This year in the midwest United States, winter has been a fickle friend. I haven’t seen the same amount of snow or ice as in recent years, but I still made sure I was prepared for it at our home. I went to my local big box hardware store in December and contemplated buying rock salt (NaCl), and NaCl/calcium chloride mixture, or just calcium chloride. Growing up my dad had switched entirely to calcium chloride because it was less damaging to the brick pavers leading to our porch and backyard. In fact, calcium chloride is generally much safer toward plants and soil than NaCl. Even though calcium chloride is much more expensive than rock salt (it was about twice the cost for 10 pounds more), that what’s I chose. Why? Because I am a chemist by trade (and chemistry teacher). I actually considered the colligative properties of calcium chloride and knew that it would lower the freezing point of water much more than NaCl. Considering the qualitative aspects of colligative properties, the more ions that the formula can be broken down into, the greater the effect. NaCl consists of 2 different ions (Na+ and Cl-) while calcium chloride, CaCl2, consists of 3 ions (Ca+2 and 2 Cl-). Quantitatively, we would discuss the van’t Hoff factor and use molality calculations to determine how much the temperature increases or decreases.
So why bring up colligative properties? About 4 years ago I was teaching AP Chemistry at a parochial high school in Indianapolis, IN. I went to an AP Chemistry training session at Butler University and learned that the newly updated AP curriculum would not include colligative properties. In fact, “since colligative properties [had] been removed from the course, the equations for freezing point depression and boiling point elevation, the formula for molality, and the Kf and Kb values for water [would be] removed” from the equations list all together (College Board). Essentially, colligative properties were becoming a non-relevant topic and/or beyond the scope of the course and exam. I found this disturbing since it is a topic that can be used in everyday life. At least, everyday life during the wintertime or when boiling water and making pasta. Still I have found colligative properties to be of great interest and still approach the topic qualitatively with my Chemistry 2 and Honors Chemistry 2 students.
Do you include colligative properties in your curriculum? If so, to what extent? Do you find it imperative as you aim to strengthen the scientific literacy of your students?
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Check this out for colligative...
We don't include it but I still like to sneek it in...
You can check out old blog post of mine.
NaCl beats CaCl2 for colligative properties
Yes, the van't Hoff factors favor the 3 ions in calcium chloride. However, the molar mass of CaCl2 is nearly twice that of NaCl. So for 1 kg of NaCl, there are 34 moles of ions, whereas 1 kg of CaCl2 only has 27 moles of ions. That's more that 25% more ions. Of course, the environmental reasons you mention may be worth the extra cost.