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December 9, 2002
A: FROM MENTOR SARA RAMSDELL
IN KY
Different chemical salts have different properties in lowering
the freezing
point of water. Some common de-icing salts are sodium chloride
[rock salt],
magnesium chloride, and calcium chloride. Some commercial
products are
blends of these salts; they also vary in cost, so you must
weigh their
relative efficiencies with their costs. For example, calcium
chloride is
more effective and will work at lower temperatures, but usually
costs more
than sodium chloride.
You also need to be sure that deicing salts will not damage
vegetation or
pavements. For example, some people suggest using fertilizer
[ammonium
nitrate] as a deicing salt, since it will not hurt vegetation;
what they
don't know is that it will chemically attack concrete and
may eat up your
sidewalk or steps! Salts may also corrode steel in bridge
decks and parking
structures, with some being worse than others.
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December 9, 2002
A: FROM MENTOR JOAN LUSK IN
RI
A pure solid (ice or snow) is in equilibrium with its liquid
phase
(water) when the temperature of each is at the freezing point
(equal
to melting point of the solid). Dissolved particles in the
liquid
lower the freezing/melting point. This is a "colligative"
property
of the solution, meaning that it depends only on the number
of
particles in solution, not their chemical nature. ("Colligative"
is
a nice fancy word to show off with! It's so obscure that it's
not in
the on-line Oxford English Dictionary, but it _is_ in Webster's.)
So
salts, sugar and the ethylene glycol found in car antifreeze
all
operate by this mechanism.
A molecule of sugar is just one particle in solution, but
a molecule
of sodium chloride in solution becomes two particles: Na+
and Cl-
ions. Calcium chloride (CaCl2) dissolves to make three particles,
Ca2+ and 2 Cl- ions, so it is 50% more effective, mole for
mole, than
NaCl. [I leave it to you to figure out the relative effectiveness
on
a weight basis, gram for gram.] Sucrose has a much high molecular
weight (342), so on a weight basis it would be less effective
than
the salts - and it would leave your porch steps pretty sticky!
To be
honest I don't know whether the CaCl2 sold for melting ice
is one of
the hydrated forms or anhydrous. On a weight basis the anhydrous
would be more effective - water molecules of hydration add
weight but
no particles to the solution.
MgCl2.6H2O (magnesium chloride hexahydrate) is also sold for
melting
ice; magnesium being a lower atomic weight than calcium gives
more
particles/gram, and it's claimed to be less corrosive than
CaCl2.
NaCl would be more toxic plants that we want to survive next
to our
walkways.
Lowering the freezing point means that ice or snow will melt,
and the
heat required to melt it come from the solution until the
whole
ice/water/solute system is at the new, lower freezing point.
For icy
sidewalks, we're mainly interested in making the solid ice
disappear.
For making ice cream, it's the colder temperature that we
want. For
keeping our car running, we want to keep the coolant mixture
liguid
and circulating freely even in very cold weather - like this
morning
in Rhode Island: 15F, -5C!
Here's a experimental design to look at freezing point depression:
http://www.woodrow.org/teachers/chemistry/institutes/1986/exp9.html
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