Name___________________________
Date___________________ Block____
Honors Biology
Carbohydrates: Chemistry & Identification Lab
Today, scientists use a combination of biology and chemistry for their understanding of life and life
processes. Thus, an understanding of some chemistry of living things is necessary. Carbohydrates
make up a large group of chemical compounds found in cells. Carbohydrates are an energy source or
are used in making cell structures.
Objectives: Upon completing this lab, you will
1. Learn how to write a molecular formula for several carbohydrates.
2. Learn how to read a structural formula for several carbohydrates.
3. Use models to construct the main types of carbohydrates.
4. Identify the three main types of carbohydrates by using chemical tests.
5. Test different food samples to determine what type of carbohydrate they are.
REMEMBER: Models do not represent the actual three-dimensional shapes of the molecules. Models
serve to help you learn how smaller molecules can be grouped into larger, more complex molecules.
Materials:
Paper models
Scissors
Test tubes
Test tube holder
Marker
Benedict’s solution
Iodine solution
droppers
hot plate
water
beaker
monosaccharide solution
disaccharide solution
goggles
polysaccharide solution
apple juice
oat solution
table sugar solution
honey solution
powdered sugar solution
Part A: Water
Any time two monomers are joined together, such as glucose and fructose joining to form sucrose, a
water molecule is formed. The diagram below is how we will represent water molecules throughout the
study of different chemical compounds found in living systems.
Part B: Carbohydrate Models
Group 1. Monosaccharides (single molecule sugars)
A single molecule sugar is called a monosaccharide. The prefix “mono” means one. However, one
molecule can have different molecular structures due to a different arrangement of atoms. Three
common monosaccharides are glucose, fructose, and galactose.
Examine the structural formulas of these three sugars (Figure 6-1) and answers the following questions
1. What three elements are present in the monosaccharides? (Write the words, not the symbols.)
_______________________________________________________________________
2. How many atoms of carbon are present in a molecule of:
Glucose? ________________ Fructose?_________________ Galactose?____________________
3. Add subscripts to the following to indicate the proper molecular formula of:
glucose: C__H__O__ fructose: C__H__O__ galactose: C__H__O__
4. Are there two times as many hydrogen atoms as oxygen atoms in a molecule of:
Glucose? _______________ Fructose? _______________ Galactose?_________________________
5. Are there two times as many hydrogen atoms as oxygen atoms in a molecule of water? __________
6. Compare the structural formula of glucose to fructose:
a. Are they exactly the same shape? ___________ b. Are they both monosaccharides? ____________
Group 2. Disaccharides (double molecule sugars)
Two monosaccharide sugar molecules can join chemically to form a larger carbohydrate molecule
called a double sugar, or disaccharide. The prefix “di-” means two. By chemically joining a glucose
molecule with a fructose molecule, a double sugar called sucrose is produced.
Use the page of paper models given to you by your teacher to complete this section.
• Cut out model of one glucose and one fructose in Figure 4.
Fold on the first dotted line, then take that crease and fold it on top of the 3rd line. This will tuck the
middle dotted line behind the first line. You should see the word “Sucrose” form at the bottom.
7. What molecule(s) disappeared (were removed) as you put the folds together? _____________How
many of those molecules were formed? ___________
8. The production of sucrose represents a dehydration synthesis (condensation) reaction. Briefly
describe what occurs during this type of reaction.
_______________________________________________________________________________
9. Write the molecular formula for sucrose by adding together the molecular formulas for glucose and
fructose and then subtracting the formula for water, H2O. (Use molecular formulas for finding this).
Formula for glucose:____________________________
+
Formula for fructose:____________________________
C___ H____ O____
Now subtract water -
Formula of sucrose =
H____ O____
C____ H____O____
Different disaccharide molecules can be made by joining other monosaccharides in different
combinations. By chemically joining a glucose molecule with another glucose molecule, a double sugar
called maltose is formed. Lactose, a common sugar found in milk, is also a disaccharide formed by
joining together one glucose and one galactose. Glucose, galactose and fructose are all considered
monomers of disaccharides.
C. Polysaccharides
Just as double sugars were formed from two single sugar molecules, polysaccharides are formed when
many single sugars are joined chemically. The prefix “poly-” means many. Starch, glycogen, and
cellulose are the three most common polysaccharides in biology. They consist of long chains of glucose
molecules joined.

Construct a starch molecule using Figure 5. Cut along the lines and fold along the lines to
create the bonds as you did with the disaccharide model. This model will represent only a small
part of a starch molecule because starch consist of hundreds of glucose molecules.
