Simply put, deicers melt ice.

In practice, there are two fundamental concepts that explain how deicers work to melt ice:

  • What is ice?
  • How does it melt?

What is Ice?Melting_curve_of_water

Ice is frozen water.  It looks like a brittle, transparent crystalline solid.  Ice molecules can exhibit up to sixteen different phases (packing geometries) that depend on temperature and pressure.  You probably didn’t know that.  Another important thing to know is that when there is ice there is water.  Even though it may not be visible, molecular water is always present on the surface of ice. The amount of water increases at higher temperatures and is reduced when temperatures are colder.

How does ice melt?

Ice melts with heat.

When ice melts, it changes from a solid (ice) to a liquid (water). In order to make something change from a solid to a liquid, you have to heat it up. That’s because when you heat something up, you give it more energy, which makes the molecules (the tiny little things that make up stuff) move faster and faster. When molecules move faster, they have a harder time holding on to each other, so the stuff (in this case, ice) becomes less solid and more liquid.  So to melt ice, you have to add heat.

(Heat can actually move through water more than 20 times as quickly as it can move through air! The reason that the ice in water doesn’t melt 20 times faster than the ice in the air does is because the ice in the air is probably sitting on something – like a glass plate, and glass is an even better conductor than water is. And metal conducts heat even better than that!)

How do deicers create heat?

A deicer starts by dissolving in water, which lower the freezing point (called the eutectic point) of the ice.  This solution (the liquid mix of deicer and water) is known as liquid brine (in Chloride-based deicers).   This brine is actually what melts ice on contact by generating heat, creating more of the solution which in turn melts more ice.  As more ice is melted, the brine becomes diluted (the more ice it melts, the more water there is which means the deicer makes up less of the liquid).

Deicer variations can melt ice in temperatures as low as -25F, however, not all deicers are the same.

Water is a better “conductor” for heat-energy than air is.  Water conducts heat-energy differently depending on the concentration of the brine and the deicer.  What that means is that the heat can travel through some brine to reach the ice more easily than it can go through other types of brine.

How Do the Most Common Deicers Compare?

Sodium chloride, calcium chloride, and magnesium chloride are the most commonly used chemicals for melting snow and ice. They are also used as anti-icers prior to snowfall or freezing conditions. Among these deicers, calcium chloride is the most effective at all temperatures. It is the preferred chemical, particularly at low temperatures.

The following information summarizes the difference between these deicers:

Composition*
Eutectic**
Lowest Practical Melting Temp
Thermodynamics
Sodium Chloride
NaCl
– 6oF (- 21oC)
at 23 wt% conc.
15oF (-9oC)
Endothermic (Absorbs heat when in contact with snow and ice)
Magnesium Chloride
MgCl2.6H2O
(46 wt% MgCl2)
-28oF (- 33oC)
at 22 wt% conc.
5oF (-15oC)
Exothermic (Releases heat when in contact with snow and ice)
Calcium Chloride
CaCl2.0.2-2H2O
(dihyrdate: 75-80
wt% CaCl2)
(anhydrous: 90-97
wt% CaCl2)
-59oF (- 51oC)
at 30 wt% conc.
-25oF (-32oC)
Exothermic (Releases heat when in contact with snow and ice)

Conclusion

Deicers melt ice by becoming brine and transferring heat.  Different deicers are needed for different applications.