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Thread: Identifying Calcite vs Gypsum

  1. #1
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    Identifying Calcite vs Gypsum

    I am looking for help from anyone who can assist me in the knowledge required to test for gypsum in my calcite. Gypsum would actually be quite preferable in this instance I believe, and I'm sure there would be some gypsum in my calcite, however, I don't know how to tell the difference.

    Calcite is simply calcium + carbon dioxide.
    Gypsum is the same but with sulfur as well I believe.

    Any thoughts from you chemists? Please?

    EM.
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    Quote Originally Posted by elixirmixer View Post
    I am looking for help from anyone who can assist me in the knowledge required to test for gypsum in my calcite. Gypsum would actually be quite preferable in this instance I believe, and I'm sure there would be some gypsum in my calcite, however, I don't know how to tell the difference.

    Calcite is simply calcium + carbon dioxide.
    Gypsum is the same but with sulfur as well I believe.

    Any thoughts from you chemists? Please?

    EM.
    Calcite = calcium carbonate. Gypsum = calcium sulfate. It's not "rocket science", really.

    As for how to know if there is some gypsum mixed in with your calcite: dissolve a sample of it in diluted acetic acid. Does it leave a white insoluble substance? Yes? Then it might be some gypsum mixed with the calcite (but then again it could also be other things that do not dissolve in that acid, like silica, for example. But the simple test will give you an idea if your calcite has many or few impurities that do not dissolve in diluted acetic acid.)

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    Nice, got any maths on the acetic acid for say... 100g of calcite? Pretty please? Since my 'incident' I haven't been able to do uni level maths anymore.
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    Quote Originally Posted by elixirmixer View Post
    Nice, got any maths on the acetic acid for say... 100g of calcite? Pretty please? Since my 'incident' I haven't been able to do uni level maths anymore.
    Just keep adding portions of the dilute acid to the powdered or coarsely crushed calcite sample until it "fizzles" no longer and you don't see any more foam/bubbles forming. That means that pretty much all the carbonates have been decomposed and soluble acetates have formed.

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    JDP, if I could bother you again, could you please explain to me the different between treating calcite with acetic acid, and treating it in acetone?
    Join me; on a voyage of stupidity, and self discovery: https://www.youtube.com/watch?time_c...&v=vccZSHroTG4

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    Quote Originally Posted by elixirmixer View Post
    JDP, if I could bother you again, could you please explain to me the different between treating calcite with acetic acid, and treating it in acetone?

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    If treating calcite with acetic acid makes calcium acetate, then treating calcite and/or calcium with acetone makes what??
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    Quote Originally Posted by elixirmixer View Post
    If treating calcite with acetic acid makes calcium acetate, then treating calcite and/or calcium with acetone makes what??
    a mess

    edit: Sorry I couldn't resist. And I figured I'd better look into this before I cat myself into trouble. I can't find solubility information for either with acetone, so my guess would be nothing at all happens.

    edit 2: gypsum is a drying agent for acetone, aparently, but doesn't dissolve. CaCO3 is listed by pubchem as insoluble in acetone.

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    The reason I ask, is because calcium acetate is one of these beautiful chemical obscurities where the dissolution rate decreases with temperature. This is an important property when identifying substances which will "solve et coagula" at low temperatures, and more importantly, repetitively.
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    It is not really an obscurity.
    When a salt is solving in a solvent the ion lattice is destroyed and the particles of the solvent surround the ions.
    The destroying needs energy (endothermic) and the surrounding sets energy free (exothermic).
    If the amount of energy needed is higher than the amount of energy set free the process needs additional energy from outside (the warmth of room temperature). If you solute for example NH4NO3 in water and stir with a thermometer you will obeserve a cooling, for the process takes additional energy from outside. If you do the same experiment with NaOH it is the other way round and a heating is obeserved.

    Usually exothermic reaction and solution processes are prefered by nature as there is always the lowest possible salary of energy is wanted by the process.
    The reason why endothermic reactions or solution processes take place anyways is because the state of chaos is preffered by nature too. Look at a building that's a hundred years old. It will consequently fall apart voluntary. Same with ion lattices in a solution.

    If the entropy (chaos) is higher than the enthalpy (energy) those processes can take place, even if it is an endothermic process.

    Now you can influence this processes by means of temperature. Endothermic processes need energy and therefore prefer higher temperatures. Exothermic reactions produce energy and therefore prefer lower temperatures. It's always a kind of chemical equilibrium between the material you start with and the material you end with. By means of energy you can shift the equilibrium to your wanted result.

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