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Thread: universal solvent

  1. #1 universal solvent 
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    Water is referred to as the “universal solvent” because it can dissolve more substances and in greater quantity than any other solvent
    yet, paradoxically, 99.9% of the earth is insoluble in water.

    Rocks dont dissolve in water because a rock (even a large rock) is a single immense (covalently bonded) molecule and therefore cannot dissolve.
    Or, if not a single molecule, then it is a collection of long polymer-like molecules (chain silicates/Geopolymers).
    This is what makes lava amorphous.

    Chem 421 - Crystalline and Amorphous Polymers
    Solid organic compounds consisting of ordinary small molecules tend to be crystalline, that is, the molecules pack themselves in regular three-dimensional arrays. Polymers are different; they can be amorphous (totally lacking positional order on the molecular scale) or semicrystalline (containing both crystalline and amorphous regions in the same sample).


    Semicrystalline polymers have true melting temperatures (Tm) at which the ordered regions break up and become disordered. In contrast, the amorphous regions soften over a relatively wide temperature range (always lower than Tm) known as the glass transition (Tg). Fully amorphous polymers do not exhibit Tm, of course, but all polymers exhibit Tg.
    If they are allowed to cool slowly, these chains (chain silicates) can fold up into regular crystal structures but they are still long chains of covalently bonded atoms.
    On the other hand, if it cools too quickly then the result is an amorphous glass, like obsidian



    MINERALS
    Silicates

    Consists of a small silicon atom with a +4 charge surrounded in tetrahedral fashion by four larger oxygen atoms each having a -2 charge (Figure).
    Net charge on the anion group is -4.
    To satisfy this charge deficiency the SiO4 tetrahedra can either bond with cations (Fe, Mg, Ca, K, Na)
    or join with other SiO4 tetrahedra through oxygen sharing.

    Chain silicates (Figure) Can be either single or double chain silicates. Single chains share two basal oxygen while the double chain shares three. Two examples of this group are the pyroxenes (single chain) and amphiboles (double chain). Since the Si-O bonds are stronger than the tetrahedra-cation bonds this subgroup has fairly good cleavage in two directions.







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  3. #2  
    Forum Cosmic Wizard
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    But silica does dissolve in water by hydrolysis. So do many other minerals.
    That is why so many ores are deposited by geohydraulic processes.

    Water under pressure is a far more effective solvent than water at surface pressure is. It even dissolves gold, especially if it has a bit of carbonic acid in it.


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