1-2: How Scientists Work

Essential Question:1. How do scientists know what they claim to

know?2. How does a scientific theory develop?

In the days before refrigeration….

….maggots were commonly found on spoiled food

Spontaneous generation = idea that living things arise from non-living things….

Maggots on meat

Mice on grain

1. Why is Redi’s experiment on spontaneous generation considered a controlled experiment?• Redi kept all variables but one the same: covering

the jar.

Vocab• Controlled experiment = only _______ variable changed at a time.• Manipulated variable = the one thing that is _________.• Responding variable = the thing(s) that are measured as response

to _________________ variable.• Controlled variables = things kept the __________ to make sure

we have fair test.

one

changed

manipulatedsame

2. How does a scientific theory compare with a scientific hypothesis?

• Hypothesis = educated guess, possible explanation for small set of observations. “iffy”

• Theory = well-tested explanation for big range of observations. Not “iffy”

Ex – earthworms attracted to fertilizer

Ex – plate tectonics, evolution

3. How do scientists today usually communicate their results and conclusions?Usually publish report in scientific journal so other scientists can review, look for mistakes, and repeat to see if they get same results.

4. How did the design of Pasteur’s flask help him successfully refute the hypothesis of spontaneous generation?

• Curved neck allowed air in, but not microorganisms. Showed living things come from other living things.

5. Evaluate the impact of Pasteur’s research on both scientific thought and society. What was the effect of Pasteur’s investigation on scientists’ ideas and people’s lives?• Disproved hypothesis of spontaneous

generation for good. • Showed infections caused by

microorganisms (tiny living things).• All milk you buy is pasteurized to kill

bacteria.

Writing in Science. Write a paragraph in which you analyze, review, and critique the spontaneous generation hypothesis. Questions to consider: What observations did the hypothesis account for? Why did it seem logical at the time? What evidence was overlooked or ignored?

Spontaneous generation explained why maggots appeared on meat. It made sense at the time because they didn’t seem to be coming from anywhere else. What they overlooked was the possibility that maggots were coming from fly eggs too small to see.

1. What does it mean to be alive?

2. What makes living things different from non-living?