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Conservation of Mass in Chemical Reactions

Matter cannot be created or destroyed in chemical reactions.

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Balancing Act

Balancing Act

Credit: Lesssemotion40
Source: http://commons.wikimedia.org/wiki/File:Raven_trick.jpg
License: CC BY-NC 3.0

This aspiring engineer is doing a balancing act. Actually, balancing acts are going on around you all the time. They’re called chemical reactions.

The Back Story

  • Chemical reactions explain many of the changes that occur around you. They explain virtually all the changes that occur in you as well. For example, chemical reactions explain how you digest and use the food you eat.
  • Credit: iweatherman
    Source: http://www.flickr.com/photos/iweatherman/9426024695/
    License: CC BY-NC 3.0

    The label shows what your body will absorb. What happens to the rest? [Figure2]

  • Chemical reactions are modeled by chemical equations. Chemical equations show which atoms and molecules start a reaction and which atoms and molecules are produced in the reaction.
  • Mass is always conserved in chemical reactions. The same atoms are present after the reaction as before the reaction occurred. They just get rearranged in the reaction.
  • Because mass is always conserved in chemical reactions, chemical equations must be balanced. Review how to balance them by watching this video: 


Can You Apply It?

Balance each of the chemical equations below. The equations represent common, real-world chemical reactions. After balancing the equations, check the reactions at the following link. Identify the reaction represented by each equation.

  1. Fe+O2Fe2O3
  2. Ca(OH)2CaO+H2O
  3. Al+Ag2SAg+Al2S3
  4. MgCl2+NaOHMg(OH)2+NaCl
  5. CH4+O2CO2+H2O

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Image Attributions

  1. [1]^ Credit: Lesssemotion40; Source: http://commons.wikimedia.org/wiki/File:Raven_trick.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: iweatherman; Source: http://www.flickr.com/photos/iweatherman/9426024695/; License: CC BY-NC 3.0

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