10. What must be removed to join each glucose together? _______________ How many of those
molecules were removed in your model? ___________
Part C. Identification of Carbohydrates
I. Chemical Tests on Known Carbohydrates
Benedict’s Test
1. Fill a 500 mL beaker half full of water. Bring the water to a boil on a hot plate. The boiling water is
called a hot water bath. CAUTION: HOT WATER!!!
2. Number three clean test tubes; 1, 2, & 3 and add a carbohydrate solution to each:
• Tube 1 – 30 drops of monosaccharide solution
• Tube 2 – 30 drops of disaccharide solution
• Tube 3 – 30 drops of polysaccharide solution
3. Add 30 drops of Benedict’s solution to each tube.
CAUTION: if Benedict’s solution spillage occurs, rinse with water and call your teacher.
4. Place the three test tubes in the hot water bath for 5 minutes.
5. Use a test tube holder to remove the tubes from the hot water bath.
6. Observe any color changes in the solutions.
NOTE: A color change may or may not occur when Benedict’s solution is added to a carbohydrate. A
change from blue to green, yellow, orange or red occurs if a monosaccharide is present. The original
blue color will remain if a disaccharide or polysaccharide is present.
7. Using the data table below, record the color of the solutions in the tubes in the data table.
8. Clean your test tubes out with soapy water and a test tube brush. You may reuse the same test
tubes for the next part of the lab.
II. Iodine Test
1. Number three clean test tubes: 1, 2, & 3 and add a carbohydrate solution to each:
• Tube 1 – 30 drops of monosaccharide solution
• Tube 2 – 30 drops of disaccharide solution
• Tube 3 – 30 drops of polysaccharide solution
2. Add 4 drops of iodine solution to each tube. DO NOT HEAT THE TEST TUBES.
CAUTION: If iodine spillage occurs, rinse with water and call your teacher immediately. .
3. Mix the contents of each tube by gently swirling.
4. Record the color of the solutions in the three tubes in the data table.
NOTE: A color change may or may not occur when iodine solution is added to a carbohydrate. A
change from its original rust color to a deep blue-black occurs if a polysaccharide is present. The
original color of the iodine remains is a disaccharide or monosaccharide sugar is present.
5. Clean your test tubes out with soapy water and a test tube brush. Place your test tubes upside down
on the test tube rack to allow them to dry. Clean up your lab station.
Results of Tests with Known Carbohydrates
Test
Tube
Carbohydrate
1
Monosaccharide
2
Disaccharide
3
Polysaccharide
Change in color after Heating with
Benedict’s Solution
Change in color after adding
Iodine
Part D. Design an Experiment. Now that you have the knowledge of how to test for
monosaccharides, disaccharides, and polysaccharides, let’s see if you can put this knowledge to good
used. Below is a list of five carbohydrates. Your job is to design and conduct an experiment to
determine the type of each carbohydrate. You must include a data table to record your results of the
tests you perform. The data table should also include the classification of each carbohydrate. Your
lab group should create a procedure and data table on the computer. When you are finished, ask
your teacher to approve your procedure before you touch any lab materials. Once your
procedure has been approved, be sure to print a copy for each person in your lab group. You must
attach your procedure and data table to your lab packet, along with your models from the pre-lab
section.
Foods to test:
Honey
Oats
Table Sugar
Apple Juice
Powdered Sugar
Post Lab Questions – use pages 93-97 in your book to help you.
1. What three elements make up all carbohydrates? _______________________________________
2. What ratio exists between the hydrogen and oxygen atoms in all carbohydrates? ______________
3. Using Benedict’s and Iodine tests, how can you tell if a carbohydrate is the following?
a. monosaccharide: _________________________________________________________________
b. disaccharide ____________________________________________________________________
c. polysaccharide __________________________________________________________________
4. Explain the difference between a monosaccharide and a disaccharide. Give an example of each.
_________________________________________________________________________________
_________________________________________________________________________________
5. Write the formula for a monosaccharide that has three carbon atoms. _______________________
6. Compare and contrast starch and cellulose (two plant polysaccharides). ____________________
________________________________________________________________________________
________________________________________________________________________________
7. How do animals store excess glucose molecules? Where in the body are they stored? ________
________________________________________________________________________________
________________________________________________________________________________
8. Explain the connection between monomers and polymers. _______________________________
________________________________________________________________________________
________________________________________________________________________________
9. a. What molecule is released during the construction of a polymer? _______________________
b. What is this reaction called? _____________________________________________________