NaCl vs. MgCl

[Matchka]

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?

[Q.E.D]

(MgCl2) as a de-icing agent as an alternative to conventional rock salt (which is predominantly NaCl) and there has been a number of articles and discussions [1-4] describing the benefits of this salt (alone or mixed with other materials) over rock salt. The major benefits cited are (i) that it will prevent ice formation at much lower temperatures (~ -33°C, -28°F) than sodium chloride (~-20°C, -5°F), (ii) that it can be used in solution as a spray rather than as crystals, thereby, increasing its effectiveness and (iii) its lower toxicity. The major disadvantage noted is the higher cost. Unfortunately, however, there are other disadvantages of MgCl2which should be taken into account when considering its use as a de-icing salt. This is because the vast majority of highway bridges, barrier walls, parking garages etc. are constructed from reinforced concrete and the scientific and technical literature is replete with articles showing the detrimental effects of Mg salts on concrete.

From here. (Warning: PDF)

[robby]

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.)

Cite.

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.

[threemae]

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.