Re: boiling vinegar

[Jack Uretsky <jlu@HEP.ANL.GOV>]

        Pure conjecture:
        I recall that water molecules bind to each other (compare the
ratio of density to molecular weight) of liquid H_{2}O, H_{2}, N_{2},
O_{2}.  So if vinegar molecules interfere with the binding they could
"catalyze" the early vaporization of some of the water molecules.
        The sequence suggests that it is the CH2 that is responsible, and
it is much less tightly bound in vinegar than in the last 2 items in the
list.  If I am anywhere near correct, then I wonder whether the difference
between 99.4 and 99.9 is significant.
                                        Regards,
                                                Jack



On Sun, 13 Apr 2003, John S. Denker wrote:

> Hi Folks --
>
> Consider the following table:
>
>                                 Pure    Aqueous Azeotrope
>                                 -----   -----------------
> Substance       Formula         Boiling         Boiling
>                                 Boint   %age    Point
> Formic acid     H-COOH          100.7   77.5    107.1
> Ethanoic acid   H-(CH2)-COOH    118.1    3.0*    76.6*
> Propanoic acid  H-(CH2)2-COOH   141.6   17.7     99.9
> Butanoic acid   H-(CH2)3-COOH   164.5   18.4     99.4
>
>    Ethanoic acid is also known as acetic acid.
>    Dilute aqueous solutions are called vinegar.
>
>    Butanoic acid is also known as butyric acid.
>
>    All data from CRC Handbook.
>
> This leads to a whole package of riddles.
>
> First, suppose I had showed you this table with
> the two starred items not filled in.  Would you
> have been able to predict anything close to the
> observed answer?
>
> Note that adding some water to formic acid raises
> the boiling point above the BP of either constituent.
>
> For the propanoic and butanoic acids, the BP of
> the azeotrope is within a fraction of a degree of
> the dominant constituent (water).
>
> So why is ethanoic acid so different?  The azeotrope
> contains only a small percentage of acid.  Adding
> a tiny amount of a high-boiling substance lowers
> the BP more than 23 degrees.
>
> To put it in simplest terms, what is going on here?
>
> I have no clue.  I've rummaged around.  I've found
> a number of "just-so stories" that allegedly explain
> the boiling of mixtures, but for every correct
> prediction they make multiple incorrect predictions,
> so they can't be taken seriously as hypotheses.
>
> I even entertained the hypothesis was that it might
> be a misprint ... 96.6 would have been more plausible
> than 76.6 ... so I did the experiment.  It's a rather
> simple experiment:  pot of vinegar plus thermometer
> plus small amount of patience.  It ain't a misprint.

--
        "What did Barrow's lectures contain?  Bourbaki writes with some
scorn that in his book in a hundred pages of the text there are about 180
drawings.  (Concerning Bourbaki's books it can be said that in a thousand
pages there is not one drawing, and it is not at all clear which is
worse.)"
                                        V. I. Arnol'd in
                                Huygens & Barrow, Newton & Hooke