point it is, too.