High School Chemistry/The Scientific Method

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What hopes and fears does this scientific method imply for mankind? I do not think that this is the right way to put the question. Whatever this tool in the hand of man will produce depends entirely on the nature of the goals alive in this mankind. Once the goals exist, scientific method furnishes means to realize them. Yet it cannot furnish the very goals. The scientific method itself would not have led anywhere, it would not even have been born without a passionate striving for clear understanding

Lesson Objectives[edit | edit source]

  • Briefly examine the history of science and the scientific method.
  • Describe the steps involved in the scientific method, and develop an appreciation for the value of the scientific method.
  • Recognize that in some cases not all the steps in the scientific method occur, or they do not occur in a specific order.
  • Explain the necessity of experimental controls, and recognize their presence in experiments.

Historical Comparisons[edit | edit source]

Introduction to Science[edit | edit source]

What is science? Is it a list of marvelous inventions and how they work? Or is it a list of theories about matter and energy and biological systems? Or is science a subject that you learn by carrying out activities in a laboratory? Science is all of these, but it is also something even more basic. Science is a method of thinking that allows us to discover how the world around us works.

To begin this study of one form of science, we will review the last 3,000 years in the history of human transportation, communication, and medicine. The following summary lists humankind's accomplishments in these areas during three periods in the last 3,000 years.

Transportation in 1000 B.C.[edit | edit source]

In 1000 B.C., people could transport themselves and their goods by walking, riding an animal, or by riding in a cart pulled by an animal (Figure 1.1). Crossing water, people could paddle a boat or have an animal walk beside the river and pull the boat (Figure 1.2). These methods of transportation required muscle power, either human muscles or animal muscles.

Figure 1.1: A horse-drawn Egyptian chariot. Chariots originated in Mesopotamia around 3000 B.C.

A few societies had designed rowboats or sailboats, which used muscle power or the force of the wind to move the boat. These early means of transportation were very limited in terms of speed and therefore, also limited in terms of distances traveled. The sail and rowboats were used on rivers and inland seas, but were not ocean-going vessels.

Figure 1.2: A photo of a wooden model of a Greek ship that has both sails and oars.

Transportation in 1830[edit | edit source]

By the year 1830, people were still walking and riding in carts pulled by animals. Iron ore was moved along canals by animals pulling barges. American pioneers crossed the United States in covered wagons pulled by animals (Figure 1.3). Large cities had streetcars pulled by horses (Figure 1.4). Ocean crossing was accomplished in sailing ships. The only improvement in transportation was the addition of springs and padded seats to carts and wagons to make the ride less jolting. In the period from 1000 B.C. to 1830, a span of 2,830 years (about 100 generations of people), there were no significant changes in the mode of human transportation.

Figure 1.3: A covered wagon of the type used by pioneers to cross the US in the mid-1800s.
Figure 1.4: The first horse-drawn street car in Seattle, Washington in 1884.

Transportation in 1995[edit | edit source]

By the year 1995, steam engines, gasoline engines, automobiles, propeller-driven and jet engines, locomotives, nuclear-powered ships, and inter-planetary rocket ships were invented (Figure 1.5). In all industrialized countries, almost anyone could own an automobile and travel great distances in very short times.

Figure 1.5: A modern jetliner.
Figure 1.6: Shopping list chiseled on a rock.

In the mid-1800s, several months were required to travel from Missouri to California by covered wagon and the trip was made at considerable risk to the traveler’s life. In 1995, an average family could travel this same distance easily in two days and in relative safety. An ordinary person in 1995 probably traveled a greater distance in one year than an ordinary person in 1830 did in an entire lifetime. The significant changes in the means of transportation in the 165 years between 1830 and 1995 (perhaps 5 generations) were phenomenal.

