Fundamentals of Chemistry 1030

Carbohydrates are a major source of energy for our bodies. Foods high in carbohydrates are potatoes, bread, pasta, and rice. One gram of carbohydrate provides 4 kcal of energy. In general, carbohydrates are polyhydroxyl compounds that contain a carbonyl group. What does polyhydroxyl mean? It means that the compound has many hydroxyl (OH) groups. What are carbonyl groups? A carbonyl group is a carbon double bonded to an oxygen (C=O). If the carbon is attached to two other carbons, in addition to the oxygen, then the carbonyl group is called a ketone (Figure A). If the carbon is bonded to a hydrogen and another carbon, then the carbonyl group is called an aldehyde (Figure B).

 Carbohydrates are grouped into three categories: monosaccharides, disaccharides, and polysaccharides.

Monosaccharides  A monosaccharide is a single "sugar" unit. The most common monosaccharides are made up of six carbon atoms and are called hexoses. Glucose is an example of a hexose (Figure C). Saccharides can exist in two interchangeable forms: an open-chain form and a closed-chain form. Glucose exists almost entirely in the closed-chain form.


Fruit juices, honey, corn syrup
Fruit juices, honey 

Disaccharides  Disaccharide carbohydrates are made up of two monosaccharide units linked together. When two glucose molecules are linked together they form the disaccharide, maltose (Figure D).

Monosaccharides units
Germinating grains glucose + glucose
Milk, yogurt, ice cream glucose + galactose
Sugar cane, sugar beets glucose + fructose
Polysaccharides  When many monosaccharide units are linked together, the resulting carbohydrate is called a polysaccharide. Three important polysaccharides are starch, cellulose, and glycogen which are all made up of glucose molecules linked in different arrangements.

Germinating grains
Muscle, liver
Plants, wood, paper, cotton 
Reducing Sugars
Carbohydrates that can undergo oxidation are called reducing sugars. All monosaccharides are reducing sugars and many disaccharides are also reducing sugars. Lactose and maltose are both reducing sugars, but sucrose is not. You will test for the presence of reducing sugars using Benedict's solution.

Laboratory Activities

         A.  Benedict's Test for Reducing Sugars

    Fill a 250-mL beaker about half full with water and begin heating it over a Bunsen burner. (Note: You will be using this boiling water bath for more than one series of tests, so you may need to add water occasionally as the water boils away.) Place 10 drops of glucose, fructose, lactose, sucrose, starch, and an unknown in separate labeled test tubes. Add 2 mL of Benedict's reagent to each sample. Place all of the test tubes in the boiling water bath for 5 minutes. The formation of a greenish to reddish-orange color indicates the presence of a reducing sugar. If the solution is the same blue-green color as the Benedict's reagent, there has been no oxidation and the sample is not a reducing sugar. Record your results. Determine if each sample as a reducing or non-reducing sugar.

    B.  Seliwanoff's Test for Ketone Hexoses

    Seliwanoff's test is used to distinguish between aldehyde and ketone hexoses (carbohydrates containing 6 carbon atoms). A ketone hexose, also called a ketohexose, will form a deep red color when reacted with Seliwanoff's reagent. An aldehyde hexose, also called a aldohexose, will show a light pink color that takes a longer time to develop when reacted with Seliwanoff's reagent.

    Place 10 drops of glucose, fructose, lactose, sucrose, starch, and an unknown in separate labeled test tubes. Add 2 mL of Seliwanoff's reagent to each sample. Place all of the test tubes in the boiling water and record the time. After 1 min, observe the colors of the solutions in each test tube. The rapid formation of a deep red color indicates the presence of a ketohexose. Record your observations as either fast color change, slow color change, or no color change. Determine if each sample is a ketohexose or not.

     C.  Iodine Test for Polysaccharides

    Starch is a polysaccharide that is easily detected by the iodine test. The many glucose units linked together to form starch molecules traps the iodine molecules and produces a dark blue-black complex. Other polysaccharides react to a lesser extent with iodine to form a red-brown or reddish-purple color. Mono- and disaccharides are too small to trap the iodine molecules, so they do not show a color change in the presence of iodine.

    Place 5 drops of glucose, fructose, lactose, sucrose, starch, and an unknown in separate labeled test tubes. Add 1 drop of iodine solution to each sample. Record your observations. A dark blue-black color is a positive test for starch.

     D.  Identifying your Unknown Carbohydrate

Using the results you obtained in the tests above, identify your unknown as either glucose, fructose, lactose, sucrose, starch or none of these. It may be helpful to combine your results from the individual tests into one table similar to the one shown below:
Benedict's Test
Seliwanoff's Test
Iodine Test
          E. Testing Foods for Carbohydrates Obtain one sugar sample, one syrup sample, and one food product. Perform the Benedict's test, Seliwanoff's test and iodine test on each sample. Record your results and describe what kinds of carbohydrates are in each sample. For example, if I test a food sample and record the following results:

Benedict's test: a reddish-orange color

Seliwanoff's test: rapid formation of a deep red color

Iodine test: turned dark blue

 Then I would conclude that my food sample contained a reducing sugar (Benedict's test), a ketohexose (Seliwanoff's test), and starch (Iodine test).

Issues to be addressed in your conclusion...