Ask A Scientist Chemistry Archive

name         Jane R.
status       other
age          40s

Question -   I am writing on behalf of my fifth grade son. For his
science experiment, he dissolved sugar in equal amounts of vinegar and
water, in separate containers and waited to see how long it would take
for them to dissolve. He thought that the acid would dissolve the sugar
more quickly, because it would be better able to change the bonds between
sugar molecules. But the sugar dissolved more quickly in the water. Can
you tell us why?
----------------------
Jane,

Because of their shape and composition, water molecules are far better at dislodging 
sugar molecules from their positions in the solid sugar crystal lattice. Vinegar 
(acetic acid) has a structure that is less able to do this. Besides, the more acetic 
acid present, the less water is there to assist the dissolving process.

Regards,
ProfHoff 850
=====================================================
Jane-
      Simple acids do not do much of anything to sugar.
Sugar's not acidic, and not basic, so acids do not react with it, helping it to dissolve.
The few acids that do react with it tend to ruin it irreversibly, and that is not dissolving, that is destroying.
Vinegar is not one of those.  Nor is Hydrochloric acid.
And they do that by forcibly adding oxygen or extracting water, not by acidity: which is donating H+ ions.


The -O-H groups in water are the important thing,
because sugar has matching OH groups all the way down its spine,
and OH groups like to stick together like this:
.............
.           .
.    H      .
.   : \     .
. -O   O-   .
.   \ :     .
.    H      .
.............
This is called hydrogen-bonding.
It's weaker than a molecular ("covalent") bond,
but stronger than the other attractions that hold liquids together.
So when it can happen, it's pretty dominant.


Water is H2O:
............
.          .
. H        .
.  \       .
.   O - H  .
.          .
............


Sugar, starch, and cellulose are mostly (HCOH)n :
.................
.        \      .
. H    (repeat) .
.  \     /      .
.   O - C - H   .
.        \      .
.      (repeat) .
.        /      .
.................


(For sucrose, n=12, but there is one good kink in the middle of the 12-chain.)


Vinegar (Acetic Acid) is:
.................
.               .
.       H   H  .
.        \ /    .
. H       C - H .
.  \     /      .
.   O - C       .
.       \\      .
.        O      .
.................
 Its OH is good, but the CH3 and -C=O parts are less cohesive to sugar
than an equal volume of additional H2O water molecules.


Alcohol dissolves some sugar, too, but also less than water.
.................
.               .
.       H   H  .
.        \ /    .
. H       C - H .
.  \     /      .
.   O - C - H   .
.        \      .
.         H     .
.................


If each OH group in sugar H-bonds to one end of a water molecule,
the other end of each water can still link to yet another water, and
the sugar nicely becomes part of water's multi-linked network of H-bonds.


On the other hand, acetic acid and alcohol each have only one OH group.
If each OH group in sugar H-bonds to a vinegar molecule, each such molecule
is no longer able to H-bond with the rest of the solvent bath.
Which, for store-bought 5% vinegar, is 95% water, and wants only H-bonds.


Jim Swenson
=====================================================