CARBOHYDRATE IDENTIFICATION
Purpose:
The purpose of this lab exercise is to identify unknown carbohydrates using classic
carbohydrate chemical tests.
Discussion:
Answer the following questions:
1. What are carbohydrates? What are their major biological roles?
2. Differentiate between monosaccharides and polysaccharides. Provide
example molecules.
3. What are the differences between fructose, glucose, lactose, starch and
sucrose?
4. How do the following tests help to tell the difference between types of
carbohydrates?
a. Iodine test
b. Benedict’s test
c. Barfoed’s test
d. Seliwanoff test
5. Complete the following flow chart to show how the identities of the unknown
carbohydrate samples will be revealed.
starch, glucose, fructose,
lactose, sucrose
Iodine Test
Positive Test Results
Negative Test Results
Benedict’s Test
Positive Test Results
Negative Test Results
Barfoed’s Test
Positive Test Results
Negative Test Results
Seliwanoff Test
Positive Test Results
Negative Test Results
Objectives:
In this lab, you will…
1. develop a procedure that uses the four tests to identify the five carbohydrates.
2. run a series of chemical tests on an unknown carbohydrate.
3. use the results of the chemical tests to identify the unknown carbohydrate.
Materials:
Chemicals:
Unknown carbohydrate solutions:
Fructose
Glucose
Lactose
Starch
Sucrose
Test solutions:
Iodine solution
Benedict’s solution
Barfoed’s solution
Seliwanoff reagent
Safety:
A number of solutions are irritants, moderately toxic and corrosive. Handle all solutions with
extreme care. Wear safety goggles at all times, and report spills immediately.
Procedure:
1. Develop the procedure and get it approved before lab day.
2. Run the approved procedure on lab day.
3. Hand in the notebook and full report one week after lab day.
Data:

Analysis:

Error:

Construct a data table for five chemicals being run through four tests.
Use your flow chart to identify the carbohydrates. Explain why each carbohydrate exhibited
the results for each test.
Ask your instructor for the true identities of the unknown carbohydrates and discuss sources
of error (even if all samples were correctly identified).