cacl2_glovesIt's January and winter in the Northern Hemisphere. cacl2_flakeIf you live far enough away from the equator, these days you may be happily sledding down snowy hills or skating on frozen ponds. The winter wonderland is enchanting and fun, but it can also be dangerously slippery underfoot and on the road. Conveniently enough, chlorine chemistry provides a product that can reduce the risk of accidents on ice and snow. That product is the chemical compound calcium chloride, CaCl2.

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Calcium chloride is a salt. Many people assume there is only one salt on Earth—the one used to season foods. The truth is there are many salts in nature besides sodium chloride, the salt that flavors popcorn and potato chips.

A Sprinkling of the Chemistry of Salts

One of the interesting chemical properties of salts is that when they dissolve in water they change completely into ions—atoms with tiny electrical charges. For example, add table salt—sodium chloride, NaCl—to your pasta pot, and along with tastier pasta, you get, for every unit of NaCl dropped into the water, equal numbers of Na+ and Cl- ions. (Na loses one electron, becoming positively charged, and Cl gains one electron, taking on a negative charge.) Similarly, when calcium chloride is added to water, Ca+2 and Cl- ions fill the water. But notice that each CaCl2 unit makes two chloride ions for every one calcium ion. So, for every unit of CaCl2 that is dissolved, three ions result—one Ca+2 and two Cl-

cacl2_temp

A Tale of Ted the Water Molecule and the Freezing Point Depression

cacl2_tedNow let's consider the water molecules that make up a puddle of water on the road on a winter day. Water molecules are shaped like tiny teddy bear heads. The large oxygen atom can be thought of as the teddy bear face; the two smaller hydrogen atoms, his ears.

Although it is not an ion, each water molecule has a net negative area (around where the teddy bear's neck would be) and a net positive area (between and above the teddy bear's ears). These molecules are constantly in motion and their speeds are determined by the temperature of the puddle--the higher the temperature, the faster the motion.

Now imagine the following scene in the puddle: Little Ted and the other water molecules have been moving around all day and speeding up as the afternoon sun has warmed the air and the puddle. But as the mild winter day wears on and evening approaches, temperatures are falling rapidly. Ted and the others are slowing down and becoming very sluggish.

cacl2_glovesThen something amazing happens. The water temperature hits 0°C (32° F) and the molecules slow down so much that they become affected by their own weak positive and negative charges. At higher temperatures the water molecules barely noticed these attractive forces, but now, it's colder and as they slow down to a crawl, the molecules are automatically arranging themselves so that the positive region of one is next to the negative region of another, and vice versa. Ted and the others are beginning to freeze in this position. The puddle is turning to ice—a danger to cars and pedestrians.

But, wait! Along comes the salt-spreader to dump some CaCl2 in the puddle that's turning to ice. We know what happens to salts in water—the puddle is soon populated by positively charged Ca+2 ions and twice as many negatively charged Cl- ions. In dissolving in the puddle, these ions increase the disorder of the puddle liquid. Because nature "prefers" things to be mixed up, the addition of the salt has the effect of putting a temporary halt to the freezing process. The puddle remains liquid at 0°C, the temperature at which pure water would freeze. Freezing now will not occur until the temperature drops even lower because the salt-water mixture is more stable than a salt-ice mixture would be. This is good news for drivers and pedestrians since wet roads are not nearly as slick as icy ones.

cacl2_mount

This little example illustrates what scientists call the freezing point depression of water. Physical chemists tell us that the greater the number of dissolved ions in water, the greater the disorder of the puddle, and the lower the freezing point. When we spread CaCl2 on roads and walkways for safety in the winter, we are taking advantage of the freezing point depression idea. And CaCl2 is a better road salt than NaCl because it makes more ions per formula unit of the salt dissolved. 


cacl2_don

This is Don Juan Pond in Antarctica, a very shallow pond (here shown during a dry spell) that is the only liquid body of water in Antarctica. This pond has so much CaCl2 dissolved in it, it never completely freezes, even in the dead of the Antarctic winter when temperatures may reach -50°C (-58°F)

Follow-Up Questions

1. Explain how and why salt is used in making ice cream.

2. Name the following salts. How many units of different ions are generated when one unit of each of the following salts is dissolved?
     a. CaF2
     b. KCl
     c. MgCl2

3. Besides lowering the freezing temperature of water, salts also raise the boiling temperature of water. Explain why this is so, using the concept that nature "prefers" disorder over order.

For a list of previous "Chlorine Compound of the Month" features, click here.

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