Changing Prevailing Theories (Teacher Copy)
Florida Sunshine State Standards Benchmark: SC.H.1.3.1
AA
The student knows that scientific knowledge is subject to modification as new information challenges prevailing theories and as a new theory leads to looking at old observations in a new way.
(Also assesses H.1.3.2, H.1.3.3, H.1.3.5) MC
Objectives: SC.H.1.3.1
I can explain how technological and scientific advances have changed our knowledge and beliefs in existing theories.
Background Knowledge:
Today, we take many things in science for granted. Many experiments have been performed and much knowledge has been accumulated that people didn’t always know.
For centuries, people based their beliefs on their interpretations of what they saw going on in the world around them without testing their ideas to determine the validity of these theories – in other words, they didn’t use the scientific method to arrive at answers to their questions. Rather, their conclusions were based on untested observations.
Two 17 th century Italian scientists questioned prevailing theories and changed the way science was conducted. Galileo Galilei was credited with introducing the scientific method since he relied on observations and experiments instead of philosophy and abstract thought to better understand the universe. Francesco Redi challenged the theory of spontaneous generation using a controlled experiment. Spontaneous generation is the theory that life forms can be generated from inanimate objects. Redi is often credited with the development of the first controlled experiment.
Materials:
Part A
Each group will need
 an Observation/Conclusion card
 a piece of chart paper (optional)
 a marker (optional)
Part B
Each student will need
 a set of sentence strips based on Francesco Redi’s experiment (pre-cut, out of order, and placed in an envelope)
 a large piece of construction paper
 a glue stick

Engage:
Show students the BrainPOP video entitled Galileo Galilei. Before viewing the video, ask the students to consider these questions:
1.
What was the biggest difference between Galileo’s method of conducting science and the methods of previous scientists such as Aristotle? (He relied heavily on the scientific method rather than philosophy and abstract theory.)
2.
What is one example in the video that shows how science is subject to modification as new information challenges prevailing theories? (Galileo used the telescope to learn more about the solar system and challenged Aristotle’s geocentric theory in support of Copernicus’ heliocentric theory.)
Explore:
Procedures
Part A
1.
Explain to students that for centuries people (including scientists) believed that nonliving things could suddenly change into living things. This theory is called spontaneous generation. They thought that special “vital” forces brought some living things into being from nonliving material. Over time, scientists have performed many experiments that have disproved this theory (e.g.: Redi,
Spallanzani, Pasteur). These experiments led to the idea that living things come from other living things.
2.
Give each group of students an observation/conclusion card. The observation card describes an observation about the natural world made by people many centuries ago. The conclusion section of the card describes a conclusion drawn by the people based on the observation.
3.
Ask students to read the observation and conclusion as a group. Then, challenge students to develop an alternative conclusion based on the fact that living things can only come from other living things. Groups may record their alternate conclusions on chart paper.
4.
Have groups select a spokesperson for their groups. Next, each spokesperson can share the observation, conclusion, and alternate conclusion with the class.
Part B
1.
Ask students the following question, “What happens to food that is left in an open trash can for a few days in the summer?” Students may say that maggots appear on the discarded food. Ask students, “Where do the maggots come from?”
2.
Explain to students that for centuries, people believed that the maggots were spontaneously generated from the meat. In 1668, Francesco Redi conducted an experiment with meat, jars and a cheese cloth cover to disprove that maggots came from meat. See the chart below which explains Redi’s experiment. (Later,
Louis Pasteur used a spiral-necked flask and gravy to disprove that microorganisms spontaneously generated. This led to the idea that living things come from other living things.)

See the chart below which explains Redi’s experiment.
3.
Give each student a set of experiment sentence strips that describe the steps of
Redi’s experiment. The sentence strips should be pre-cut and out of order. To facilitate the lab, consider placing each set of strips in an envelope.
4.
Each student should arrange the experiment sentence strips in order from the beginning to end according to the scientific method. They should also match the steps of the experiment with their step in the scientific method. For example, the sentence strip that discusses data collected during the experiment would be matched with the data strip. Students should not glue to strips to the construction paper yet.
5.
Have students discuss and justify their experimental order making sure it fits the established steps of the scientific method.
6.
Once sentence strips are arranged correctly, students can glue the sentence strips on the construction paper.
7.
Answer any questions that students have about Redi’s famous.
Explain:
Have students discuss and justify their experimental order, making sure it fits the established steps of the scientific method.

Elaborate:
Have students research the famous experiment of Spontaneous Generation in Soup experiment of Louis Pasteur (1864) and explain how he challenged the prevailing theory of Spontaneous Generation.
Evaluate:
Students will journal in their science notebooks:
1.
In your science notebook, write a hypothesis for Redi’s experiment using an If … then statement.
2.
Outline the conclusion after Redi’s experiment and then explain the impact the new information had on science.

