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Secondary II • 2yr.


I can't figure out what chemical changes are and how to recognize them. Could you explain me what they are please?

Thanks a lot!


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Explanations (1)

  • Explanation from Alloprof

    Explanation from Alloprof

    This Explanation was submitted by a member of the Alloprof team.

    Team Alloprof • 2yr.

    Hi Zen Zombie!

    Thank you for your question!

    Chemical changes modify the nature and characteristic properties of matter. New substances are formed as a result of a chemical change. An example of a chemical change is photosynthesis, where carbon dioxide and water react in the presence of energy to become oxygen and glucose.

    Here are five clues to recognize chemical changes:

    1) The formation of a gas (effervescence):

    During a reaction between two substances, a gas may be produced. In some cases, bubbles or a characteristic color may appear.

    2) The production or absorption of heat:

    In some reactions there may be a noticeable change in temperature. For example, during the combustion of a wooden log, much energy is released in the form of heat, suggesting the presence of a chemical change. Conversely, when heat is absorbed, the temperature cools. In some chemical reactions, the final temperature is cooler than the initial temperature, suggesting that there has been a chemical change.

    3) The production of bright light

    Some chemical reactions can be identified by their emission of bright light, in other words, of luminous energy. For example, the light of combustion reactions is the telltale sign of a chemical change.

    4) A significant color change

    The products of some chemical reactions do not have the same color as the reactants. In some cases a color change indicates that a chemical change has taken place. For example, when heating some salts, the color at the start and end of the heating is different, thus suggesting a chemical change.

    A note of caution: Dilutions are not chemical changes. Though the color of the solution may change, no new substances are created. Water simply dilutes the substances it dissolves, perhaps changing their tone, but not their color in a significant way.

    5) Formation of a precipitate

    When non-solid solutions are mixed, an insoluble compound–a precipitate–can form. This shows there has been a chemical change.

    If you have any other questions, don't hesitate to let us know! 😊

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