Brainiacs

[barkwindjammer]

A science topic:
any chemistry students please lend your expertise

we were having a discussion over a couple of pints last night,
Using rock salt on snow

Q: is the salt more effective when spread on top of the snow and ice
or, is it more effective if the snow is cleared then the salt is put down?
this is in relation to pavements as the large ploughs are used on the roads


#-o

 

[Richard T]

As neither a chemistry or science student, I can say that it's very much better to clear down as close to the surface as possible before spreading rocksalt.
When I was a council tree surgeon and it was too cold for climbing they would have us clearing the snow from central Solihull ... I must have done a few miles in the '80s, snowboard, shovel, chuck the salt off a shovel. Happy days. :deer

 

[SketchUp Guru]

The salt lowers the freezing point of water which is why it melts the snow/ice. A fiite amount of salt, though will only be effective on a limited amount of snow/ice. Put a single crystal of salt on an ice cube and put it back in the freezer. Watch what happens.

It would make more sense to mechanically remove as much snow as possible before applying the salt so the salt has less water to melt. Around here we also try to get down to bare pavement where ever possible to let the pavement absorb some solar infrared. This will warm the pavement and help to melt the snow/ice.

 

[Gary]

Salt is most effective with vinegar on cod and chips. :ho2

 

[SketchUp Guru]

Or better, on the rim of a glass filled with Tequila, Cointreau and lime juice.

 

[Steve Maskery]

Yep, mechanically clear as much as you can so the salt is most effective on what little is left.

Salt lowers the freezing/melting point of water and raise the boiling point. So when you have a pan of boiling water on the stove and throw a handful of salt into it, it stops boiling straight away. It's not because the salt is cold, it's because it disolves almost instantly, so the water is not water any more, it's saline. So it has to get to 102 to boil again. Or 107. Or, IIRC, 112C if it is saturated. The more salt in it the higher the boiling point.

Similarly for freezing, the more salt the lower the temp has to get before it freezes again.

Incidentally, when you add salt to ice the ice actually gets colder, even though it is melting. The energy required to melt the ice has to come from somewhere, so it takes it from itself, as it were.

Many other salts work in the same way. Mr Fahrenheit tried lots of experiments until he could get no colder. The lowest he got was -17.77C, which is what he called Zero. Today we know that we can get much colder and we have a value for absolute zero, which scientists have got very close to. They get there by a repeated cycle of cooling, making one cold reaction and using that to cool the next and so on.

And for your homework, class...

Cheers
Steve
B.Sc. Chemistry 1979.

 

[treeturner123]

Incidentally, the lowest temperature at which water can exist as a liquid is a lot lower than you might guess. Any offers?

Shall I tell you?

Approx -42 degrees C.

Mind you it must be VERY pure so that there are no ‘specs’ of dust for example round which ice can start forming. When water does freeze at this temperature, the process is so fast that the human eye can hardly see it happen! There was an example on the BBC science site a while ago and I'm sure if you trawl youtube.....

Phil

 

[Steve Maskery]

And in Siberia:
http://www.youtube.com/watch?v=k_k13fo4pr4

S

 

[SketchUp Guru]

I did that last winter out my back door to show my son.

 

[LuptonM]

Probably putting the salt down before as it going to have a better chance of dissolving and turning the little bit of snow/ice that lands into liquid water.

If u put the salt on top it might find it harder to dissolve and the salt might just lay on top of the snow- u need a little water to get the salt working