Introduction: Substances in food that can be used by an organism for energy or for growth and repair are called nutrients. Nutrients include carbohydrate, proteins, fats, minerals, and vitamins. Cells of all organisms are composed of the nutrients and water. There are several chemical tests that can be used to determine which nutrients, if any, are present in a substance. You will use some of these tests in this activity.
We will use these indicators:
1) Benedict’s Solution tests for simple sugars (monosaccharides)
2) Lugol’s Iodine Solution tests for starch ( a polysaccharide)
3) Biuret’s Solution tests for protein
HCl (hydrolysis rxn of unknown)
Beaker (400 mL)
Test tube holder
Hot water Bath
Clean test tubes
Test tube holder*
*Safety equipment must be used when handling all chemicals and/or hot tubes and solutions
Procedure and Observation:
Part I. Testing for Carbohydrates Starches and sugars belong to the group of organic compounds called carbohydrates. Starch molecules are composed of many simple sugar molecules linked together. Starch molecules and simple sugar molecules have different chemical behaviors. For this reason, different chemical reagents, or testing agents, are used to test for starch and for simple sugars.
You will use different chemical tests to determine the nature of four unknown samples (#1 - #4).
I. A solution of iodine, Lugol’s iodine solution, is the reagent used to test for the presence of starch. Lugol’s solution is yellow brown. However, it reacts chemically with starch to form a blue-black substance called iodine of starch.
1. Add two drops of the four unknowns and water to wells on a spot plate.
2. Add one drop of iodine solution to each of the four samples. 3. What are your determinations about the four unknown samples to this point in the exercise?
Light yellowbrown = Negative test for starch
Purple, violet or blueblack ± precipitate = Positive test for starch
II. Benedict’s solution, a clear blue solution of sodium and copper salts, is used to test for the presence of reducing sugars. Reducing sugars have a free ‘ COOH ‘ which can reduce other compounds. In the presence of reducing sugar, Benedict’s solution changes color to green (low), yellow (medium) and brick red (high), depending on the amount of reducing sugar present. Reducing sugars are those that have a free or potentially free aldehyde or ketone. In a solution of pH 8 or higher the sugar is capable of reducing certain weak oxidizing agents such as cupric hydroxide along with a resultant oxidation of the carbonyl group of the sugar. Both Benedicts and Barfoeds tests identify reducing sugars. The following reaction is an example:
(reducing sugar) glucose + 2Cu(OH)2 --------- > Cu2O + 2H2O + glucose
(oxidized cupric ions: blue) (reduced cuprous ions: red)
1. Label a clean test tube for each of the remaining unknown solutions and water. Using a hand pipette, add 5 mL of each solution into the test tubes. Then set it in the test tube rack/beaker.
2. To each test tube add 2 ml of Benedict solution.
3. Carefully place both test tubes in a 950 water bath for 3 minutes. During the heating, observe the contents of each tube for any color changes.
4. At the end of 3 minutes, remove your test tubes with a test tube holder and place then in the test tube/beaker rack. Caution: Avoid getting splashed with hot water. Never touch hot test tubes with bare hands.
5. What are your determinations about the unknown samples to this point in the exercise?
Light blue =