Adding salt helps to melt road ice and freeze ice cream —
why the different effect? Eye-color genetics, Unhappy scorpions
 When your roads or sidewalks are frozen, you pour salt on
them to make the ice melt. But you also put salt in an ice cream freezer
to make the ice cream freeze. Why does the salt that you put in the ice
cream freezer have a different effect than when you pour it on the frozen roads?
Catherine, Dubach, Louisiana
Strawberry ice cream. Photo courtesy of Lotus Head and Wikipedia.
Actually, the effect is the same in those two situations, but we
look at different
aspects. We put salt on icy roads, because we know salty
water freezes at a lower temperature than water, so the brine will melt. And
that's what we want — to get rid of the ice on the road. We don't notice that,
in the process of melting the road ice, we have made the road colder, because it takes energy to melt the brine.
That heat energy comes from the road.
But that's exactly what we want for the ice cream. As the brine in the
ice-cream freezer melts, it takes heat from the custard, makes the custard colder and, eventually, freezes the
custard into ice cream. We definitely notice the custard getting colder, whereas we
didn't the road. Moreover, the ice cream will continue to get colder until the temperature drops to the freezing point of
brine — about -6 degrees Fahrenheit (-21 C).
"The nature of water is remarkable," comments
physicist
Rod Nave, professor at Georgia State University. Consider a kilogram
(about a quart) of water. It takes only 1 food calorie to raise that water
1 degree Celsius (1.8 F). But, at 0 C (32 F), we must extract 80 food
calories to freeze the water, and add 80 food calories to melt that kilogram
of ice.
The calorific energy is huge: 80 food calories is almost enough energy
to fuel a person running a mile. "This remarkably large energy associated with
the freezing/melting transition of water leads to some extraordinary scenarios
like the one you have described —
if you force water to melt by lowering the freezing temperature, it's going to
grab a lot of energy from somewhere!" emails Nave.
Further Reading:
Homemade ice cream maker (and heat thermodynamics), Sci-Toys.com
Comment
Nice explanation on salt's role in melting and freezing.
Sukumar, Chennai, India
What color eyes should two blue-eyed people make?
Chastity, Somewhere, World
Many colors are possible for the offspring of blue-eyed
parents, including a child with brown eyes. Blue eyes are most likely.
Further Reading:
Eye color
mocks easy rules. Mom has green eyes, Dad blue. How did I get brown?
WonderQuest
What do scorpions do when it rains?
Cly, Bristol, New Hampshire
Scorpions are most prolific and diverse in warm, dry lands, although they
exist almost anywhere on the globe. So, when
spring and summer rains fall on dry-land species, these scorpions seek drier places — perhaps up a slope. Houses and buildings near
arroyos and river beds that are normally dry, are also possible targets.
Further Reading:
Scorpions, by Russell Wright, Oklahoma State
University
(Answered April 21, 2008)
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