ELi ON EARTH: De-Icing Solutions

In response to the recent snows and freezing rain, East Lansing residents brought out shovels and de-icing agents to clear their walks and driveways. ELi on Earth (EoE) has already covered the creation of ice in the form of hail, and this week EoE is going to look at using salt to melt ice on your sidewalk and driveway, including which product to choose if you’re conscious of the environment in East Lansing.

Removing snow and ice with a shovel is the most effective and environmentally sound way to clear your walk and driveway, but sometimes a shovel or even a tough metal scraper is just not practical. When mechanically removing snow and ice does not work, East Lansing residents can turn to chemistry to help melt snow and ice.

Melting ice with chemistry depends on a couple of simple principles: (1) ice is a solid; and (2) impurities make solids melt at a lower temperature than pure solids do. Let’s take a moment to think about that second point.

When any liquid substance freezes into a solid, the molecules of the solid stack in a regular pattern. There are many examples of this kind of stacking in our daily lives, for example, the stacking of oranges at a grocery store. Each orange fits into the space of the four oranges in the level below it. As long as the bottom layer of oranges is kept in place, the oranges are pretty stable. But if you put a couple of boxes of crackers (representing “impurities” in this example) in the middle of the stack, then the overall stack of oranges is not nearly so stable. It is easier to knock down the “impure” stack of oranges.

In the example of the oranges, the amount of energy it takes to knock down the stack is just like the amount of energy it takes to melt the stack of water molecules in ice. The less energy it takes to disturb a solid at a molecular level, the lower the melting point of the solid. That’s why introducing impurities like salt into ice helps melt ice—because it lowers the melting point of ice.

There are two common chemical agents for melting snow and ice on your sidewalks and driveway, regular salt (sodium chloride: NaCl) and calcium chloride (CaCl₂). At the basic level, both of these chemicals melt ice the same way. The surface of ice always has molecules of water joining and leaving the ice. If more molecules of water are joining the ice, the water is freezing; and if more molecules are leaving the ice, the ice is melting. If the same number of molecules are joining and leaving, the ice and water are in equilibrium.

Both NaCl and CaCl₂ dissolve easily in water – they dissolve so easily that just the very thin amount of liquid water on the surface of the ice is enough to dissolve some NaCl or CaCl₂. This is where the chemistry gets clever. The NaCl and CaCl₂ act as impurities in the surface water just above the ice, so the water has a lower melting temperature (and therefore lower freezing temperature) than the ice next to the water. The molecules that were previously moving back and forth between the ice and water cannot rejoin the ice because the surface water won’t freeze, so more and more water leaves the ice: the ice melts. The ice itself does not have to have impurities if the water next to it has NaCl or CaCl₂ dissolved in it.

There are limits to this process—and this translates to a limit on the usefulness of salt to melt ice. If the temperature is below the freezing point of the water with NaCl or CaCl₂, then no overall melting can happen. For regular rock salt (NaCl), this temperature is about -17C (0 degrees F) and for CaCl₂ the lowest temperature is -52C (-62 degrees F). Below these temperatures, no amount of NaCl or CaCl₂ can melt the ice.

The reduction of ice’s melting point happens for any substance dissolved in water. Alcohol, sugar, ethylene glycol (the toxic and functional substance in car anti-freeze) all lower ice’s melting point the same way. They could all work as de-icing substance, but you probably wouldn’t want to put them on your sidewalks.

Most people and transportation systems use rock salt for deicing because it is cheap and readily available, but CaCl₂ works at lower temperatures, is probably easier on East Lansing roadside plants, does less damage to concrete, and does not cut the feet of pets and wild animals of East Lansing. You can get it at a number of local stores, including Meijer. As you consider how to remove ice and snow, remember that the City of East Lansing has a policy on removing ice and snow from your sidewalk. That policy applies to city and landlord sidewalks, too.

The photo above shows rock salt on the left, a block of ice in the middle, and calcium chloride on the right. The video below is a timelapse of the ice melting.  The ice is clearly melting faster over the calcium chloride (CaCl₂₎, on the right side. There are three main reasons the ice is melting faster on the right side: the calcium chloride is softer and finer than the rock salt and therefore makes better contact with the ice; calcium chloride dissolves faster in water than rock salt does; and calcium chloride gives off heat when it dissolves in water and that heat helps melt the ice.

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