Old 02-05-2004, 04:09 PM
Matchka Matchka is offline
Join Date: Jun 2003
Location: Lakewood, CO
Posts: 719
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?
I didn't know the sponge was supposed to be wet.
Old 02-05-2004, 04:15 PM
Q.E.D. Q.E.D. is offline
Charter Member
Join Date: Jan 2003
Location: Richmond, VA
Posts: 22,536
(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 (~ -33C, -28F) than sodium chloride (~-20C, -5F), (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)
SnUgGLypuPpY -- TakE BaCk tHe PiT!
Old 02-05-2004, 08:49 PM
robby robby is offline
Charter Member
Join Date: Dec 2000
Location: Connecticut, USA
Posts: 5,130
Calcium chloride (CaCl2) is even more effective than magnesium chloride (MgCl2), 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, CaCl2 and MgCl2 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 MgCl2. "MgCl" [sic] is never correct.
"I don't want to achieve immortality through my work. I want to achieve it through not dying." --Woody Allen
Old 02-05-2004, 10:01 PM
threemae threemae is offline
Join Date: Jun 1999
Location: Philadelphia, PA
Posts: 3,760
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.
You know, doing what is right is easy. The problem is knowing what is right.

--Lyndon B. Johnson


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Forum Jump

All times are GMT -5. The time now is 07:47 AM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2018, vBulletin Solutions, Inc.

Send questions for Cecil Adams to: cecil@straightdope.com

Send comments about this website to: webmaster@straightdope.com

Terms of Use / Privacy Policy

Advertise on the Straight Dope!
(Your direct line to thousands of the smartest, hippest people on the planet, plus a few total dipsticks.)

Publishers - interested in subscribing to the Straight Dope?
Write to: sdsubscriptions@chicagoreader.com.

Copyright 2018 STM Reader, LLC.

Copyright © 2017