Communication in 1000 B.C.[edit | edit source]

Essentially, people’s only means of communicating over large distances (more than 15 miles) in 1000 B.C. was to send hand-carried messages (Figure 1.6). Some societies, for short distances, had developed the use of smoke signals, light signals, or drum signals, but these methods were useless for long distances. Since the means of communicating required hand-carried messages, the speed of communication was limited by the speed of transportation. Sending messages over distances of 1,000 miles could require several weeks and even then delivery was not guaranteed.

Communication in 1830[edit | edit source]

Figure 1.7: A Pony Express rider, circa 1861.

By the year 1830, people's means of communication over large distances was still the hand-carried message. While the paper and ink used to write the message had been improved, it still had to be hand-carried. In the United States, communication between New York and San Francisco required more than a month. When a new president was elected, Californians would not know who it was for a couple of months after the election.

For a short period of time, the Pony Express was set up and could deliver a letter from St. Louis, Missouri to Sacramento, California in eleven days, which was amazing at the time (Figure 1.7). The means of communication in 1830 was essentially the same as in 1000 B.C.

Communication in 1995[edit | edit source]

By the year 1995, the telegraph, telephone, radio, television, optical fibers, and communication satellites were invented (Figure 1.8). People could communicate almost anywhere in the industrialized world instantaneously. Now, when a U.S. president is elected, people around the globe know the name of the new president the instant the last vote is counted.

Figure 1.8: A modern cell phone.

Astronauts communicate directly between the earth and the moon. An ordinary person in an industrialized country can speak with people around the world while simultaneously watching events occur in real time globally. There have been truly extraordinary changes in people’s ability to communicate in the last 165 years.

Medical Treatment in 1000 B.C.[edit | edit source]

Medical treatment in 1000 B.C. consisted of a few natural herbs and some superstitious chants and dances. The most advanced societies used both sorcerers and herbalists for medical treatment. Some of the natural herbs helped the patient and some did not. Cleaning and bandaging wounds decreased opportunity for infection while some herbs such as sesame oil demonstrated moderate antiseptic properties. Dances, chants, incense burning, and magic spells were absolutely useless in curing illnesses. At some point in time, bloodletting was added to the physician’s repertoire (Figure 1.9). Bloodletting was accomplished by cutting the patient and allowing the blood to drip out or by applying leeches (which doctors often carried with them). However, bloodletting was not helpful to the patient, and in many cases, it was harmful. Bloodletting was flourishing by 500 B.C. and was carried out by both surgeons and barbers. It wasn’t until around 1875 that bloodletting was established as quackery.

Figure 1.9: Physician letting blood from a patient.

In those times, for an ordinary person, broken bones went unset and injuries like deep cuts or stab wounds were often fatal due to infection. Infant mortality was high and it was common for at least one child in a family to die before adulthood. The death of the mother in childbirth was also quite common.

In the Middle Ages, knowledge of germs, hygiene, and contagion was non-existent. People who were seriously ill might have their disease blamed on the planets going out of line (astrology) or "bad odors", or retribution for sins, or an imbalance in body fluids. Cures could involve anything from magic spells, bleeding, sweating, and vomiting to re-balance bodily fluids. Between 1340 AD and 1348 AD, the Black Death (bubonic plague) was responsible for killing in the vicinity of half the population of Europe. The bacterium causing the disease was carried by fleas, but, of course, none of this was known by the physicians of the time. Efforts to stop the plague included burning incense to eliminate "bad odors", causing loud noises to chase the plague away (the constant ringing of bells or firing of cannons), and a number of people used self-flagellation to attempt to cure the disease.

Medical Treatment in 1830[edit | edit source]

Medical treatment in 1830 remained in the form of natural herbs and bloodletting. During this time, the ability to set broken bones and to amputate limbs was also developed. Amputation saved many lives from infection and gangrene. Gangrene occurs when the blood supply to tissue is interrupted and the tissue dies. The dead tissue remains part of the body, invites infection, and causes death as the poisons from the rotting tissue are carried through the body. Once gangrene afflicted an arm or leg, the poison from the limb would eventually kill the patient. During the American Civil War (1861 - 1863), a common means of treatment for wounds in field hospitals was amputation. Along with amputation was the ability to cauterize wounds to stop bleeding.

