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Thread: Fractional Distillation

  1. #1 Fractional Distillation 
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    Fractional Distillation separates mixtures OR compounds? Or both? A bit confused here. Fractional distillation - Wikipedia, the free encyclopedia
    Fractional distillation is the separation of a mixture into its component parts, or fractions, such as in separating chemical compounds


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    It separates mixtures, it (shouldn't) break down any compounds. The quote you give is meaning it separates a mixture of compounds into it's component parts so you get a series of fractions each containing a pure compound.


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    Ah okay thanks a lot for the verification!
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    Quote Originally Posted by PhDemon View Post
    It separates mixtures, it (shouldn't) break down any compounds. The quote you give is meaning it separates a mixture of compounds into it's component parts so you get a series of fractions each containing a pure compound.
    Sort of. It is not too often that the fractions are pure compounds.
    ..fractions within which different compounds are concentrated... might be more accurate, do you think?
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  6. #5  
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    True, but I pitched the answer at the level of the question, I didn't want to confuse him with practical difficulties...
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    Quote Originally Posted by PhDemon View Post
    True, but I pitched the answer at the level of the question, I didn't want to confuse him with practical difficulties...
    Fair enough.
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    Quote Originally Posted by Warron View Post
    Quote Originally Posted by PhDemon View Post
    It separates mixtures, it (shouldn't) break down any compounds. The quote you give is meaning it separates a mixture of compounds into it's component parts so you get a series of fractions each containing a pure compound.
    Sort of. It is not too often that the fractions are pure compounds.
    ..fractions within which different compounds are concentrated... might be more accurate, do you think?
    Ah so the fractions may also contain different compounds?
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  9. #8  
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    Quote Originally Posted by molecool View Post
    Quote Originally Posted by Warron View Post
    Quote Originally Posted by PhDemon View Post
    It separates mixtures, it (shouldn't) break down any compounds. The quote you give is meaning it separates a mixture of compounds into it's component parts so you get a series of fractions each containing a pure compound.
    Sort of. It is not too often that the fractions are pure compounds.
    ..fractions within which different compounds are concentrated... might be more accurate, do you think?
    Ah so the fractions may also contain different compounds?
    Fractionation separates substances by boiling point, so if you start with a mixture containing many compounds, some of which have very similar boiling points, you will in practice be unlikely to get perfect separation of them all, but may instead get a series of fractions that contain narrow ranges of compounds, grouped by boiling point range.

    The fractional distillation of petroleum in a crude distiller is perhaps the classic example of this. You start with thousands of hydrocarbons, not to mention other organic compounds with hetero atoms, all mixed, and from the distiller you get fractions with narrow boiling ranges, but which still may each contain many individual species.

    However if you distill a mixture of just two compounds with fairly widely differing boiling points, say ethanol and water, you can get fractions that are almost pure substances.
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  10. #9  
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    The word compound seems to getting used two different ways here.
    To a chemist a compound is a molecule composed of different elemental chemicals.
    Like how the elements sodium and chlorine combine to make the compound salt, or how iron and oxygen combine to make rust.

    Liquid solutions of compounds in solvents are not usually thought of as compounds.
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  11. #10  
    KJW
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    Quote Originally Posted by exchemist View Post
    Quote Originally Posted by molecool View Post
    Quote Originally Posted by Warron View Post
    Quote Originally Posted by PhDemon View Post
    It separates mixtures, it (shouldn't) break down any compounds. The quote you give is meaning it separates a mixture of compounds into it's component parts so you get a series of fractions each containing a pure compound.
    Sort of. It is not too often that the fractions are pure compounds.
    ..fractions within which different compounds are concentrated... might be more accurate, do you think?
    Ah so the fractions may also contain different compounds?
    Fractionation separates substances by boiling point, so if you start with a mixture containing many compounds, some of which have very similar boiling points, you will in practice be unlikely to get perfect separation of them all, but may instead get a series of fractions that contain narrow ranges of compounds, grouped by boiling point range.

