Weymouth, Mass. — I've just warmed my hands over some calcium chloride pellets in a simple but impressive experiment in the relative merits of sidewalk de-icers.
I poured a little water into a plastic pouch containing the pellets and within seconds could feel the considerable warmth resulting from the chemical reaction. In fact, the pouch temporarily got too hot to hold.
Winter can be quite a beautiful time of year, but it has its challenges. One of these is the ice that forms on sidewalks and paths leading to the front door. There are several ways of combating this problem, all of which have their drawbacks.
Wood ash or sand sprinkled over the path is a moderately effective way to improve traction. But this approach only covers up the problem; it does not remove it. The great advantage to the gardener is that these two substances pose no threat to lawns or shrubbery that abut the path.
Common chemical approaches to de-icing involve sprinkling sodium chloride (common rock salt) or pellets of calcium chloride over the path. Both can have damaging consequences to growing plants when heavy concentrations are applied, but rock salt is the greater offender, simply because more of it has to be used to get the desired ice-free result.
Typically the choice in chemical de-icers is limited to sodium or calcium chloride, both naturally occurring salts that penetrate the ice to form a brine solution. So the consumer had best look beyond the brand name on the bag and check out the chemical composition.
Less calcium chloride needs to be used on an icy path for the simple reason that it generates heat (as the experiment above so clearly showed) when it comes into contact with moisture. In technical terms this is called an exothermic reaction, and it enables calcium chloride to work on ice in Fahrenheit temperatures as low as 25 degrees below zero.
Sodium chloride melts ice with heat drawn slowly from the atmosphere rather than generated by chemical reaction. As a result, common rock salt becomes ineffective the moment temperatures drop below 10 degrees F.
Tests have shown that while a certain amount of calcium chloride will melt 100 grams of ice in 30 minutes at 20 degrees F., an equal amount of sodium chloride will melt only 20 grams. Applied in equal amounts at 10 degrees F., the calcium salt melts 75 grams of ice and the rock salt only 10 grams.
Meanwhile, it is worth noting that plants growing in soils rich in organic matter are more able to survive the overly salty diet inflicted on them by sidewalk de-icing.