Carbohydrates:.
Chemistry And Identification
Today, scientists use a combination of b~olagy and chemistry for their ~mderstanding of life and
t~e processes. Thus, ~u understandLng of some che~t~ of liv~g t~gs is necessa~.
C~5ohy~ m~e up a ltrge ~oup o} chemical compo~ds lo~d in ceils, C~bohyd:at~s are
In duis investigation, you will
RE~ER: Models do not represent ~e ac~ t~ee-~ension~ shapes o} the molecules.
Models sere ~o help you learn how sma~e~ molecules c~ be gouped ~m largeL mare complex
molec~es.
Materials
¯ paper models
s~issors
test tubes
test tube holder
~lass m~rkin~ pencil or label~
Benedict’s solution
iodine sq~.,ution
H
hot plate
beaker (Pyrex)
monosacchzride solution
~saccb~aide solu~on
H
H’-C--O--H ,
I
H
HH"
0 H
H
FIGURE 6-1
Part C. Identi£icafion of Carbohydrates
I~ Chemical Tests on Known Carbohydrates
Benedict’s Test
® Fill a 500 mL beaker hal/fuI1 of water. Bring the
water to a boil on a hot plate. The bailing water is
called a hot water bath {Figure 6-2). CAUTION:
Water is very hot.
polysaccha~de solution
apple iuice
oat soIu~ion .~. ’" ..
table sugar soldfi,.c~. ;
honey solution
powdered sugar solution
® Observe any color changes in the ~olunons.
NOTE: A color change may or may not occur when
Benedict’s solution is added to a carbohydrate and
then heated. A change from blue to green, yellow,
orange, or red occurs if a monosaccharide is
present. The original blue color will remain after
heating if a disaccharide or polysaccharide is
present.
’,
FIGURE 6-2
o Number three clean teat tubes one to three.
Using l:igure 6-5 as a guide and a clean dropper for
each tube, add the following:
Tube 1--30 drops of monosaccharide solution
Tube 2--50 drops of disaccharide solurion
Tube 3--30 drops Of poIysaccharide solution
~? drops Benedict’s
e Record in Table 6-I the color of the solutions in
the tubes in the column marked "Benedict’s Color
A~ter Heating."
Iodine Test
Number three clean test tubes one to three.
Using l:igure 6-4 as a guide and a clean dropper for
each tube, add the following:
Tube 1--30 drops of monosaccharide solution
Tube 2--30 drops-of disaccharide solution
Tube 3--30 drops of polysaccharide solution
’ solution in each<b~
FIGURE 6-4
30 dr~ps
monosaccharide
solution
FIGURE 6-3.
30 drops
disaccharide
solution
30 lrops
polysaccharlde
solution
"~ - _~\
4 drops
solution
"-’~iodine
30 drops
30 drops
30 drop~
rnonosaccharlde disacchaHde
polysaccharide
solution
solution
solution
~OD{NE TEST
BENEDICT’S TEST
¯ Add 30 drops of Benedict% solution to each tube.
e Add 4 drops of iodine solution to each tube.
CAUTION:// io~e s.pillage occurs, rinse wfth
water and ca]] your teacher immerHately.
CAUTION: If Bened2ct’s solution spillage occurs,
rinse with water an~ ca~ your t~ac~e~.
Mix the contents of each tube by gently swirl.g.
o Place the t~ee test robes into the hot water
for five minutes.
~ Use a test tube holde~ to remov~ the tubes
the hot water bath.
CAUTION: Water and test robes ar~ ve~ ~ot.
HenCe test tubes o~]y ~th a test tube holder.
o Record in Table 6-I the color of the solutions in
the three tubes in the column mmrked "Iodine
color." NOTE: A color change may or may not
occur when iodine solution is added to a carbohydrate. A change from its original rest co!~ to deep
blue-black occurs ff a polysaccharide is present.
The original color of the carbohydrate remains if a
dfsacchmride or monosacchmride sugar is present.
TABLE 6-1. RESULTS OF TESTS WITH KNOWN CARBOHYDRATES
24
TUBE NUMBER
CARBOHYDRATE TYPE
1
Monosaccharide
2
DisacCharide
3
Polysaccharide
BENEDICT’S COLOR
AFTER HEATING
IODINE COLOR
Name
Date
II. Chemical Te~ts on Unknown Carbohydrate~
Place all five test tubes into a hot water bath for
Having tested known caxbohydrates, you are ~ive minutes.
now ready to test some unknow-n subst.~mces. By
comparing results of the Benedict’s and iodine ® Remove the test robes ~rom the bath with a test
tests in Table 6-1, you should be abie to classify tube holder a~..l note any color changes. Record the
known substances as either monosaccharides, color of the solutions in Table 6-2.
disacchatides, or polysaccharides.
® Using Figure 6-6 as a guide, prepare five more
® Number £ive clean test tubes I to 5. Using Figure test tubes containing the same substances just
6-5 as a guide and a clean dropper £or each tube, add used (honey, oats, and so on}. Do not add
BenedSct’s solution.
the IolIowing:
tube 1--20 drops of honey
tube 2-20 drops of liquid oats
tube 3--20 drops of table sugar solution
tube 4---20 drops of apple iuice
in each
tube 5--20 drops of powdered sugar solution
20dro
honey
0 dnl~
liquid
® Add 4 drops of iodine solution to each tube and
mix by swirling.
20d~1~ 20~
solution
FIGURE 6-5
juice
~lutlon
BENEDICT’S TEST
® Add 30 drops of Benedict’s solution to each test
tube.
Note any color changes ~.ud record in Table 6-2.
On the basis of your results, classify each
carbohydrate as a monosaecharide, disaccharide or
polysaccharlde and record answers in Table 6-2.
TABLE 6-2. RESULTS OF TESTS WITH UNKNOWN CARBOHYDRATES
CARBOHYDRATE
Honey
Oats
Table sugar
Apple
Powdered sugar
BENEDICT’S COLOR
IODINE COLOR
TYPE OF CARBOHYDRATE
Analysis
Useyour results from Parts A and B to answer questions 1 to 5.
1. Name the three categories of carbohydrates studied in t.his investigation.
2. What three elements are present in all carbohydrates?
3. Give two examples each of sugars that axe
(a) monosaccharides.
(b] disaccharldes.
[c} polysaccharides,
4. (a} How many times larger is the number of hydrogen atoms than oxygen atoms in all carbohydrates?
(b} In water?
5. "Mono-" means one, "di-" means two, and "poly-" means many. Why are these terms used in
describing the three types of sugars?
Uge your results from Part C to answer questions 6 to 9.
6. How can you tell by using Benedict’s a~ad "iodXne solutions k[ a sugar is a
(a) monosaccharide?
(b) disacchazide?
(c) polysaccharide?
7. A certaIn sugar has no change in color when tested with Benedict’s solution.
(a) Can you tell what type of saccharide it is?
(b) Explain:
8. A certain sugar has a color change in Benedict’s solution.
(a) Can you tell what type of saccharide it is?
(b) Explain:
Give an example of a food that is a
(a) monosaccharide.
(b) Eisaccharide.
(c) polysaccharide.