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Copyright Pearson Prentice Hall

1-2 How Scientists Work

Mystery Worms

A teacher collected some beetles from a rotting log A teacher collected some beetles from a rotting log and placed them in a container of dry oatmeal in her and placed them in a container of dry oatmeal in her classroom. She kept the box covered with a light classroom. She kept the box covered with a light cloth so that the beetles could not escape. She also cloth so that the beetles could not escape. She also asked one of her students to add potato and apple asked one of her students to add potato and apple pieces once a week to provide food and moisture for pieces once a week to provide food and moisture for the beetles. After several weeks, the student the beetles. After several weeks, the student reported that there were some strange-looking, reported that there were some strange-looking, wormlike organisms in the container.wormlike organisms in the container.

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1.1. Formulate a hypothesis that might explain the Formulate a hypothesis that might explain the presence of the “worms” in the container.presence of the “worms” in the container.2.2. How could you test your hypothesis?How could you test your hypothesis?3.3. Identify the variables in your proposed Identify the variables in your proposed experiment. Identify the control in your proposed experiment. Identify the control in your proposed experiment.experiment.

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DefinitionsDefinitions::

Hypothesis – a possible explanation for something Hypothesis – a possible explanation for something observed.observed.

Inference – A conclusion made based on something Inference – A conclusion made based on something you already know.you already know.


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Designing an Experiment

How do scientists test hypotheses?

Whenever possible, 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.


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The process of testing a hypothesis includes:

1. Asking a question

2. Forming a hypothesis

3. Setting up a controlled experiment

4. Recording and analyzing results

5. Drawing a conclusion


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1. Asking a Question (What do you want to know?)

Many years ago, people wanted to know how living things came into existence. They asked:

How do organisms come into being?


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2. Forming a Hypothesis

One early hypothesis was spontaneous generation (aka: abiogenesis), 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.


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3. Setting Up a Controlled Experiment

The variables that are not changed are called the controlled variables.

The variable that is deliberately changed is called the manipulated variable.

The variable that is observed and that changes in response to the manipulated variable is called the responding variable.


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Redi’s Experiment

Controlled Variables:jars, type of meat,location, temperature,time

Covered jarsUncovered jars


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Redi’s Experiment

Manipulated Variable:Gauze covering that keeps flies away from meat

Responding Variable:whether maggots appear Maggots appear.

Severaldays pass.

No maggots appear.


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Redi’s Experiment


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4. Recording and Analyzing Results

Scientists keep written or electronic records of their observations, or data, so that the experiment can be repeated.


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5. 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.


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Repeating Investigations


Scientists repeat experiments to be sure that the results match those already obtained.


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Needham's Test of Redi's Findings

Needham challenged Redi’s results by claiming that spontaneous generation could occur under the right conditions.


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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.


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Spallanzani's Test of Redi's Findings

Gravy is boiled. Flask isopen.

Gravy is teeming with microorganisms.

Gravy is boiled.Flask issealed.

Gravy is free of microorganisms.


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Spallanzani found that if the gravy in Needham’s experiment was boiled and sealed, it would not produce microorganisms. He showed that Needham wrongly assumed he had heated the gravy enough to kill any living microorganisms.


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Pasteur's Test of Spontaneous Generation

• Louis Pasteur conclusively disproved the hypothesis of spontaneous generation.

• Pasteur showed that all living things come from other living things.


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Pasteur’s Experiment

Broth is boiledBroth is freeof microorganismsfor a year.

Curved neck isremoved.

Broth is teeming with microorganisms.


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Pasteur’s Experiment

Pasteur showed that spontaneous generation would not even occur if air was available. His experiment showed that living things can only come from living things (biogenesis) and he finally put the theory of spontaneous generation to rest.


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When Experiments Are Not Possible

When Experiments Are Not Possible

It is not always possible to do an experiment to test a hypothesis. For example:

• Wild animals must be observed without disturbing them.

• Ethical considerations prevent some experiments.

By carefully planning alternative investigations, scientists can discover reliable patterns that add to scientific understanding.


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How a Theory Develops

How does a scientific theory develop?


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As evidence from numerous investigations builds up, a hypothesis may become so well supported that scientists consider it a theory.

In science, the word theory applies to a well-tested explanation that unifies a broad range of observations.


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No theory is considered absolute truth.

As new evidence is uncovered, a theory may be revised or replaced by a more useful explanation.

- or -Continue to: Click to Launch:

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1–2

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1–2

In an experiment, the variable that is deliberately changed is called the

a. control.

b. manipulated variable.

c. responding variable.

d. constant control.

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1–2

The mistaken belief that living organisms can arise from nonliving matter is called

a. biogenesis.

b. Pasteur's theory.

c. spontaneous generation.

d. Spallanzani’s hypothesis.

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1–2

Which of the following was the manipulated variable in Redi’s experiment?

a. the kind of meat used

b. the temperature the jars were kept at

c. the gauze covering on some jars

d. the kind of fly that visited the jars

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1–2

A well-tested explanation that unifies a broad range of observations is a

a. hypothesis.

b. variable.

c. control.

d. theory.

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1–2

A scientific explanation does not become a theory until

a. a majority of scientists agree with it.

b. it has been supported by evidence from numerous investigations and observations.

c. it is first proposed as an explanation.

d. it is published in a textbook.

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