KEY CONCEPT
Scientific ideas are
based on evidence.
Sunshine State
STANDARDS
SC.H.1.3.3: The student
knows that science disciplines differ from
one another in topic,
techniques, and outcomes, but that they
share a common purpose, philosophy, and
enterprise.
SC.H.1.3.5: The student
knows that a change in
one or more variables
may alter the outcome
of an investigation.
SC.H.1.3.7: The student
knows that when similar investigations give
different results, the
scientific challenge is
to verify whether the
differences are significant by further study.
FCAT VOCABULARY
independent variable
p. 15
dependent variable p. 15
VOCABULARY
experimental group p. 15
control group p. 15
model p. 16
theory p. 19
BEFORE, you learned
NOW, you will learn
• Different sciences focus on
different topics but have a
common approach
• Scientific methods can be
used to analyze information,
solve problems, and evaluate
conclusions
• About variables and controls
in scientific experiments
• About four different types of
scientific inquiries
• How new evidence can change
widely accepted ideas
EXPLORE Effects of Changes in Procedures
How many drops of water can a coin hold?
PROCEDURE
1
Put two different types of coins on a paper
towel.
2 Use an eyedropper to put drops of water on
MATERIALS
•
•
•
•
2 different coins
paper towel
eyedropper
water
the coins. Count how many drops each coin
can hold before the water overflows.
WHAT DO YOU THINK?
• Does it matter what type of coin you use or
which side of the coin you use? Does it matter how
far the eyedropper is above the coin?
• How would keeping detailed records help others
reproduce your results?
Observations provide scientific evidence.
Observations are a way of learning about the world. You continually
make observations with your senses as you see, hear, touch, smell, and
taste things. You can increase your ability to observe by using equipment, such as a magnifying glass. When you keep records, you can
compare observations made at different times or places.
reminder
You can find detailed information about different units
and unit conversions on
pages R20–R21.
A measurement is an observation expressed as a number and a
unit. Suppose some bicyclists tell you they normally bike a distance of
15. This information is meaningless because a unit, such as kilometers
or miles, is missing. In the United States, many people use nonmetric
units of measurement, such as inches and quarts. Scientists usually use
units of the metric system, such as centimeters and liters. The formal
name of the standard scientific system is the International System of
Units, or SI.
14 Chapter 1: The Nature of Science
An inquiry is a way to gather evidence.
A laboratory experiment is one type of scientific inquiry. You will read
about other types later in this section. In a laboratory, scientists have
more control over factors in experiments. Scientists can get clearer
results as they test their ideas about cause-and-effect relationships.
Variables and Controls
In an experiment, each repetition of a procedure is called a trial. A
factor that changes or can be changed in a trial is called a variable.
Suppose you are testing how salt affects the freezing temperature
of water. Variables include the amounts of water and salt as well as
temperature. A controlled variable is one that you keep constant in
all trials. You use the same amount of water in each trial, so it is a
controlled variable. A factor that you change individually is an
independent variable. In your experiment, you change the amount of
salt used in each trial. The amount of salt used is an independent variable. A factor that depends on another factor is a dependent variable.
The temperature at which the water freezes is a dependent variable,
because it depends on the amount of salt used.
VOCABULARY
Add magnet word diagrams for independent
variable, dependent variable, experimental group,
and control group to
your notebook.
An experiment may have variables that are uncontrolled.
These variables can change by chance. If one part of your
freezer is sometimes colder than another part, your experiment has an uncontrolled variable. To decrease the effects of
uncontrolled variables on your results, you can make several,
or multiple, trials in which the same value is used for the
independent variable. You could make multiple trials for each
amount of salt used.
Some experiments, such as tests on living organisms, have
many uncontrollable variables. Suppose you want to test the
effect of a new fertilizer. Because plant seeds are slightly different from one another, you might plant a number of seeds
in pots that contain the new fertilizer. Each seed becomes a
trial. You have made an experimental group —a set of trials
with the same value of the independent variable.
