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Recognizing Chemical Reactions

Describes the four main signs that a chemical reaction has occurred.

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Only Change Is Constant

Only Change Is Constant

Credit: Liz West
Source: http://www.flickr.com/photos/calliope/1643107709/
License: CC BY-NC 3.0

The leaves on these trees have changed color. This happens every fall, year after year. There’s an old saying that the only thing in life that is constant is change. If you think about it, you’ll probably agree that change goes on around you all the time.

The Back Story

  • Changes like leaves turning color in the fall are chemical changes. They involve chemical reactions. Bonds break in chemicals and new bonds form. Substances change into new chemical compounds.
  • Changes like water freezing to ice are physical changes. They do not involve chemical reactions. Physical changes occur because of forces, pressure, or temperature.
  • Use this interactive online tool to explore some of the physical and chemical changes in the world around you: http://www.wisc-online.com/objects/ViewObject.aspx?ID=SCE204
  • Credit: Jared Tarbell
    Source: https://www.flickr.com/photos/generated/7854132326/
    License: CC BY-NC 3.0

    Broken glass is an example of a physical change [Figure2]

Show What You Know

Learn more about physical and chemical changes at the link below. Then answer the questions that follow.

  1. Define chemical change.
  2. List three real-world examples of chemical change.
  3. Define physical change.
  4. What are two major types of physical change? What is an example of each?
  5. List three additional real-world examples of physical change.
  6. Chemical and physical changes often work together. Describe an example of this.
  7. What evidence suggests that leaves changing color in the fall is a chemical change?
  8. What other signs provide evidence of chemical reactions and show that a chemical change has occurred?

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Image Attributions

  1. [1]^ Credit: Liz West; Source: http://www.flickr.com/photos/calliope/1643107709/; License: CC BY-NC 3.0
  2. [2]^ Credit: Jared Tarbell; Source: https://www.flickr.com/photos/generated/7854132326/; License: CC BY-NC 3.0

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