Even though bloodletting did not help patients, it continued in use through 1830. There is a tale (which may or may not be true) that George Washington was suffering from pneumonia and his doctors removed so much blood trying to cure him that they actually caused his death.

Medical Treatment in 1995[edit | edit source]

By 1995, medical science had discovered chemical medicines, antiseptic procedures, surgery, and probably most important of all, vaccination, the ability to prevent disease rather than cure it after it had been contracted (Figure 1.10).

Figure 1.10: Receiving a vaccination.

Diseases that had killed and crippled hundreds of thousands of people in the past are seldom heard of today (polio, smallpox, cholera, bubonic plague, etc.). These diseases have been controlled by scientific understanding of their causes and carriers and by vaccination. Average life expectancy has nearly doubled in the last 165 years. Both infant mortality and death during childbirth rates have dropped to less than 25% of what they were in 1830.

Methods of Learning About Nature[edit | edit source]

Opinion, Authority, and Superstition[edit | edit source]

Why did humans make so little progress in the 2,800 years before 1830 and then such incredible progress in the 160 years after?

Socrates (469 B.C. - 399 B.C.), Plato (427 B.C. - 347 B.C.), and Aristotle (384 B.C. - 322 B.C.) are three of the most famous Greek philosophers. Plato was a student of Socrates and Aristotle was a student of Plato. These three were probably the greatest thinkers of their time. Aristotle's views on physical science profoundly shaped medieval scholarship and his influence extended into the Renaissance (14th century). Aristotle's opinions were the authority on nature until well into the 1300s.

Unfortunately, many of Aristotle's opinions were incorrect. Aristotle was, without a doubt, a brilliant man, however, he used a method for determining the nature of the physical world that is inadequate for that task. His method was logical thinking, not making observations in the natural world. This led to many errors in Aristotle’s thinking on nature. Let's consider just two of Aristotle's opinions as examples.

In Aristotle’s opinion, men were bigger and stronger than women, and therefore, it was logical that men would have more teeth than women. Therefore, Aristotle concluded it was a true fact that men had more teeth than women. Apparently, it never entered his mind to actually look into the mouths of both genders and count their teeth. Had he done so, he would have found that men and women have exactly the same number of teeth.

In terms of physical science, Aristotle thought about dropping two balls of exactly the same size and shape but of different masses to see which one would strike the ground first. In his mind, it was clear that the heavier ball would fall faster than the lighter one and he concluded that this was a law of nature. Once again, he did not consider doing an experiment to see which ball fell faster. It was logical to him, and in fact, it still seems logical. If someone told you that the heavier ball would fall faster, you would have no reason to disbelieve it. In fact, it is not true and the best way to prove this is to try it. Eighteen centuries later, Galileo decided to actually get two balls of different masses, but with the same size and shape, and drop them off a building (according to legend, the Leaning Tower of Pisa), and actually see which one hit the ground first. When Galileo actually did the experiment, he discovered, by observation, that the two balls hit the ground at exactly the same time. Aristotle's opinion was, once again, incorrect. The method of the philosopher, pure logic, is more appropriate for the unobservable, that is, for studying things outside the realm of science.

Leaning Tower of Pisa experiment
Leaning Tower of Pisa experiment

In the 16th and 17th centuries, innovative thinkers were developing a new way to discover the nature of the world around them. They were developing a method that relied upon making observations of phenomena and insisting that their explanations of the nature of the phenomena corresponded to the observations they made. In order to do this, they had to overcome the opinions of the ancient Greeks, the authority of the church, and the superstitions of ordinary people.