But how can you know whether the new fertilizer is better
than the one you have been using? Make a control group, a group
used for comparison in an experiment. Add the currently used fertilizer to some pots, then put seeds in them. This group is the control
group. The control group and the experimental group are treated
equally except for the fertilizer used. Therefore, any change in plant
growth is likely to be caused by the difference in fertilizers.
Check Your Reading
This scientist is studying how changes in the
amount of light and
water affect experimental groups of
plants and insects.
How is a control group different from an experimental group?
Chapter 1: The Nature of Science 15
Types of Scientific Inquiries
Doing science does not always require laboratory equipment. Indeed,
many types of scientific inquiries cannot take place in a lab. Some
common types of inquiries are described below and on the next page.
In this physical model,
spheres are used to
represent atoms in
a molecule.
In a laboratory, scientists can regulate
most conditions. Variables are easier to control, and uncontrolled
variables are easier to reduce or correct for. The subject of a laboratory experiment can be tiny, such as an inquiry into the
arrangement of a substance’s atoms. Or it can be huge, such as
an inquiry into tree growth in a field enclosed by a greenhouse.
1
Laboratory Experiments
2
Inquiries that are done outside a laboratory are called
fieldwork. Variables are more difficult to control, or they may not
be controlled at all. Scientists do fieldwork when, for example, they
want to investigate animal behavior in the wild. Some subjects—
such as relationships among rock layers in a mountain—are too
large to be studied without doing fieldwork.
3
A survey involves examining a part of a group and then
using the results to draw conclusions about the whole group.
This type of survey requires data from direct observations, not
from asking questions of people. For example, scientists might
want to know the numbers of organisms in the soil of an area. It
is not possible to count all the organisms without destroying the
area. Instead, the scientists could examine samples of the soil at
different locations. They would infer that the rest of the soil held
similar numbers of organisms.
4
Models
Fieldwork
Surveys
Some things are too large, are too small, are too far away,
or take too long to study directly. Other things are too dangerous
or too expensive to test directly. For example, scientists cannot
study directly how stars form. Such things are often studied by
using models, which are representations of objects or processes.
In a computer model, mathematical equations and techniques are
used to represent the behavior of things like stars or volcanic eruptions. In a physical model, objects are used to represent other
objects—ones that are difficult or impossible to work with directly.
A physical model might be used to test a design for a new aircraft.
Scientists often use several types of inquiries together. A survey
might be part of a larger program of fieldwork. Data from several
types of inquiries might be used to build a model. Then scientists can
control variables in the model that would be impossible to control in
real life. The results might suggest new effects to look for in fieldwork
or might suggest new experiments to design.
Check Your Reading
16 Chapter 1: The Nature of Science
What questions do you have about different types of
scientific inquiries?
Scientific Inquiries
The type of inquiry scientists choose to use depends on the topic
they are investigating. Often, scientists use more than one type
of inquiry as they study different aspects of a topic.
1
Laboratory Experiments
When scientists do a laboratory experiment, they
can more easily change just one variable at a
time, so they get clearer results. The photograph
shows a scientist using laboratory equipment to
analyze a substance.
3
Surveys
This researcher is working with an ice core
collected in Antarctica. Each core gives information about what climatic conditions were like
when the ice formed.
2
Fieldwork
Scientist Jane Goodall began fieldwork in 1960
to study chimpanzees. At that time, scientists
thought that only humans used tools. Goodall’s
observations of chimpanzees in their natural
habitat showed that they use tools too.
4
Models
This computer model represents a protein
molecule. The model, which is much larger than
the actual molecule, allows scientists to study the
structure of the molecule in detail.
Why might scientists who are studying the formation of tornadoes in
thunderstorms do fieldwork and use models?
Chapter 1: The Nature of Science 17
MAIN IDEA WEB
Add to your notebook
information about how
scientific ideas change.
New evidence can change scientific ideas.
