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Chemical Reactions

Investigation 2 – PreLab









Remember that everything in the environment, whether naturally occurring or man-made, is composed of chemicals, and research of chemicals has led to the discovery and development of new and improved paints, medicines, cosmetics, electronic components, and thousands of other products.


Forensic chemists analyze evidence from crime scenes. After identifying and characterizing things like paint chips, blood, glass, and hair (among others), they give expert testimony in court that helps the jury determine a verdict.


This Investigation is designed to help you to answer the following Focus Questions:

  • In a chemical reaction, do the amounts of the reactants affect the amounts of products produced? 
  • How does the Law of Conservation of Matter relate to chemical reactions? 

Note: These questions are located in your SDR at the beginning and end of the Investigation.



As a class, read the Background(s) in the Investigation. When finished, discuss the following concepts as a class:

  • In a chemical reaction, the chemical bonds that hold the atoms of the reactants together are broken and reformed in a different arrangement to form the products.
  • When all of the reactants are consumed, all the atoms originally found in the reactants can now be found in the products.
  • The observation that all the matter of the reactants is converted into the matter of the products is referred to as the Law of Conservation of Matter.
  • If more of one reactant is present compared to the other, the reactant present in the lesser amount is completely consumed in the reaction, and the reaction stops.



The following list includes Key Terms that are introduced in the Investigation Background(s). They should be used, as appropriate, by teachers and students during everyday classroom discourse.

  • chemical bonds
  • atoms
  • react
  • matter
  • Law of Conservation of Matter
  • consumed
  • unreacted

Note: Definitions to these terms can be found on the Introduction page to the CELL.



    •  The following chemical equation describes the reaction that you will analyze in this Investigation. 

    CHEM2 equation

    • Notice that for every atom of magnesium metal that is consumed as a reactant, 2 molecules of HCl are consumed. This ratio of the two reactants produces one molecule of each of the two products, MgCl2 and H2.
    • Predict the answers to the following questions.
      • Are the reactants always completely used up in a chemical reaction?
      • Does the matter making up the reactants disappear?
      • What happens if there is more of one reactant than the other reactant?
      • How do you know when a chemical reaction has stopped?

    Note: You will answer these questions after you have completed the Investigation.

    • In this Investigation, you will study how the amount of each reactant affects the amount of the products that are produced.
    • Play the video below. Remember to follow along with your SDR and make any notes that you think might be helpful in the lab.
    • After the video, divide into your lab groups to discuss strategy for the lab. For example, you may assign certain group members to perform specific functions during the lab.

    Note: The purpose of the video is to allow you to anticipate the laboratory experience you will soon encounter. You should leave this PreLab session with a firm idea of what to expect and how to perform in the lab.

    Note: Homework is posted below the video.


You should review the Investigation and video in preparation for the Lab.