In this part, students change the conditions for the yeast and identify variables to change. If your students are unfamiliar with dependent and independent variables, see more about them here.
Having demonstrated what happens with one bottle of yeast, water and two cubes of sugar, and how it compares to a similar setup with baking soda, challenge your students to come up with ways to change the conditions for the yeast. As a class, you will run an experiment to find yeast's optimal growth conditions.
As a class, talk about what factors you can vary in the experimental setup. Each student should choose just one variable to change. Variables in this setup include:
- amount of yeast
- amount of sugar
- amount of water
- temperature of water
- temperature of environment
Keep track of what variables the students choose to change, and encourage the students to change their variable differently. Or, to keep things simple, you can also assign variables to groups. So, for example, no more than three students should increase the amount of sugar added to the bottle to 40 mL of sugar. If a fourth student wants to increase the sugar, encourage them to choose a different larger value. This variation will give you a better spread of data as a class. Not all the variables need to be tested. In fact, you could have the students only test the results with different water temperatures, or different amounts of sugar. If you have enough bottles, each student or lab pair/group can set up two or three bottles with different values for the variable.
You will set up a control similar to the yeast bottle in the class demo.
Before you run the experiment, the students should write a hypothesis. Ask the students:
If time permits, students should write their own procedures. The more they write for themselves, the more thinking they do about how their actions will affect their results.
Keep the focus on objective description, with the mantra, "If you don’t know what you saw, then you don’t know what it means."
Students follow the procedure to set up their bottle, changing one aspect of the setup and keeping all others constant. They observe and record their immediate impressions and observe the results of what happened with the yeast lab setup overnight.
For students who used different temperatures of water in their experiment, comparing their results to this Exploratorium bread baking chart may be helpful.