Scientists make various types of inquiries as they explore a topic. They
develop, test, and either discard or improve hypotheses. Scientists look
for relationships among different types of evidence and develop ideas
that account for as much of the evidence as possible.
Science is always an ongoing process. If new evidence is discovered
that does not support an idea or if scientists begin to think about old
evidence in a new way, then the idea can be overturned. An example is
how scientific understanding of the solar system has changed.
In the mid-1900s, scientists thought that the solar system was stable
and unchanging. They understood the motions of planets and moons
very well. Scientists did not think that small objects in the solar system
could cause large changes to moons or planets, including Earth.
At one time, scientists
used models such as this
one to study the solar
system. Now, scientists
realize that such models
do not represent many
important processes.
New Evidence
Over time, different types of scientists made observations that seemed
inconsistent with the idea of an unchanging solar system. Major challenges to this idea developed from inquiries into topics that seemed
unrelated, including
• circular features on the Moon
• rock layers on Earth
• the extinction of the dinosaurs
The Moon’s surface shows
many circular features
created by the impact of
small objects.
Together, these inquiries provided evidence
that the accepted idea could not explain.
Scientists studying Earth’s moon noticed
circular features on its surface. They did not
know if the features were caused by volcanoes
or by collisions of smaller objects with the
Moon. Then, astronauts exploring the Moon
brought back samples of rock that showed
that the features were from collisions, or
impacts. Most of the impacts had taken place
a very long time ago. Spacecraft showed that
other planets and moons had been hit many
times by small objects. By comparison, evidence of impacts on Earth’s surface seemed rare. Just a few circular
features on Earth were shown to be caused by impacts.
18 Chapter 1: The Nature of Science
Meanwhile, scientists studying rock layers on Earth found an
element—iridium (ih-RIHD-ee-uhm)—in a thin layer that formed
about 65 million years ago. Iridium is more common in rocks from
space than in rocks formed on Earth. The scientists developed a
hypothesis that a huge rock from space hit Earth 65 million years ago.
They suggested that the impact destroyed the rock and threw huge
amounts of iridium-rich dust into the atmosphere. The dust spread
around the world before settling on Earth’s surface.
Models of nuclear explosions suggested that large clouds of dust
would make the weather cooler by reflecting some of the sunlight that
warms Earth’s surface. The scientists suspected that
dust from the impact could suddenly have produced cooler weather on Earth. Scientists who
study the patterns of life on Earth already knew
that many species, including dinosaurs, had
become extinct about 65 million years ago. Perhaps
the extinctions were related to the impact.
The scientists who were studying impact features added information about the age of the
iridium layer and the timing of the extinctions to
their knowledge. At first, the hypothesis that a rock
from space had hit Earth 65 million years ago
seemed unlikely to many scientists, because they
held the idea that the solar system was stable and
unchanging.
The extinction of the dinosaurs is now thought
to be related to the impact of a space object
with Earth about 65 million years ago.
Building a New Idea
Part of the work scientists did to test the new hypothesis involved
looking for an iridium layer in other 65-million-year-old rocks. The
scientists found the layer in locations around the world. They also
looked for and found evidence of additional impacts. By the late
1980s, many scientists accepted that large numbers of small objects
had hit and changed Earth, other planets, and moons. They realized
that more impacts are likely in the future. Scientists now think that
the solar system is dynamic and changing.
The hypothesis that an impact of a space rock with Earth occurred
65 million years ago developed into a theory. A theory is a well-tested
explanation that brings together many sets of scientific observations.
Theories are the big ideas of science. Details of a theory can change as
more evidence is discovered. More rarely, a theory is overturned by
new evidence and eventually replaced with a new theory.
Check Your Reading
How do scientific ideas change?
Chapter 1: The Nature of Science 19
Results and conclusions may be challenged.
