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Heats of Fusion and Solidification

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Some Heat, Please!
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Some Heat, Please!

Credit: Pete Birkinshaw
Source: http://www.flickr.com/photos/93001633@N00/1458555513/
License: CC BY-NC 3.0

Squirrels are probably well equipped to handle the cold of a snowy winter.  Luckily for humans, we’ve learned to take advantage of chemistry to heat the way.

News You Can Use

  • Nearly every chemical process is accompanied by the flow of heat, which itself generally leads to a change in temperature.  Some transformations are endothermic and cause the surroundings to decrease in temperature.  That is clearly a reaction you would want to avoid if you were cold during the winter.  
  • Alternatively, there are processes that are exothermic and thus increase the temperature of the surroundings.  A useful example of this is the crystallization of sodium acetate from a supersaturated solution, which has been exploited commercially to produce hand warmers.
  • Credit: Wikipedia
    Source: http://en.wikipedia.org/wiki/Hand_warmer
    License: CC BY-NC 3.0

    A sodium acetate hand warmer with a metallic disc [Figure2]

     

  • To see the dramatic crystallization, watch this video: http://www.youtube.com/watch?v=K7iM0lu61zg

Can You Apply It? 

With the links below, learn more about supersaturated solutions and exothermic processes.  Then answer the following questions.

  1. What mass of KCl (potassium chloride) can dissolve in 100g of water at 25 °C?
  2. KCl doesn’t readily form supersaturated solutions.  What happens to prevent the formation of supersaturated solutions? 
  3. Name some ways to cause crystallization to occur in a supersaturated solution.
  4. Describe a procedure for preparing a supersaturated solution.

Image Attributions

  1. [1]^ Credit: Pete Birkinshaw; Source: http://www.flickr.com/photos/93001633@N00/1458555513/; License: CC BY-NC 3.0
  2. [2]^ Credit: Wikipedia; Source: http://en.wikipedia.org/wiki/Hand_warmer; License: CC BY-NC 3.0

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