    The fractional distillation of petroleum in a crude distiller is perhaps the classic example of this. You start with thousands of hydrocarbons, not to mention other organic compounds with hetero atoms, all mixed, and from the distiller you get fractions with narrow boiling ranges, but which still may each contain many individual species.

    However if you distill a mixture of just two compounds with fairly widely differing boiling points, say ethanol and water, you can get fractions that are almost pure substances.
    And to complicate matters even further, even if the fractional distillation is perfectly ideal, many mixtures of pure compounds form azeotropes that distil as if they were pure compounds.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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  12. #11  
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    Quote Originally Posted by KJW View Post
    Quote Originally Posted by exchemist View Post
    Quote Originally Posted by molecool View Post
    Quote Originally Posted by Warron View Post
    Quote Originally Posted by PhDemon View Post
    It separates mixtures, it (shouldn't) break down any compounds. The quote you give is meaning it separates a mixture of compounds into it's component parts so you get a series of fractions each containing a pure compound.
    Sort of. It is not too often that the fractions are pure compounds.
    ..fractions within which different compounds are concentrated... might be more accurate, do you think?
    Ah so the fractions may also contain different compounds?
    Fractionation separates substances by boiling point, so if you start with a mixture containing many compounds, some of which have very similar boiling points, you will in practice be unlikely to get perfect separation of them all, but may instead get a series of fractions that contain narrow ranges of compounds, grouped by boiling point range.

    The fractional distillation of petroleum in a crude distiller is perhaps the classic example of this. You start with thousands of hydrocarbons, not to mention other organic compounds with hetero atoms, all mixed, and from the distiller you get fractions with narrow boiling ranges, but which still may each contain many individual species.

    However if you distill a mixture of just two compounds with fairly widely differing boiling points, say ethanol and water, you can get fractions that are almost pure substances.
    And to complicate matters even further, even if the fractional distillation is perfectly ideal, many mixtures of pure compounds form azeotropes that distil as if they were pure compounds.
    Indeed, and oops, I find my example above, of ethanol in water does in fact form such an azeotrope at ~ 95:5.
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  13. #12  
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    Thanks very much for all of your neat explanations!
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    Oh one more thing - in fractional distillation, 'boiling points' are the temperatures at which individual fractions condense, right?
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    The boiling point is the temperature at which a substance boils (the clue is in the name ), to condense it the temperature must be lower than the boiling point...
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  16. #15  
    KJW
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    Quote Originally Posted by molecool View Post
    Oh one more thing - in fractional distillation, 'boiling points' are the temperatures at which individual fractions condense, right?
    Quote Originally Posted by PhDemon View Post
    The boiling point is the temperature at which a substance boils (the clue is in the name ), to condense it the temperature must be lower than the boiling point...
    In fractional distillation, the temperature is taken above the fractionating column where the vapour exits to the condenser. Similarly, for boiling point determination of a pure substance, the temperature is taken above the liquid of the condensing vapour.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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  17. #16  
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    Quote Originally Posted by molecool View Post
    Oh one more thing - in fractional distillation, 'boiling points' are the temperatures at which individual fractions condense, right?
    As KJW says, the boiling point is measured just before the condenser stage. However, in general, fractions are taken off over a boiling point range. Just how narrow that range is depends on the fractionating power of the equipment and the complexity of the mix.

    Fractional distillation is not really separation by boiling point. It is separation by the difference in composition in the a liquid mixture and the vapour above it. In a fractionating column, this difference is exploited many times over. The greater the number of times it is exploited (the number of theoretical plates) the greater the separating power of the process.

    The concept that distillaton is separation by boiling point can lead to the misunderstanding that the different components are boiled off from the liquid mixture one at a time, in sequence of boiling point. I have seen many cases of this misunderstanding. This isn't really what happens, but a fractional distillation process approximates this outcome in many cases.
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