14 Chapter 1: The Nature of Science

BEFORE, you learned

• Different sciences focus on different topics but have acommon approach

• Scientific methods can be used to analyze information,solve problems, and evaluateconclusions

NOW, you will learn

• About variables and controls in scientific experiments

• About four different types ofscientific inquiries

• How new evidence can changewidely accepted ideas

KEY CONCEPT

Scientific ideas arebased on evidence.

EXPLORE Effects of Changes in Procedures

How many drops of water can a coin hold?PROCEDURE

Put two different types of coins on a papertowel.

Use an eyedropper to put drops of water onthe coins. Count how many drops each coincan 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 othersreproduce your results?

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MATERIALS• 2 different coins• paper towel• eyedropper• water

Observations provide scientific evidence.Observations are a way of learning about the world. You continuallymake observations with your senses as you see, hear, touch, smell, andtaste things. You can increase your ability to observe by using equip-ment, such as a magnifying glass. When you keep records, you cancompare observations made at different times or places.

A measurement is an observation expressed as a number and aunit. Suppose some bicyclists tell you they normally bike a distance of15. This information is meaningless because a unit, such as kilometersor miles, is missing. In the United States, many people use nonmetricunits of measurement, such as inches and quarts. Scientists usually useunits of the metric system, such as centimeters and liters. The formalname of the standard scientific system is the International System ofUnits, or SI.

Sunshine StateSTANDARDSSC.H.1.3.3: The studentknows that science dis-ciplines differ fromone another in topic,techniques, and out-comes, but that theyshare a common pur-pose, philosophy, andenterprise.SC.H.1.3.5: The studentknows that a change inone or more variablesmay alter the outcomeof an investigation.SC.H.1.3.7: The studentknows that when simi-lar investigations givedifferent results, thescientific challenge isto verify whether thedifferences are signifi-cant by further study.

FCAT VOCABULARYindependent variable p. 15

dependent variable p. 15

VOCABULARYexperimental group p. 15control group p. 15model p. 16theory p. 19

reminder

You can find detailed infor-mation about different unitsand unit conversions onpages R20–R21.

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An inquiry is a way to gather evidence.A laboratory experiment is one type of scientific inquiry. You will readabout other types later in this section. In a laboratory, scientists havemore control over factors in experiments. Scientists can get clearerresults as they test their ideas about cause-and-effect relationships.

Variables and ControlsIn an experiment, each repetition of a procedure is called a trial. Afactor that changes or can be changed in a trial is called a variable.

Suppose you are testing how salt affects the freezing temperatureof water. Variables include the amounts of water and salt as well astemperature. 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 acontrolled variable. A factor that you change individually is an

In your experiment, you change the amount ofsalt used in each trial. The amount of salt used is an independent vari-able. A factor that depends on another factor is a The temperature at which the water freezes is a dependent variable,because it depends on the amount of salt used.

An experiment may have variables that are uncontrolled.These variables can change by chance. If one part of yourfreezer is sometimes colder than another part, your experi-ment has an uncontrolled variable. To decrease the effects ofuncontrolled variables on your results, you can make several,or multiple, trials in which the same value is used for theindependent variable. You could make multiple trials for eachamount of salt used.

Some experiments, such as tests on living organisms, havemany uncontrollable variables. Suppose you want to test theeffect of a new fertilizer. Because plant seeds are slightly dif-ferent from one another, you might plant a number of seedsin pots that contain the new fertilizer. Each seed becomes atrial. You have made an —a set of trialswith the same value of the independent variable.

But how can you know whether the new fertilizer is betterthan the one you have been using? Make a a groupused for comparison in an experiment. Add the currently used fertiliz-er to some pots, then put seeds in them. This group is the controlgroup. The control group and the experimental group are treatedequally except for the fertilizer used. Therefore, any change in plantgrowth is likely to be caused by the difference in fertilizers.

Check Your Reading How is a control group different from an experimental group?

control group,

experimental group

dependent variable.

independent variable.

This scientist is study-ing how changes in theamount of light andwater affect experi-mental groups ofplants and insects.

VOCABULARYAdd magnet word dia-grams for independentvariable, dependent vari-able, experimental group, and control group toyour notebook.

16 Chapter 1: The Nature of Science

Types of Scientific InquiriesDoing science does not always require laboratory equipment. Indeed,many types of scientific inquiries cannot take place in a lab. Somecommon types of inquiries are described below and on the next page.

Laboratory Experiments In a laboratory, scientists can regulatemost conditions. Variables are easier to control, and uncontrolledvariables are easier to reduce or correct for. The subject of a lab-oratory experiment can be tiny, such as an inquiry into thearrangement of a substance’s atoms. Or it can be huge, such asan inquiry into tree growth in a field enclosed by a greenhouse.

Fieldwork Inquiries that are done outside a laboratory are calledfieldwork. Variables are more difficult to control, or they may notbe controlled at all. Scientists do fieldwork when, for example, theywant to investigate animal behavior in the wild. Some subjects—such as relationships among rock layers in a mountain—are toolarge to be studied without doing fieldwork.