Scientists must accurately describe the work that led to the results and
conclusions they report. Such openness allows other scientists to judge
the work’s credibility and to reproduce the work if they want to. If a
report lacks this information, scientists often reject the results or conclusions. If a report is complete, then any scientist can decide whether
or not to accept the results.
Sometimes scientists accept the results of an inquiry but not the
conclusions. Even when scientists think that results are valid, they
might think that the conclusions are not supported by enough evidence.
The conclusion that a space object collided with Earth 65 million
years ago was not widely accepted until many types of evidence were
discovered that all supported the same conclusion.
reading tip
The word controversy refers
to a dispute between people
holding opposite views. If
people agree, a subject is
noncontroversial, but if they
disagree, it is controversial.
When scientists report that they have discovered evidence that
challenges a widely accepted idea, their results may lead to controversy.
A period of uncertainty and argument may follow while scientists try
to confirm or disprove the results. If further results and other evidence
support the challenge, more and more
scientists begin to reject the accepted idea.
Controversial Results
Many factors can lead to controversy. A
new result can be controversial at first.
If a new result does not support earlier
results or theories, scientists first test
whether the new result can be reproduced. Then they try to understand
how it fits with what they already know.
Sometimes, a new result or idea changes
how scientists think about an entire
aspect of the natural world.
Prions, such as the one shown
in this computer model, are
proteins that can multiply and
cause diseases by causing normal
proteins to change their shapes.
The hypothesis that prions (PREE-ahnz), a type of protein, can
cause infectious diseases was very controversial. Prions are now
thought to cause infections such as mad cow disease, a fatal disorder
that affects the structure of cows’ brains. A form of the disease can be
passed to people when they eat meat from infected cows.
RESOURCE CENTER
CLASSZONE.COM
Learn more about special
proteins known as prions.
Most scientists at first did not accept that prions could multiply
and cause infections. Unlike bacteria or viruses, prions cannot
reproduce because they have no genetic material. Therefore, scientists
did not believe that a few prions passed to an organism could cause an
infection. In 1982, however, a U.S. scientist named Stanley Prusiner
hypothesized that prions can cause the shape of normal proteins to
20 Chapter 1: The Nature of Science
change, making new prions. Although some scientists are still skeptical
about Prusiner’s hypothesis, he was awarded the Nobel prize in
medicine in 1997.
Some controversies occur when groups of scientists get different
results when doing similar inquiries. For example, teams at two laboratories might analyze the same substance but find different materials. A
controversy could result over which results are correct. The controversy
ends if an explanation is found. Perhaps some samples of the substance
were contaminated, or a procedure was performed incorrectly.
Controversies can also be the result of bias. Scientists can be biased
if they become too attached to particular ideas. They may unknowingly choose data, design inquiries, or make assumptions that support
these ideas. People are more likely to suggest the possibility of bias
when results are personally unfavorable to them. For example,
research showing that a material is harmful may be challenged by a
manufacturer that uses the material.
Check Your Reading
reading tip
The word bias comes from
the French word biais,
meaning “slant.” A biased
person might not consider
an idea fairly or might not
consider all possibilities.
Give two reasons why scientific results might
be controversial.
Noncontroversial Results
Results that can be easily reproduced are rarely controversial. For
example, a chemist might analyze a substance with common equipment
and accepted methods. The results of the analysis are noncontroversial
because scientists who question them can repeat the analysis themselves. Results that are similar to previous results or that confirm a
theory in a new way are likely to be noncontroversial.
KEY CONCEPTS
CRITICAL THINKING
1. What is the purpose of
controlled variables in an
experiment?
4. Apply What types of inquiries
would be most appropriate for
studying how mountains form?
Explain.
2. Briefly describe four common
types of scientific inquiries.
3. How can a widely accepted
theory be replaced with a
new theory?
CHALLENGE
6. Identify Limits What sorts of
problems might be associated
with using models to represent
actual features or processes?
5. Synthesize How can a scientific controversy help advance
understanding of the natural
world?
Chapter 1: The Nature of Science 21