The science behind salt melting ice

Published: Jan. 7, 2018 at 2:34 AM CST
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Salt and Ice

Here in Wisconsin, we know to throw salt on icy sidewalks and driveways but why does salt melt ice?

Ice, as we know, is frozen water. Water is made of a special collection of atoms. Specifically, two hydrogen atoms, and one oxygen atom. As the temperature of pure liquid water starts approaching its freezing point (32 degrees F, or 0 degrees C), the molecules start to arrange themselves into ice crystals. At the freezing point, there’s an equal number of molecules joining to form ice, and detaching from ice to make liquid water.

Now dissolve some salt – or other substance – into the water. A salt molecule is made of a sodium atom and a chlorine atom. When dissolved, these sodium and chlorine ions don’t keep water molecules from leaving the ice, but they DO interfere with water molecules attaching to the ice.

This means the freezing point drops. For salt, this can be as low as -6 F (-21 C).

For other chemicals, like calcium chloride (CaCl2), it can be as low as -60 F (-51 C)!

Even weirder, the temperature of the water / salt mixture DROPS! It takes energy for water molecules to go from ice to liquid, and to break the salt molecules into separate ions. Where does this energy come from? The heat energy of the mixture!

Now you know why you add salt to the ice in your ice cream maker. As the salt melts the ice, the salt / water mixture (or brine) that is formed is COLDER than the ice by itself. So your ice cream freezes faster!

The Wonders of Physics Annual Shows - 35th Anniversary

Programming on Wisconsin Public Radio in its early years often consisted of variety-type shows. So, in honor of 100 years of Wisconsin Public Radio, and 35 years of The Wonders of Physics, we present an old-time radio variety show with the greatest demonstrations from The Wonders of Physics.

The next public presentations of The Wonders of Physics:

Saturday, February 10, 2018 1, 4, and 7 pm

Sunday, February 11, 2018 1 and 4 pm

Saturday, February 17, 2018 1, 4, and 7 pm

Sunday, February 18, 2018 1 and 4 pm

These presentations will be held in 2103 Chamberlin Hall, 1150 University Avenue, Madison, WI. Shows last a bit over an hour and are suitable for all ages.

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Our program serves thousands of kids every year, and there are thousands more passionate young scientists that we are unable to reach. We are committed to sharing all the wonders that physics has to offer, but we need your help. Click

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L.R. Ingersoll Physics Museum - 100th Anniversary

The L.R. Ingersoll Physics Museum was conceived by Professor Snow and Professor Ingersoll in 1917 and was fully established in 1918 when the construction of Sterling Hall was competed. Which made the physics museum one of the first such museums in this country. Following Professor Ingersoll death in 1958, the physics department renamed the museum to Leonard R. Ingersoll Physics Museum to honor his dedicated for the success of the museum. Keeping with tradition, all exhibits are designed by the faculty and staff, then constructed by the departmental shops, staff, and students. Some of them were purchased and then modified to be used in the museum.

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