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Chemical reaction equation, historical question

  1. Apr 17, 2014 #1
    I am reading a chemistry book printed in 1805. The chemical reaction equations are written using the equality symbol = instead of the arrow →, which is used in modern times.
    Anyway sometimes it is still possible to see the "old fashioned" way:

    Does anyone know why the equality symbol was abandoned, and when did it happen
    in the history of chemistry? Are there reasons why this change was needed?
    I know only a little about chemistry, I think this is a very basic question, but I cannot
    seem to find the complete solution myself. I can think that maybe the = was replaced by → because chemical reaction equations are not mathematical equations, there is no equality
    in the equation in mathematical sense.

    If the chemical equations are not mathematics, then why the addition symbol + has not
    been replaced by something else? The addition is a mathematical operation, so should
    it be understood to mean also a chemical reaction? Something is added into something
    else, perhaps this is an universal concept applicable not just in mathematics.
  2. jcsd
  3. Apr 17, 2014 #2


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    An arrow indicates direction, whereas an equality sign does not.
  4. Apr 17, 2014 #3


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    As long as a reaction is not in equilibrium, the reaction proceeds in one or the other direction. Hence it is more convenient to use arrows. In some situations, it is also necessary to distinguish formally between reactands and products, e.g. in calculating the potential of a electrochemical half cell, you divide by convention the product of the concentration of the products by that of the reactands.
  5. Apr 17, 2014 #4
    Ok, there is a direction in chemical equation, reactants are cause and products are effect,
    hence there is causality. But not in mathematical equation, there is symmetry in mathematical
    equation instead of causality. 1+1→2 does not make sense because 2 is not caused by 1+1,
    instead there is symmetry: 1+1=2 and 2=1+1.

    Perhaps this is a weakness of mathematics, it does not seem offer causality.
  6. Apr 17, 2014 #5


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    You gave the answer to your question yourself.
    And there are instances where mathematics offers a cause and effect.
    Mathematical Induction
  7. Apr 17, 2014 #6


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    I found this.

    chemistry and symbols

    1789 Lavoisier uses "=" sign for a chemical equation.
    1884 Vant Hoff uses double arrows
    1901 single arrow to designate direction, products and reactants
    http://www.chemistryviews.org/SpringboardWebApp/userfiles/chem/image/2012/2012_November/Arrow [Broken]

    http://www.chemistryviews.org/SpringboardWebApp/userfiles/chem/image/2012/2012_November/Arrow [Broken]

    Other uses of arrows in chemistry shown, past and present.
    Last edited by a moderator: May 6, 2017
  8. Apr 18, 2014 #7


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    Nice links. This timeline was very interesting→
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