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Classes of Crystalline Solids

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Why Won't My Salt Melt?

Why Won't My Salt Melt?

Credit: Timothy Takemoto
Source: http://www.flickr.com/photos/nihonbunka/90233423/
License: CC BY-NC 3.0

Place a pad of butter in a skillet on a warm stove, and it will quickly melt into an oily liquid that can be poured over your baked potato. However, if you put a pile of salt in a skillet, it will not liquefy, even if you crank the heat way up. Why not? The difference lies in the types of solid that you are trying to melt. The types of bonds that must be broken in order to melt a solid have a strong influence on its melting point.

Why It Matters

  • The nonpolar fat molecules in butter are held together in the solid state by relatively weak London dispersion forces. Only a little bit of energy is required to loosen these intermolecular interactions enough to allow the formation of a free-flowing liquid, so its melting point is relatively low.
  • Credit: Joe Shlabotnik
    Source: http://www.flickr.com/photos/joeshlabotnik/2845512621/
    License: CC BY-NC 3.0

    Because butter is soft and easy to mold, it can be used to create sculptures. However, the sculptures must be refrigerated to prevent it from melting [Figure2]

     

  • The ions in a salt crystal are held together by strong ionic bonds. Much more energy is required before the ions can break free of their positions within the solid crystal.
  • Although NaCl cannot be melted on a common stove, molten (liquid) sodium chloride can be achieved at higher temperatures, as seen in the following video:

http://www.youtube.com/watch?v=eEHdLGPi-FE

Explore More

 With the links below, learn more about how the structures of different types of solids affect their physical properties, such as their melting points. Then answer the following questions.

  1. An unknown solid melts when heated to body temperature by holding it in your hand. The solid does not conduct electricity. What type of solid (network covalent, metallic, ionic, or molecular) is this substance likely to be?
  2. Solid mixtures of metals are known as alloys. Would brass (a copper-zinc alloy) still take the form of a metallic solid? What defining property of metallic solids can we look at to determine the answer?
  3. Butter is a molecular solid, but it is not crystalline. Are there examples of molecular solids that are also crystals?
  4. At low enough temperatures, the heavier noble gases can be solidified. Which type of solid (network covalent, metallic, ionic, or molecular) best describes solid xenon? (Hint: Think about the properties of the solid, and note that it may not perfectly fit into any of these categories.)

Image Attributions

  1. [1]^ Credit: Timothy Takemoto; Source: http://www.flickr.com/photos/nihonbunka/90233423/; License: CC BY-NC 3.0
  2. [2]^ Credit: Joe Shlabotnik; Source: http://www.flickr.com/photos/joeshlabotnik/2845512621/; License: CC BY-NC 3.0

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