Surveys A survey involves examining a part of a group and thenusing the results to draw conclusions about the whole group.This type of survey requires data from direct observations, notfrom asking questions of people. For example, scientists mightwant to know the numbers of organisms in the soil of an area. Itis not possible to count all the organisms without destroying thearea. Instead, the scientists could examine samples of the soil atdifferent locations. They would infer that the rest of the soil heldsimilar numbers of organisms.

Models Some things are too large, are too small, are too far away,or take too long to study directly. Other things are too dangerousor too expensive to test directly. For example, scientists cannotstudy directly how stars form. Such things are often studied byusing which are representations of objects or processes.In a computer model, mathematical equations and techniques areused to represent the behavior of things like stars or volcanic erup-tions. In a physical model, objects are used to represent otherobjects—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 surveymight be part of a larger program of fieldwork. Data from severaltypes of inquiries might be used to build a model. Then scientists cancontrol variables in the model that would be impossible to control inreal life. The results might suggest new effects to look for in fieldworkor might suggest new experiments to design.

Check Your Reading What questions do you have about different types of scientific inquiries?

models,

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In this physical model,spheres are used torepresent atoms in a molecule.

Chapter 1: The Nature of Science 17

The type of inquiry scientists choose to use depends on the topicthey are investigating. Often, scientists use more than one typeof inquiry as they study different aspects of a topic.

Scientific Inquiries

Why might scientists who are studying the formation of tornadoes inthunderstorms do fieldwork and use models?

Surveys3

Laboratory Experiments1 Fieldwork2

Models4

When scientists do a laboratory experiment, theycan more easily change just one variable at atime, so they get clearer results. The photographshows a scientist using laboratory equipment toanalyze a substance.

Scientist Jane Goodall began fieldwork in 1960to study chimpanzees. At that time, scientiststhought that only humans used tools. Goodall’sobservations of chimpanzees in their naturalhabitat showed that they use tools too.

This computer model represents a proteinmolecule. The model, which is much larger thanthe actual molecule, allows scientists to study thestructure of the molecule in detail.

This researcher is working with an ice core collected in Antarctica. Each core gives informa-tion about what climatic conditions were likewhen the ice formed.

18 Chapter 1: The Nature of Science

New evidence can change scientific ideas.Scientists make various types of inquiries as they explore a topic. Theydevelop, test, and either discard or improve hypotheses. Scientists lookfor relationships among different types of evidence and develop ideasthat account for as much of the evidence as possible.

Science is always an ongoing process. If new evidence is discoveredthat does not support an idea or if scientists begin to think about oldevidence in a new way, then the idea can be overturned. An example ishow scientific understanding of the solar system has changed.

In the mid-1900s, scientists thought that the solar system was stableand unchanging. They understood the motions of planets and moonsvery well. Scientists did not think that small objects in the solar systemcould cause large changes to moons or planets, including Earth.

New EvidenceOver time, different types of scientists made observations that seemedinconsistent with the idea of an unchanging solar system. Major chal-lenges to this idea developed from inquiries into topics that seemedunrelated, including

• circular features on the Moon

• rock layers on Earth

• the extinction of the dinosaurs

Together, these inquiries provided evidence that the accepted idea could not explain.

Scientists studying Earth’s moon noticedcircular features on its surface. They did notknow if the features were caused by volcanoesor by collisions of smaller objects with theMoon. Then, astronauts exploring the Moonbrought back samples of rock that showedthat the features were from collisions, orimpacts. Most of the impacts had taken placea very long time ago. Spacecraft showed thatother planets and moons had been hit manytimes by small objects. By comparison, evi-dence of impacts on Earth’s surface seemed rare. Just a few circularfeatures on Earth were shown to be caused by impacts.

At one time, scientistsused models such as thisone to study the solar system. Now, scientistsrealize that such modelsdo not represent manyimportant processes.

The Moon’s surface showsmany circular features created by the impact ofsmall objects.

MAIN IDEA WEBAdd to your notebookinformation about how scientific ideas change.

Chapter 1: The Nature of Science 19

Meanwhile, scientists studying rock layers on Earth found an element—iridium (ih-RIHD-ee-uhm)—in a thin layer that formedabout 65 million years ago. Iridium is more common in rocks fromspace than in rocks formed on Earth. The scientists developed ahypothesis that a huge rock from space hit Earth 65 million years ago.They suggested that the impact destroyed the rock and threw hugeamounts of iridium-rich dust into the atmosphere. The dust spreadaround the world before settling on Earth’s surface.

Models of nuclear explosions suggested that large clouds of dustwould make the weather cooler by reflecting some of the sunlight thatwarms Earth’s surface. The scientists suspected thatdust from the impact could suddenly have pro-duced cooler weather on Earth. Scientists whostudy the patterns of life on Earth already knewthat many species, including dinosaurs, hadbecome extinct about 65 million years ago. Perhapsthe extinctions were related to the impact.

The scientists who were studying impact fea-tures added information about the age of theiridium layer and the timing of the extinctions totheir knowledge. At first, the hypothesis that a rockfrom space had hit Earth 65 million years agoseemed unlikely to many scientists, because theyheld the idea that the solar system was stable andunchanging.

