Ticker for January 26, 2000

                
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January 26, 2000   January 26, 2000   January 26, 2000   January 26, 2000


How Does Rock Salt Work, Anyway?

The Ticker received the following e-mail from a loyal reader earlier
this afternoon:

"How come adding rock salt to your ice cream maker makes the
ice cream freeze and putting it on the road makes ice melt?"

That's a good question, and here's the answer: in both of these
scenarios, humans take advantage of the same scientific properties
to achieve two different objectives.

Adding sodium chloride (otherwise known as table salt) to water
acts to depress the freezing point of the salt-water solution.
In other words, salt water freezes at a lower temperature than
fresh water.  The exact temperature depends on the concentration
of salt and the type of salt used.

When rock salt is added to an ice cream maker, the resulting salt
water solution can bathe the metal canister at a temperature less
than 32F.  As the human adds ice, the temperature drops below 32F,
but the salt water solution doesn't freeze.  The result?  Harder
ice cream!

When rock salt is added to the street, it depresses the freezing
point of any water which dissolves it.  This salt water solution
can exist as a liquid at lower temperatures than fresh water.  The
result?  Salty water, instead of clean ice, if the solution is
strong enough to withstand the surface temperature.


Speaking of Melting Ice ...

Pouring table salt on snowy (or pre-snowy) roads isn't the only way
to melt ice.  Sodium chloride is used because it is cheap and easy
to obtain in large quantities.  But, as any New Yorker with a car
can tell you, salt can be quite corrosive.  And as hard as it is on
cars, it's just as hard on roadways and bridge decks.  This is costly
in the long run.

So, alternative methods to road salting are desirable.  One type of
alternative is using a different kind of salt.  Some salts are more
effective than others at lowering freezing points, and some salts are
more environmentally friendly (and road-, car-, and bridge-friendly).
However, these salts are typically much more expensive than ordinary
sodium chloride.

Another alternative is to heat the surface itself.  This may sound
like an impossible task, but we live on one of the most consistent
heaters around!  That's right, the Earth!  Engineers at Oklahoma State
University are designing a bridge deck heating system that draws heat
from groundwater to keep the bridge deck above freezing temperatures.
The process is natural and cost-effective.  And, since the system
only borrows heat from the water, and doesn't use the water itself,
it is much more environmentally safe than salting.

Be looking for more about the OSU "SmartBridge" Project in the months
to come.