Explain to the class that before a scientist starts an experiment, he or she must first have a prediction about what the outcome of the experiment will be. This prediction is known as a hypothesis. A hypothesis is not simply a guess, however. Instead it is a prediction based on prior knowledge of or experience with the subject. For example, if a gardener wanted to find out if it was really necessary to fertilize his zucchini plants, he or she might grow twelve zucchini plants, but fertilize only half of them. In this case, the hypothesis being tested might be, "Fertilized zucchini plants will produce more zucchinis than unfertilized zucchini plants." The data collected to support or refute the hypothesis would be the total number of zucchinis produced by the fertilized plants, compared to the total number produced by the unfertilized plants.

Point out to students that in the zucchini experiment, the gardener collected data that involved numbers. In science this is usually the case, because numbers can easily be compared and they are based on things that actually happened, as opposed to things that the experimenter thought happened.

Then, explain briefly how the photosynthesis experiment will be set up, and ask the class what hypothesis will be tested. It shouldn't take them long before they come up with a statement such as, "The plants that receive more light will produce more bubbles than the plants that receive less light."

These steps should be performed with some or all of the room lights turned off. The room should not be dark, and there should be adequate light for students to see easily, but the room should not be brightly lit.

1. Each team needs to fill a beaker with about 500 ml of aged water for the Elodea. To this water they should add a scant ¼ teaspoon of sodium bicarbonate (baking soda). This provides a source of carbon dioxide for the plants, since they can't get it from the atmosphere like terrestrial plants do. Students should stir the water until the sodium bicarbonate is dissolved and the water looks clear.
2. Each team should obtain enough sections of Elodea plants so that it has about 18-24 inches of total plant length. These should be arranged so that all of the plants will be at least 1½" under the water in the beaker. String or twist ties can be used to hold them together, and then a small rock should be added to keep the plants from floating to the surface. Point out to students that the more area exposed to the light above the plant, the more photosynthesis can occur within the leaves. If students form clumps of Elodea, many of the leaves will be shaded by those above, and thus may not be able to conduct as much photosynthesis. It would be better to form the plants into loops that cover the entire bottom of the beaker, instead of a single clump in the middle of the beaker.

1. As soon as the plants are arranged in the beaker, the team should start timing for five minutes. Two team members should have their eyes glued to the beaker for those five minutes, watching for bubbles to rise to the water surface. Any bubbles that rise should be announced to the third team member, who will keep count (tally marks will help with this) as well as monitor the time until the five minutes are up. The bubbles are fairly large, about 2 mm in diameter, and so are easily seen when they rise to the surface.
2. When all the teams have counted bubbles for five minutes (it is quite possible that some teams will see no bubbles at all), turn on the room lights and have students position the desk lamps directly above the beakers. The light bulbs should only be a few inches above the beakers. Once the lights are in place, the teams should again begin timing and counting bubbles for five minutes.

1. Make a large chart on the board in which the teams can fill in the number of bubbles they observed in each of the two light conditions.
2. Once it is filled in, have students work individually to determine the mean, median, mode, and range of each of the two sets of data. Allow enough time so that all students arrive at the same answers.
3. Provide students with grid paper and ask them to make a vertical bar graph that compares the mean number of bubbles in the two light conditions. Be sure that students include a title, labels on the axes, and a legend if different colors were used for the two bars of the graph. Then show them how they can indicate the ranges of the data by adding a vertical line segment to the center top of each bar, with the lower end of the line segment situated at the lowest number of bubbles observed by a team, and the upper end of the line segment at the highest number of bubbles observed by a team.

Ask students what these numbers tell them about the amount of photosynthesis that occurred in each of the two light conditions. In other words, was the hypothesis the class tested supported or not?

Then ask students how they know that the bubbles they saw rise to the surface were bubbles of oxygen. They may answer that they know photosynthesis produces oxygen, so the bubbles must have been oxygen. However, without a way to determine the chemical composition of the bubbles, it is only an assumption that the bubbles contain oxygen. They might just as well have been bubbles of nitrogen, or carbon dioxide, or some other gas from some other process that was occurring in the plants instead of photosynthesis. Nevertheless, since the plants were exposed to light, the bubbles were most likely made up of oxygen. Point out that it is important for scientists to make sure they recognize the difference between what they know about an experiment and what they assume about it.

• What things are needed in order for photosynthesis to occur?
• What are the products of photosynthesis?
• Where in the plant does photosynthesis occur?
• Why do plants need water in order to survive?

Provide a graph of data from an experiment similar to the one they performed, and ask them to draw a conclusion from it. For example, the data could represent the heights of corn plants, half of which were grown in the shade of a forest and half of which were grown in an open field.