Changing Prevailing Theories (Student Copy)
Objectives: SC.H.1.3.1
I can explain how technological and scientific advances have changed our knowledge and beliefs in existing theories.
Background Knowledge:
Today, we take many things in science for granted. Many experiments have been performed and much knowledge has been accumulated that people didn’t always know.
For centuries, people based their beliefs on their interpretations of what they saw going on in the world around them without testing their ideas to determine the validity of these theories – in other words, they didn’t use the scientific method to arrive at answers to their questions. Rather, their conclusions were based on untested observations.
Two 17 th century Italian scientists questioned prevailing theories and changed the way science was conducted. Galileo Galilei was credited with introducing the scientific method since he relied on observations and experiments instead of philosophy and abstract thought to better understand the universe. Francesco Redi challenged the theory of spontaneous generation using a controlled experiment. Spontaneous generation is the theory that life forms can be generated from inanimate objects. Redi is often credited with the development of the first controlled experiment.
Materials:
Part A (per group)
 an Observation/Conclusion card
 a piece of chart paper (optional)
 a marker (optional)
Part B (per student)
 a set of sentence strips based on Francesco Redi’s experiment
 a large piece of construction paper
 a glue stick

View the BrainPOP video entitled Galileo Galilei. While viewing the video, answer these questions:
1.
What was the biggest difference between Galileo’s method of conducting science and the methods of previous scientists such as Aristotle?
2.
What is one example in the video that shows how science is subject to modification as new information challenges prevailing theories?
Procedures:
Part A
1.
Get an observation/conclusion card from your teacher. The observation card describes an observation about the natural world made by people many centuries ago. The conclusion section of the card describes a conclusion drawn by the people based on the observation.
2.
With the other members of your group, read the observation and conclusion.
Then, work together to develop an alternative conclusion based on the fact that living things can only come from other living things. Record your group’s alternative conclusion on chart paper using a marker given to you by your teacher.
3.
Select a spokesperson for your group. Have your spokesperson share the observation, conclusion, and alternate conclusion with the class.

Part B
1.
Answer the following question, “What happens to food that is left in an open trash can for a few days in the summer?”
2.
Get a set of experiment sentence strips that describe the steps of Francesco
Redi’s famous experiment about Spontaneous Generation.
3.
Arrange the experiment sentence strips in order from the beginning to end according to the scientific method. Also, match the steps of the experiment with the appropriate step in the scientific method. For example, the sentence strip that discusses data collected during the experiment would be matched with the data strip. Do not glue the strips to the construction paper yet.
4.
Discuss and justify the experimental order making sure it fits the established steps of the scientific method.
5.
Once sentence strips are arranged correctly, glue the sentence strips on the construction paper.
Evaluate:
1.
In your science notebook, write a hypothesis for Redi’s experiment using an If … then statement.
2.
Outline the conclusion after Redi’s experiment and then explain the impact the new information had on science.

Observation/Conclusion #1
Observation: Every year in the spring, the Nile River flooded areas of Egypt along the river, leaving behind nutrient-rich mud that enabled the people to grow that year’s crop of food. However, along with the muddy soil, large numbers of frogs appeared that weren’t around in drier times.
Conclusion: Muddy soil gave rise to the frogs.
Observation/Conclusion #2
Observation: In many parts of Europe, medieval farmers stored grain in barns with thatched roofs. As a roof aged, it was not uncommon for it to start leaking. This could lead to spoiled or moldy grain, and of course there were many mice around.
Conclusion: The mice came from the moldy grain.
Observation/Conclusion #3
Observation: In the cities, there were no sewers, no garbage trucks, no electricity, and no refrigeration. Sewage flowed in the gutters along the streets. In the morning, the contents of the chamber pots were tossed out the nearest window. Food was purchased and prepared on a daily basis, and when people were done eating a meal, the bones and left-overs were tossed out the window, too. Many cities also had major rat problems.
Conclusion: The sewage and garbage turned into the rats.
Observation/Conclusion #4
Observation: Since there were no refrigerators, the mandatory, daily trip to the butcher shop, especially in summer, meant battling the flies around the carcasses.
Conclusion: The rotting meat that had been hanging in the sun all day produced the flies.

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Observation:
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There are flies around meat carcasses at the butcher shop.
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Question:
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Where do the flies come from? Does rotting meat turn into or produce the flies?
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Prediction:
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Rotten meat does not turn into flies. Only flies can make more flies.
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Hypothesis:
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If meat cannot turn into flies, then rotting meat in a sealed (fly-proof) container should not produce flies or maggots.
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Testing:
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Wide-mouth jars each containing a piece of meat were left open or covered with cheese cloth (gauze) while all other variables were kept the same.
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Control group —
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These jars of meat were set out without lids so the meat would be exposed to whatever might be in the butcher shop.
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Experimental group(s) —
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One group of jars had cheese cloth (gauze) placed over them.
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Replication (multiple trials) —
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Several jars were included in each group.
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Data:
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In the gauze-covered jars, no flies or maggots were observed in these jars.
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In the control group of jars, flies were seen entering the jars. Later, maggots, then more flies were seen on the meat.
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Conclusion(s):
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Only flies can make more flies. In the uncovered jars, flies entered and laid eggs on the meat. Maggots hatched from these eggs and grew into more adult flies. In the covered jars, no flies, maggots, or eggs could enter, thus none were seen in those jars. Maggots arose only where flies were able to lay eggs. This experiment disproved the idea of spontaneous generation.