Summer Research Program for Science Teachers
Bergen Academy
2000
Activity #1 Related to Natural Products
Activity 2: Extraction of Caffeine from Tea Leaves
Activity 3: Growing Barley and Detecting the Presence of Amino Acids
Introduction to Activity 1: Natural products are substances produced in living systems that may be useful to humankind, especially in the field of medicine. For example, taxol is a substance derived from Pacific yew trees that has been shown to be effective against certain types of cancer. The extraction of such substances is necessary before their therapeutic value can be evaluated.
This action plan presents three activities related to natural products. One of these is the extraction of a natural product, caffeine, from tea leaves. This has been chosen since the starting ingredients are relatively easy to come by, and the student will still find a reasonable level of challenge. The microwave oven was chosen to provide energy for heating the tea bags, to emphasize the importance of the microwave oven in the field of organic chemistry. Microwave-assisted chemical reactions are increasingly important, especially for organic chemistry. Many chemical reactions that used to take hours now take minutes or seconds with the use of microwave techniques. A simple demonstration of the microwave is included in the action plan. A third activity is the growing of barley under controlled conditions, since it is often necessary to grow plants (or animals) to provide raw material from which natural products can be extracted. The growing of plants offers ample opportunity for open-ended activities
I. Operation of the
Microwave Oven for Chemistry Experiments
The microwave oven has been used increasingly by chemists to provide the activation energy for chemical reactions, especially in the field of organic chemistry. There are many advantages to this technique. First, it eliminates some of the disadvantages of a gas burner, such as the high-temperature flame. Second, the microwave oven generally reduces heating time. Many people are familiar with the time-saving aspects of microwave ovens in the heating of water, but some chemical reactions are not merely sped up, but altered by the use of the microwave oven. For example, the percent yield of certain products may be greater when microwave irradiation is used. Third, the quantities of reagents are typically much smaller, which is good for the environment. Fourth, the delivery of energy is very efficient, since the reaction vessels (typically glass beakers covered with petri dishes) are almost transparent to microwaves, so a greater percentage of the energy available actually reaches the reactants. [9-12 Content Standard E- Understandings about science and technology] This reduction in energy use is also good for the environment.
The object here is to demonstrate to students how the microwave oven delivers energy at different power levels. Students might assume for example that an oven at 40% power is delivering only 40% as much energy per second as it does at 100% power. In actual fact, however, the oven turns on for about 4 seconds and off for about 6 seconds when set at 40% power, so that the average power may be 40%, but this is achieved by alternating periods of 100% power and zero power.
Materials and Equipment
Microwave oven (generally a 1000-W model, but a lower watt model is may be used instead)
250-mL beaker salt
ordinary incandescent light bulb water
stopwatches whiteboards (one for each group)
dry-erase markers (for whiteboards)
erasers for whiteboards
teaspoon
[Teaching Standard D- Make accessible science materials]
Before conducting this experiment, ensure individuals (such as wearers of pacemakers) who should not be near operating microwave ovens are out of the room. [Teaching Standard D- Ensure a safe working environment]
1. Select a student volunteer to operate the microwave oven. Divide the rest of the class into groups of four or five.Each group will consist of a director, a spokesperson (who will explain group ideas to the rest of the class), a recorder (who will write the group’s ideas on the whiteboard), and a researcher (who will use all available means, such as books and the Internet, to find information relevant to the project). Five-member groups will have two researchers. [Teaching Standard E- Nurture collaboration among students] Explain to the class that a beaker holding an aqueous NaCl solution and an ordinary incandescent light bulb will be placed in the microwave oven, and that the oven will be operated at 10% power for 20 seconds.
2. Place the microwave oven so that the class can see inside the oven.
3. Explain to the students that they must monitor what happens inside the microwave oven. Have groups list on their whiteboards items they will monitor as the oven operates. If a group includes time as an item, then that group will be provided with a stopwatch.
4. The volunteer fills the beaker about half way with water and mixes in a teaspoon of salt.
5. The volunteer places the bulb threads down in the beaker, and then positions the beaker on the turntable in the oven.
6. The volunteer sets oven power for 10% at 20 seconds, and starts the oven.
7. While the microwave oven runs for the specified time, have the groups monitor the parameters designated in step 3. (The microwave oven will go on, then off, for brief intervals, causing the bulb, with no attached wires, to illuminate, then go dark, in each cycle.)
8. When the oven stops, the groups must describe what they saw and propose explanations, which will be written on the whiteboards.
9. On its whiteboard, each group will propose a hypothesis to be tested, and a method to test it. [9-12 Content Standard A- Design and conduct a scientific investigation] One objective here is to have students make hypotheses that are simple enough that they can be readily and safely tested. For example, the hypotheses need not explain in detail how the microwave energy was converted to the visible light energy of the incandescent bulb, since such hypotheses might not be easily tested under classroom conditions. One acceptable hypothesis might be: “When the oven is at 50% power, the bulb will be dark and illuminated for equal durations.” Another hypothesis might be that the bulb will light regardless of whether there is salt in the solution or not. Yet another hypothesis might be that the threads of the bulb need not touch the water for illumination to occur. The students will be required to devise procedures for testing their hypotheses. (For reasons of safety, of course, the teacher will place restrictions on these procedures, but the variations may include the placement of the light bulb in and out of the beaker of water, and salt added to or excluded from the water. If the light bulb is placed out of the water, there should still be a beaker of water positioned in the oven to absorb energy.) After each group has presented its hypothesis and method of testing it, the class will vote on which hypothesis and method will be used next.
10. Subsequent trials, designed to test hypotheses, may be performed, in which one condition is changed from a previous trial. In other words, a controlled experiment is performed with the previous trial serving as the control. Ensure that each procedure devised includes a heat sink (a beaker of water or saltwater) somewhere in the oven for each trial, and limit durations to 20 seconds.