Final answer:
The amount of flour used in the baker's experiment would be a constant, not an independent variable. Other concepts discussed include the dependent variable in a study with boys and doughnuts, independent and dependent variables in a bacteria growth experiment, and the use of a placebo in controlling experiment expectations.
Step-by-step explanation:
In the baker's investigation to determine which preservative will most effectively improve the shelf life of his bread, the amount of flour used would be a constant. A constant is a factor that does not change during the experiment and is kept the same across all test subjects to ensure that the results are due to the varied conditions (the independent variables) and not other factors.
Regarding the other questions, in the study where ten-year-old boys are fed doughnuts, the dependent variable would be 'the weight gained' since it’s the factor being measured that may change in response to the independent variable (the doughnuts). Similarly, in the experiment where bacteria were grown on two types of gel, the independent variable would be the type of gel used, while the dependent variable would be the number of bacteria observed after 24 hours.
Lastly, when researchers want to control for the effects participant expectations might have on an experiment, they may administer a placebo, which is a substance with no therapeutic effect used as a control in testing new drugs.