In the opinion of the ancient Greeks, the earth was the center of the universe and did not move, while the sun, moon, planets, and stars revolved around the Earth in orbits. The astronomer Ptolemy (around 150 A.D.) observed the positions of the planets and recognized that the positions where he observed the planets did not match up with the positions predicted by the orbits of the Greeks. Ptolemy designed new orbits that had circles within circles and complicated retrograde motion (planets moving backward in their orbits at certain times). His descriptions came closer but still could not accurately predict where the heavenly orbs would be on a given night. It wasn’t until Nicolaus Copernicus (1473 - 1543) suggested a heliocentric (sun-centered) system that the positions of the planets came close to matching predictions. Copernicus was hesitant to publish his ideas—some say because he feared ridicule from his peers and others say because he feared persecution by the church—but eventually, he sent his work for publication just before his death.

Copernicus' heliocentric theory remained latent for the next 50 years until the it was supported by a scientist named Giordano Bruno, who was promptly prosecuted and burned at the stake by Cardinal Bellarmini in 1600. The heliocentric theory was against the beliefs of the church, and the majority, at the time. The most famous supporter of the Copernican system was Galileo Galilei (1564 - 1642) who developed an improved telescope (1610) and turned it toward the sky. Galileo published a small work describing what he had seen with his telescope and how his observations supported the Copernican theory. His book was banned by the church in 1616, and Galileo was instructed not to write about the subject any further. In 1632, Galileo published another work, again supporting the Copernican theory and was arrested and prosecuted by the church. Under threat of death, he was forced to recant, and was punished by house arrest for the remainder of his life.

But the method of learning by experimenting, observing, and hypothesizing had been launched and many scientists would not turn back. There are still supporters of the methods of opinion, authority, and superstition. With free speech, anyone can present a statement as fact, but without standing the test of the scientific method, that statement is not science.

The Scientific Method[edit | edit source]

Scientists frequently list the scientific method as a series of steps, however, not all steps occur in order. The scientific method is listed in a series of steps in Table 1.1 and represented in the flowchart (Figure 1.11) because it makes it easier to study.

Table 1.1: The Steps in the Scientific Method
Step Number Step Description
1 Identify the problem or phenomenon that needs explaining. This is sometimes referred to as "defining the problem". This activity helps limit the field of observations.
2 Gather and organize data on the problem. This step is also known as "making observations".
3 Suggest a possible solution or explanation. A suggested solution is called a hypothesis.
4 Test the hypothesis by making new observations.
5 If the new observations support the hypothesis, you accept the hypothesis for further testing. If the new observations do not agree with your hypothesis, you discard the hypothesis, add the new data to your observations list, and return to step 3.
Figure 1.11: Flowchart of the steps in the Scientific Method

When the results of several experiments support the hypothesis, you might think that the work is finished. However, for a hypothesis to be useful, it must withstand repeated testing. Other scientists must be able to repeat the experiments using the same materials and conditions and get the same results—this quality is known as reproducibility. Scientists submit reports of research to other scientists, usually by publishing an article in a scientific journal, so the work can be verified.

An Example of the Scientific Method[edit | edit source]

Table 1.2
Will Burn Won't Burn
tree limbs rocks
chair legs bricks
pencils marbles
baseball bat hubcaps

(Source: Richard Parsons)

Suppose you are required to maintain a large campfire and you are completely unfamiliar with the property of objects that makes them combustible (able to burn). The first step in the scientific method is to define the problem. What property of objects make them combustible?

The next step is to gather data on the problem. So, you begin to collect objects at random and put them into the fire. You must keep good records of what objects were tried and whether or not they burned. Table 1.2 shows a list of organized data (observations).

Table 1.3
Will Burn Won't Burn
tree limbs rocks
chair legs bricks
pencils marbles
baseball bat hubcaps
broom handle iron pipes
  soda bottles
  tin cans

The list of organized observations helps because now you can collect only the items on the "will burn" list and not waste the effort of dragging items that won't burn back to the fire. However, you would soon use up all the items on the "will burn" list and it is necessary to guess what property the "will burn" objects have that cause them to burn. If you had that answer, you could bring objects that may not be on the "will burn" list but that have the "will burn" property and keep the fire going.

