All Weather News

Salt Rocks! on an Icy Sidewalk

19 Nov 2017, 9:06 pm

You may already know that you can put salt on a sidewalk to keep if from becoming icy, but do you know why this works?

Pure water freezes at 32°F (0°C) but water mixed with salt will have a lower freezing point. Ocean water has a lower freezing point than freshwater in lakes and rivers.  

So salt on an icy sidewalk will help to melt the ice, all you need is a little water for this to work.  Fortunately, even if the ice on your sidewalk as solid as a rock there will still be a thin film of liquid water to combine with salt and lower the freezing point.  The salt will actually pull water out of the ice to continue and enhance the process.  Salt readily dissolves in the water it absorbs making salt water that freezes at a much lower temperature.

This method of de-icing does have limitations and is still dependent on temperature.  A 10-percent salt solution with water freezes at 20 F (-6 C), and a 20-percent solution freezes at 2 F (-16 C). That means if you are dealing with below zero temperatures this winter, salt may not cut it.  Sprinkling a little sodium chloride or table salt on ice when it’s 0°F may only result in ice coated with a layer of salt.  On the other hand, if you put the same salt on ice at 15 F, the salt will be able to prevent melting ice from re-freezing.

Salt is typically the go too melting agent used on roads and highways but magnesium chloride and calcium chloride are also used.

Magnesium chloride works down to 5 F while calcium chloride works down to -20 F.

Meteorologist Mike Morrison

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