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

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

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

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

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

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

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

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

Designing an Experiment Redi’s Experiment Copyright Pearson Prentice Hall

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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