Particle of Sweetness
Lab Activities
Chapter 20 Natural World
Composition of Carbohydrates
The first part of the experiment in Ch 20
was intended to show two things. First,
the heating of carbohydrates. Large
molecules can be broken down into
smaller molecules. Sucrose is gently
heated. As more heat is added the bonds
in the carbohydrate are broken. Water
vapor forms on the rim of the test tube.
This is how the proof works. Using an
equation (not balanced):
C12H22O11 + heat ---->
H2O + C
you should be able to see that sucrose, a
molecule with 45 atoms, was broken down,
or decomposed into water (3 atoms big)
and carbon. Large to small.
Water vapor condensing proves that two of
the atoms in a carbohydrate are oxygen and
hydrogen. (Later this year water will be
broken down with electricity into hydrogen
and oxygen).
In addition to water forming, carbon remains
at the bottom of the test tube.
So the three atoms that make up
carbohydrates are carbon, hydrogen, and
oxygen. This is NOT proof for the ratio,
just that these three atoms comprise sugar.
Heating of the sugar also showed that large
molecules can be broken down into smaller
molecules, catabolism.
It is important to also note that as atoms are
removed from the sucrose the properties of
the sucrose change. Both the color and
taste change as the substance is heated. The
initial heating results in only a phase change,
so there is no change in the physical
properties of sucrose.
Sucrose is a Disaccharide
Sucrose is a sugar formed by the combining
of the monosaccharides of glucose and
fructose. The bond between them can be
broken and sucrose can be converted
back to these monosaccharides. To test
for this you tested for glucose using the
Benedict’s Test.
Benedict’s Test
Benedict’s test is useful in forming an
operational definition for glucose. It does
two things, it tells you if glucose is present
with a color change, and depending on
the color it forms, it tells you the amount
of glucose that has formed.
Benedict’s Test
Benedict’s solution plus glucose will form a
color change. If the concentration is low,
a light green color will form. This color
will change to yellow, then brown, and
finally a bright orange-red color. Orangered is the greatest concentration of
glucose.
Sucrose being broken down into glucose again
illustrates catabolism. Sucrose, a disaccharide,
was broken down into a monosaccharide, and
fructose. Although you do not have a chemical
test for fructose, it was formed. (You can see
that Fructose is formed with the Selwinoff
reaction).
Proof for Glucose from Sucrose
Results from Lab
The results from the table show the effects
of various chemicals with Benedict’s
solution. The various combinations of
chemicals rule out that the Benedict’s
could have reacted to any chemical
except the glucose. Therefore the
glucose formed had to have come from
the sucrose. Again, large molecules can be
catabolized to form smaller molecules.
Here you can see the positive test for
GLUCOSE with the Benedict’s Test. Note the
change in color from yellow to dark brown.
Benedict’s shows not only that glucose is
present, but in what concentration.
In addition to this, sucrose, a disaccharide is
composed of two monosaccharides.
Glucose, which you proved present with
Benedict’s and fructose. The presence of
Fructose is shown by reacting with
Seliwanoff Reagent.
Positive Selawanoff reaction for Fructose.
Same atoms, but not isomers
A second heating of a sugar is done.
Glucose is heated in a test tube in the
same manner that sucrose was heated.
Like with sucrose, gentle heating resulted
in a phase change but not a chemical
change. Continued heating formed water
vapor and after intense heating, carbon
remained at the bottom of the test tube.
This means that both glucose and sucrose
contain similar atoms. Similar atoms does
not mean isomers. In order to be isomers
you need to have the same chemical
formula and different structural formulas.
Sucrose and glucose do not have the same
molecular formulas.
Molecules vs. Ions
From earlier labs you learned that ions are
electrolytes. Ions are atoms that have an
unequal number of protons and electrons
so they have a charge.
Molecules are compounds that are bonded
covalently, so ions cannot form. This is
why you hear of a molecule of water, not
a compound of water.
Crystal Formation
Crystals form from a repeated internal
arrangement of atoms. Although sugar
molecules are neutral, the covalent bonds
do not result in equal sharing of electrons.
This results in the molecules becoming
DIPOLAR. This means that the molecules
have poles, like magnets. These opposite
poles attract and allow the molecules to
attract to each other in the crystal.
The bonds formed when these dipolar
substances form is not one of the stronger
bonds in chemistry. That is why the
molecules of sugar will separate so easily in
water to form a solution.
It is important to note that dissolving does
not determine whether or not a substance
contains ions, it is whether or not the
substance forms electrolytes when it
dissolves.
So why did the sugar crystals melt when
earlier this year the KI crystals did not
break down with intense heat? The
difference is the type of bond formed in the
crystals. Sugar is organic and the molecules
form weak polar bonds. KI is ionic and the
individual atoms make more bonds and
stronger bonds between each other.