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Homework Help: Heterogeneous equilibria: concentration of solids issue

  1. Jun 27, 2016 #1
    1. The problem statement, all variables and given/known data

    Here is the quote from the chemistry textbook by Tro:

    However, since carbon is a solid, its concentration is constant—it does not change.

    Adding more or less carbon to the reaction mixture does not change the concentration

    of carbon. The concentration of a solid does not change because a solid does

    not expand to fill its container. The concentration of a solid, therefore, depends

    only on its density, which (except for slight variations due to temperature) is constant

    as long as some solid is present. Consequently, pure solids—those reactants or

    products labeled in the chemical equation with an (s )—are not included in the

    equilibrium expression.

    2. Relevant equations
    3. The attempt at a solution

    I will be grateful for your help because I do not understand everything that is said in the above quote.
    Please, correct my assumptions or explain what I miss:
    - I understand that the concentration of the solid will not change no matter how much solid we add because the concentration implies the amount of dissolved substance. Hence, if we have solid, it doesn't dissolve in that substance, and the concentration in the solution will be the same. That's why the textbook states that adding more carbon to the solution will not change its concentration.
    - What I don't understand is the next thing that is being stated: the concentration depends on the density, which is constant. I assume the author says that no matter how much carbon we add to the solution the density will stay the same. How can that be, or do I miss some basic understanding? Density is the mass/volume, i.e. how much of substance's particles are present in a given volume. If I add more carbon, I will increase the density, won't I?
    - I would interpret the phrase 'concentration is constant' as 'after some solid has been formed, at a certain period, if that solid did dissolve to some extent, the concentration will become stable no matter how much additional solid is added'.
    Please, help to understand this topic.
    Thank you!
  2. jcsd
  3. Jun 27, 2016 #2
    They are referring to the concentration of the solid carbon, not the concentration of the carbon dissolved in the solution.
  4. Jun 27, 2016 #3
    Thank you for your reply! I am confused by the idea that no matter how much of carbon we add, the density stays the same. How can this be? Or am I reading the passage incorrectly?
  5. Jun 27, 2016 #4
    The density of the solid carbon stays the same (assuming that the carbon is considered incompressible).
  6. Jun 27, 2016 #5
    I think I understand now, and see what has caused my confusion. Thank you!
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