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Lesson 10 Endothermic and Exothermic Reactions

  Evidence of a Chemical Reaction

What specific evidence will indicate that a chemical reaction has occurred?



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B10.2 Yellow precipitate forming in a test tube
There are several key pieces of evidence that can be collected that indicate that a chemical reaction has occurred.

  1. A new substance with unique properties is created.
  2. The process is difficult to reverse.
  3. A gas is produced.
  4. A precipitate is formed when two solutions are combined.
  5. A new odour is detected.
  6. A colour change is observed.
  7. A change in temperature is measured.

At times, you may observe one of these pieces of evidence; however, it may not definitively indicate that there was a chemical reaction. Think of a pot of water boiling; even though gas bubbles are produced, it is not a chemical change but instead a phase change from liquid water to water vapour.

A key technique to help you identify if the evidence supports the identification of a chemical change is to ask yourself, “Was a new substance formed?” and “Is the process difficult to reverse?” In the case of the boiling water, no new substance was produced; it is still water, and by simply cooling the water vapour, you will recreate liquid water.

Watch these examples of chemical changes.

Is There a Chemical Change? Part I @ YouTube CDPEatUCO 


Watch this video to observe evidence of a chemical reaction.

Is There a Chemical Change? Part II @ YouTube CDPEatUCO 


Watch this video to observe evidence of a chemical reaction.

Is There a Chemical Change? Part IV @ YouTube CDPEatUCO 


Watch this video to observe evidence of a chemical reaction.


  Read This

Please read pages 15, 17, and 84 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on identifying evidence that indicates a chemical reaction occurred. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice question to check your understanding of the concept you just learned. Make sure you write a complete answer to the practice question in your notes. After you have checked your answer, make corrections to your response (where necessary) to study from.

  1. Write down four observations that could provide evidence that a chemical reaction has taken place.

    Four observations that could indicate that a chemical reaction has occurred are a colour change, the formation of a gas, the formation of a precipitate, and a temperature change.
  2. What is the only real proof that a chemical reaction has occurred?

    The only real proof is if a new substance is formed that has properties different than the reactants.