Organic CompoundsHeidi Haugen
The foods you eat are made of organic compounds. You can perform chemical tests to learn what foods contain carbohydrates, lipids, and/or proteins.
8 test tubes
raw egg white
- Read all the directions for this activity before you begin your work.
- Put on your safety goggles, lab apron, and plastic gloves.
- Put 8 test tubes in your test-tube rack. Label each test tube by putting masking tape near the top edge of the test tube. Use a pencil to write one of the seven food substances on each label. Mark the eighth label water. The water is your control.
PART I: Testing for Carbohydrates (STARCH)
PART II: Testing for Carbohydrates (SUGAR)
- Use a medicine dropper to put ~10 drops of each food in the test tube with the matching label. Add 3-4 drops of Lugol's solution (iodine) to each test tube.
- Starch is one form of carbohydrate. If the substance in your test tube contains starch, it will turn a blue-black color when it mixes with the iodine solution.
- Observe the contents of your test tubes and Record the amount of starch present (0, +, ++, +++, ++++) in your data chart. The food which contains the most starch should be recorded as ++++.
- Empty and wash each test tube and return it to your test tube rack.
- Use a medicine dropper to put ~10 drops of each food into the test tube with the matching label. Add 10 drops of Benedict's solution to each test tube. CAUTION: BenedictÍs solution is poisonous. Do not get any in your mouth and do not swallow any!
- Use a test-tube holder to carefully place the test tubes in the hot water bath your teacher has prepared. Heat the test tubes for 2 to 3 minutes. CAUTION: Use a test-tube holder to handle hot test tubes. Point the open end of a test tube away from yourself and others.
- Use a test -tube holder to return the hot test tubes to the test-tube rack. If the
substance in your test tube contains sugar, Benedict's solution will change color. See Table 1 below:
Table 1: Appearance of Substance after Adding Benedict's Solution
Sugar in Food
- Observe your test tubes (using white paper as a background). Record the amount of sugar present, in your data table.
- Empty your test tubes, clean them thoroughly, and return them to the test tube rack.
PART III: TESTING FOR LIPIDS
- Use a medicine dropper to put ~1 drop of each food onto the newsprint.
- Observe and compare/contrast the translucence of each food substance. Record the information, in order of translucence (0, +, ++, +++, ++++) in your data chart. The food which contains the most lipids should be recorded as ++++.
PART IV: TESTING FOR PROTEIN
- Use a medicine dropper to put ~10 drops of each food on the test tube with the matching label. Use a medicine dropper to carefully add 10 drops of Biuret reagent to each test tube. CAUTION: Biuret reagent can burn your skin. Wash off spills and splashes immediately with plenty of water while calling to your teacher.
- Observe the contents of each test tube (using white paper as a background). If the food contains proteins, it will turn a pinkish purple. Record the amount (0, +, ++, +++, ++++) of protein for each food substance in your data table. The food which contains the most protein should be recorded as ++++.
- Empty the test tubes and clean them thoroughly. Before leaving the laboratory, clean up all materials and wash your hands thoroughly.
Answer the following questions on a separate sheet of paper:
- Which compound is most common in foods that come from plants? Which compound is most common in foods that come from animals?
- Does water contain any of the organic compounds you tested? Explain why water was used as the control.
- If you wanted to reduce the amount of fat in your diet, what foods would you avoid?
- Which foods tested would your body use for a quick burst of energy? For energy when no carbohydrates are available? For building body parts?
- Write a Knowledge Claim. Include a comparison of your predictions and the actual results.