Posted by: BibleScienceGuy | January 16, 2019

Why Does Salt Melt Ice?

(3 Minute Read)

Niagara’s Bridal Veil Falls in Winter

Ice melts if the temperature rises above 32°F. Why does ice melt if you throw salt on it, even if the temperature is as low as 15°F? What happens if you use sugar, flour, ashes, or sand instead?

Why Does Salt Melt Ice?

By the design of the Great Creator, water freezes and ice melts at 32°F. At this temperature, an icy road usually has a thin layer of water molecules interacting with the ice molecules beneath. The ice is freezing the water at the same rate the water is melting the ice. When salt is added, it lowers the water’s freezing point. The result is that the ice can’t freeze the water, but the water continues to melt the ice, reducing the ice on the road.

How does salt lower the freezing point of water? By the design of the Great Creator, when dissolved in water, salt (NaCl) splits into sodium and chlorine ions which form weak chemical bonds with water molecules (H2O). This solution stays liquid at much lower temperatures than pure water because the sodium and chlorine ions “get in the way” and impede crystallization of the water molecules into ice because the ions have bonded with water molecules.

Salt is sprinkled over winter roads because it lowers the freezing point of water (32°F) or the melting point of ice (32°F) thereby reducing the slippery ice hazard. It’s most effective to salt roads close to 32°F when there is water with the ice. Over 20 million tons of salt are used every year to melt snow and ice.

BibleScienceGuy at our campsite’s picnic table loaded with a foot of snow.

The more salt that is dissolved in water, the more it lowers the freezing point (impedes crystallization) until the water is saturated with salt. Under lab conditions, this minimum temperature is –6°F. On the street, salt can melt ice only down to about 15°F. There is no point in salting roads below 15°F.

Common table salt (NaCl) isn’t the only salt that can be used. Other salts are more effective (but more expensive) in lowering the melting point of ice. For example, calcium chloride (CaCl2) lowers it to –20°F.

Calcium chloride melts ice better than table salt because it breaks down into three ions instead of two: one calcium ion and two chlorine ions. This results in more ions bonding to water molecules and impeding the crystallization of the water molecules into ice.

When salt water freezes, the salt moves to the surface. Icebergs are free of salt below the surface, despite floating in salty oceans. Thus icebergs are a potential source of fresh water. Some have considered towing icebergs to alleviate water shortages.

Homeschool Science Project

Figure out what sugar, flour, ashes, and sand do to the freezing point of water.

Experiment by putting water in a compartment of an ice cube tray and sugar water solutions of varying concentrations in other compartments. Use flour, sand, and ashes with water in the remaining tray compartments. Check every 15 minutes to observe the freezing rates. Explain your results.
(Hint: Do popsicles freeze faster or slower than ice in your freezer?)

For another experiment, take a tray of ice cubes. On top of separate cubes in the tray put salt, sugar, flour, sand, and ashes. Return the tray of frozen cubes to the freezer. Put a thermometer in the freezer so you’ll know the temperature. Check periodically to see which additives cause the most melting.

Report and explain your results in the comments below.

Making ice cream in a zip-lock bag

Ice Cream in a Baggie

Has the “ice cream urge” ever hit you when the freezer was empty and you couldn’t bring yourself to drive to the store?

Do your children sometimes beg for ice cream at the most inconvenient times?

Here’s a solution to this urgent problem that applies what we’ve learned about ice and salt.

Make your own double-scoop of flavorful ice cream with these four simple steps in only 10-15 minutes!

  1. Put 1/4 cup sugar, 1/2 cup milk, 1/2 cup whipping cream, 1/4 teaspoon vanilla in a quart zip-lock bag with desired flavorings (strawberries, chocolate chips, cherries, crumbled oreos, etc). Double-bag and seal.
  2. Fill a gallon zip-lock bag 1/3 full of ice. Add a cup of salt.
  3. Place the sealed quart bag into the gallon bag and seal.
  4. Shake the gallon bag gently for 10-15 minutes until the ice cream is firm.

Heat energy from the mix melts the ice, thereby cooling the cream mix as its heat is used to melt the ice. The cream mix freezes (at a lower temperature than water) because the ice-salt mixture has a lower freezing/melting point than pure ice.

Raise the Freezing Point of Water

It’s easy to lower the freezing point of water by adding salt. It’s not so easy to do the reverse — raise the freezing point of water.

Nobody has figured out how to make water freeze at temperatures above 32°F. In fact, many scientists think it cannot be done. This and the science behind it could be another fruitful research topic for a Nobel Prize — like figuring out why ice is slippery.

If somebody could figure out how to raise the freezing point of water economically, it could be a very useful energy-saving device in a variety of contexts. For example, one could make ice in refrigerators with less energy.

Questions to Ponder

1. When winter’s snow and ice frustrate your plans, how do you react? Does your response honor the Great Creator?
2. How does winter weather remind you of the Great Creator?
3. In what ways could raising the freezing point of water be useful?

Share your thoughts on these questions in the comments below. It could encourage or help another reader.

For Christ and His Kingdom.

Read the prequel:
Why Is Ice Slippery?

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©William T. Pelletier, Ph.D.
“contending earnestly for the faith”
“destroying speculations against the knowledge of God”
“for the defense of the gospel”
(Jude 1:3; 2 Cor 10:5; Phil 1:16)
Wednesday January 16, 2019 A.D.

God thunders with His voice wondrously,
Doing great things which we cannot comprehend.
For to the snow He says, “Fall on the earth,”
And to the downpour and the rain, “Be strong.”
He seals the hand of every man,
That all men may know His work.

(Job 37:5-7)

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