NaCl vs. MgCl

Both are used as roadway de-icers.
Which is most effective at:

  1. melting existing road ice
  2. preventing road ice

How does ambient temperature and altitude affect their de-icing qualities?

From here. (Warning: PDF)

Calcium chloride (CaCl[sub]2[/sub]) is even more effective than magnesium chloride (MgCl[sub]2[/sub]), and both are much more effective than sodium chloride (NaCl). NaCl is far cheaper, though, which is why it is so widely used.

All three compounds lower the freezing point of water, a phenomena known as “freezing point depression.” However, CaCl[sub]2[/sub] and MgCl[sub]2[/sub] both liberate heat as well when dissolved in water. This exothermic reaction adds to their effectiveness. (The dissolving of NaCl in water is endothermic.)


P.S. Altitude is basically irrelevant. Ambient temp is addressed in the link.

P.P.S. The proper formula for magnesium chloride is MgCl[sub]2[/sub]. “MgCl” [sic] is never correct.

Wow, I didn’t know that dissolving MgCl2 or CaCl2 were exothermic processes. You learn something new everyday.

Anyway, the reason that both of these salts are more effective than NaCl is because the freezing point depression is a colligative property, a property that is affected by the number of molecules disolved within it. Basically, MgCl2 breaks down into three ions, NaCl breaks down into two. It is fifty percent more colligative action per original salt molecule, plus the other benefits described above.