1. Copyright Pearson Prentice Hall
1-2 How Scientists Work
Copyright Pearson Prentice Hall

2. Designing an Experiment
How do scientists test hypotheses?
A hypothesis should be tested by an experiment in which only one variable is changed at a time.
All other variables should be kept unchanged, or controlled.
Copyright Pearson Prentice Hall

3. Designing an Experiment
Purpose: asking a question
Forming a hypothesis
Setting up a controlled experiment
Recording and analyzing results (Data)
Drawing a conclusion
Copyright Pearson Prentice Hall

4. Designing an Experiment
*Asking a Question
Many years ago, people wanted to know how living things came into existence. They asked:
How do organisms come into being?
Copyright Pearson Prentice Hall

5. Designing an Experiment
*Forming a Hypothesis
One early hypothesis was spontaneous generation, or the idea that life could come from nonliving matter.
For example, most people thought that maggots spontaneously appeared on meat.
In 1668, Redi proposed a different hypothesis: that maggots came from eggs that flies laid on meat.
Copyright Pearson Prentice Hall

6. Designing an Experiment
Setting Up a Controlled Experiment
The variable that is deliberately changed is called the independent variable.
The variable that changes in response to the independent variable is called the dependent variable.
Copyright Pearson Prentice Hall

7. Designing an Experiment
Redi’s Experiment
Uncovered jars
Covered jars
Controlled Variables:
jars, type of meat,
Location, temperature,
time
In a controlled experiment, only one variable is tested at a time. Redi designed an experiment to determine what caused the sudden appearance of maggots. In his experiment, the manipulated variable was the presence or absence of the gauze covering. The results of this experiment helped disprove the hypothesis of spontaneous generation.
Copyright Pearson Prentice Hall

8. Designing an Experiment
Redi’s Experiment
Independent Variable:
Gauze covering that keeps flies away from meat
Several days pass.
Dependent Variable:
whether maggots appear
Maggots appear.
No maggots appear.
Copyright Pearson Prentice Hall

9. Designing an Experiment
Redi’s Experiment
Copyright Pearson Prentice Hall

10. Designing an Experiment
Recording and Analyzing Results
Written records or data.
Drawings.
Computers to record their work.
Copyright Pearson Prentice Hall

11. Designing an Experiment
Drawing a Conclusion
Scientists use the data from an experiment to evaluate a hypothesis and draw a valid conclusion.
Redi’s results supported the hypothesis that maggots were produced by flies, not spontaneous generation.
Copyright Pearson Prentice Hall

12. Repeating Investigations
Scientists repeat experiments to be sure that the results match those already obtained.
Copyright Pearson Prentice Hall

13. Repeating Investigations
*Needham's Test of Redi's Findings
Needham challenged Redi’s results by claiming that spontaneous generation could occur under the right conditions.
Copyright Pearson Prentice Hall

14. Repeating Investigations
*Needham’s Test of Redi’s Findings
Needham sealed a bottle of gravy and heated it.
After several days, the gravy was swarming with microorganisms.
Needham concluded that these organisms came from the gravy by spontaneous generation.
Copyright Pearson Prentice Hall

15. Repeating Investigations
Spallanzani's Test of Redi's Findings 
Gravy is boiled.
Gravy is boiled.
Spallanzani’s experiment showed that microorganisms will not grow in boiled gravy that has been sealed but will grow in boiled gravy that is left open to the air. Interpreting Graphics What variable was controlled in this experiment?
Copyright Pearson Prentice Hall

16. Repeating Investigations
Spallanzani's Test of Redi's Findings 
Flask is open.
Flask is sealed.
Copyright Pearson Prentice Hall

17. Repeating Investigations
Spallanzani's Test of Redi's Findings 
Gravy is teeming with microorganisms.
Gravy is free of microorganisms.
Copyright Pearson Prentice Hall

18. Repeating Investigations
Pasteur's Test of Spontaneous Generation
Louis Pasteur finally disproved the hypothesis of spontaneous generation.
Pasteur showed that all living things come from other living things.
Copyright Pearson Prentice Hall

19. Repeating Investigations
Pasteur’s Experiment
Broth is free of microorganisms for a year.
Curved neck is removed.
Broth is teeming with microorganisms.
Pasteur’s experiment showed that boiled broth would remain free of microorganisms even if air was allowed in, as long as dust and other particles were kept out.
Broth is boiled
Copyright Pearson Prentice Hall

20. Copyright Pearson Prentice Hall
How a Theory Develops
How a Theory Develops
As evidence from investigations build up, a hypothesis may become so well supported that scientists consider it a theory.
No theory is considered absolute truth.
As new evidence is uncovered, a theory may be revised or replaced by a more useful explanation.
Copyright Pearson Prentice Hall

21. Copyright Pearson Prentice Hall
1–2
In an experiment, the variable that is deliberately changed is called the
control.
manipulated variable.
responding variable.
constant control
Copyright Pearson Prentice Hall

22. Copyright Pearson Prentice Hall
1–2
The mistaken belief that living organisms can arise from nonliving matter is called
biogenesis.
Pasteur's theory.
spontaneous generation.
Spallanzani’s hypothesis.
Copyright Pearson Prentice Hall

23. Copyright Pearson Prentice Hall
1–2
Which of the following was the manipulated variable in Redi’s experiment?
the kind of meat used
the temperature the jars were kept at
the gauze covering on some jars
the kind of fly that visited the jars
Copyright Pearson Prentice Hall

24. Copyright Pearson Prentice Hall
1–2
A well-tested explanation that unifies a broad range of observations is a
hypothesis.
variable.
control.
theory.
Copyright Pearson Prentice Hall

25. Copyright Pearson Prentice Hall
1–2
A scientific explanation does not become a theory until
a majority of scientists agree with it.
it has been supported by evidence from numerous investigations and observations.
it is first proposed as an explanation.
it is published in a textbook.
Copyright Pearson Prentice Hall