Building a New IdeaPart of the work scientists did to test the new hypothesis involvedlooking for an iridium layer in other 65-million-year-old rocks. Thescientists found the layer in locations around the world. They alsolooked for and found evidence of additional impacts. By the late1980s, many scientists accepted that large numbers of small objectshad hit and changed Earth, other planets, and moons. They realizedthat more impacts are likely in the future. Scientists now think thatthe solar system is dynamic and changing.

The hypothesis that an impact of a space rock with Earth occurred65 million years ago developed into a theory. A is a well-testedexplanation that brings together many sets of scientific observations.Theories are the big ideas of science. Details of a theory can change asmore evidence is discovered. More rarely, a theory is overturned bynew evidence and eventually replaced with a new theory.

Check Your Reading How do scientific ideas change?

theory

The extinction of the dinosaurs is now thoughtto be related to the impact of a space objectwith Earth about 65 million years ago.

20 Chapter 1: The Nature of Science

Results and conclusions may be challenged.Scientists must accurately describe the work that led to the results andconclusions they report. Such openness allows other scientists to judgethe work’s credibility and to reproduce the work if they want to. If areport lacks this information, scientists often reject the results or con-clusions. If a report is complete, then any scientist can decide whetheror not to accept the results.

Sometimes scientists accept the results of an inquiry but not theconclusions. Even when scientists think that results are valid, theymight think that the conclusions are not supported by enough evidence.The conclusion that a space object collided with Earth 65 millionyears ago was not widely accepted until many types of evidence werediscovered that all supported the same conclusion.

When scientists report that they have discovered evidence thatchallenges a widely accepted idea, their results may lead to controversy.A period of uncertainty and argument may follow while scientists tryto confirm or disprove the results. If further results and other evidencesupport the challenge, more and more scientists begin to reject the accepted idea.

Controversial ResultsMany factors can lead to controversy. Anew result can be controversial at first.If a new result does not support earlierresults or theories, scientists first testwhether the new result can be repro-duced. Then they try to understandhow it fits with what they already know.Sometimes, a new result or idea changeshow scientists think about an entireaspect of the natural world.

The hypothesis that prions (PREE-ahnz), a type of protein, cancause infectious diseases was very controversial. Prions are nowthought to cause infections such as mad cow disease, a fatal disorderthat affects the structure of cows’ brains. A form of the disease can bepassed to people when they eat meat from infected cows.

Most scientists at first did not accept that prions could multiplyand cause infections. Unlike bacteria or viruses, prions cannot reproduce because they have no genetic material. Therefore, scientistsdid not believe that a few prions passed to an organism could cause aninfection. In 1982, however, a U.S. scientist named Stanley Prusinerhypothesized that prions can cause the shape of normal proteins to

reading tip

The word controversy refersto a dispute between peopleholding opposite views. Ifpeople agree, a subject isnoncontroversial, but if theydisagree, it is controversial.

Prions, such as the one shown in this computer model, are proteins that can multiply andcause diseases by causing normalproteins to change their shapes.

RESOURCE CENTERCLASSZONE.COM

Learn more about specialproteins known as prions.

change, making new prions. Although some scientists are still skepticalabout Prusiner’s hypothesis, he was awarded the Nobel prize inmedicine in 1997.

Some controversies occur when groups of scientists get differentresults when doing similar inquiries. For example, teams at two labora-tories might analyze the same substance but find different materials. Acontroversy could result over which results are correct. The controversyends if an explanation is found. Perhaps some samples of the substancewere contaminated, or a procedure was performed incorrectly.

Controversies can also be the result of bias. Scientists can be biasedif they become too attached to particular ideas. They may unknowing-ly choose data, design inquiries, or make assumptions that supportthese ideas. People are more likely to suggest the possibility of biaswhen results are personally unfavorable to them. For example,research showing that a material is harmful may be challenged by amanufacturer that uses the material.

Check Your Reading Give two reasons why scientific results might be controversial.

Noncontroversial ResultsResults that can be easily reproduced are rarely controversial. Forexample, a chemist might analyze a substance with common equipmentand accepted methods. The results of the analysis are noncontroversialbecause scientists who question them can repeat the analysis them-selves. Results that are similar to previous results or that confirm atheory in a new way are likely to be noncontroversial.

Chapter 1: The Nature of Science 21

KEY CONCEPTS1. What is the purpose of

controlled variables in anexperiment?

2. Briefly describe four commontypes of scientific inquiries.

3. How can a widely acceptedtheory be replaced with a new theory?

CRITICAL THINKING4. Apply What types of inquiries

would be most appropriate forstudying how mountains form?Explain.

5. Synthesize How can a scien-tific controversy help advanceunderstanding of the naturalworld?

CHALLENGE6. Identify Limits What sorts of

problems might be associatedwith using models to representactual features or processes?

reading tip

The word bias comes fromthe French word biais,meaning “slant.” A biasedperson might not consideran idea fairly or might notconsider all possibilities.