The third step in the scientific method is to suggest a hypothesis. Your guess about what property the "will burn" objects have that makes them combustible is a hypothesis. Suppose you notice that all the items on the "will burn" list are cylindrical in shape and therefore, you hypothesize that "cylindrical objects burn". The fourth step in the scientific method is to test your hypothesis. To test this hypothesis, you go out and collect a group of objects that are cylindrical including iron pipes, soda bottles, broom handles, and tin cans. When these cylindrical objects are placed in the fire and most of them don’t burn, you realize your hypothesis is not supported by these new observations. The new observations are the test, and your hypothesis has failed the test. When the new observations fail to support your hypothesis, you reject your original hypothesis, add your new data to (Table 1.3), and make a new hypothesis based on the updated observations list. In the schematic diagram of the scientific method, a failed test returns the scientist to step 3, make a new hypothesis.

Suppose your new hypothesis is "wooden objects burn". You find this hypothesis more satisfactory since all the wooden object you try will burn. Your confidence may grow that you have discovered a "law of nature". Even with your somewhat successful theory, you might be ignoring a large stack of old car tires, objects made of fabric or paper, or perhaps containers of petroleum. You can see that even though you are quite satisfied with your theory because it does the job you want it to do, you actually do not have a complete statement on the property of objects that make them burn, because there is still unexamined (and perhaps unknown) evidence. This is often the case in science.

You can see from this example that the ”solution” does not become what we think of as a "fact", but rather becomes a tentatively accepted theory which must undergo continuous testing and perhaps adjustment. No matter how long a tentative explanation has been accepted, it can be discarded at any time if contradictory observations are found. As long as the theory is consistent with all observations, scientists will continue to use it. When a theory is contradicted by observations, it is discarded or modified to explain the new observations. Even though the terms hypothesis, theory, and fact are used somewhat interchangeably at times, a theory will continue to be used while it is useful and will be called into question when contradictory evidence is found. Theories never become facts.

One common generalization about theories is that "theories are much easier to disprove than to prove". A common example is the hypothesis, "all swans are white". You may observe a thousand white swans and every observation of a white swan supports your hypothesis, but it takes only a single observation of a black swan to disprove that hypothesis. To be an acceptable scientific hypothesis, observations that disprove the hypothesis must be possible. That is, if every conceivable observation supports the hypothesis, then it is not an acceptable scientific hypothesis. To be a scientific hypothesis, it must be possible to refute the concept. This property is known as falsifiability. Again, things that are unfalsifiable are outside the realm of science.

Some Basic Terminology[edit | edit source]

  • A hypothesis is a guess that is made early in the process of trying to explain some set of observations. There are scientists who object to calling a hypothesis a "guess". The primary basis for the objection is that someone who has studied the subject under consideration would make a much better guess than someone who was completely ignorant of the field. Perhaps we should say that a hypothesis is an "educated guess".
  • A theory is an explanation that stands up to everyday use in explaining a set of observations. A theory is not proven and is not a "fact". A scientific theory must be falsifiable in order to be accepted as a theory.
  • A law describes an observable relationship, that is, observations that occur with a predictable relationship to each other. It is only after experience shows the law to be valid that it is incorporated into the field of knowledge.

The Scientific Revolution[edit | edit source]

The explosion of achievement in the last 160 years was produced by using a new method for learning about nature. This sudden and massive achievement in understanding nature is called the Scientific Revolution and was produced using the scientific method.

The British historian, Herbert Butterfield, wrote a book called The Origins of Modern Science. In the preface to the book, Butterfield wrote:

The Revolution in science overturned the authority of not only the Middle Ages but of the ancient world … it ended not only in the eclipse of scholastic philosophy but in the destruction of Aristotelian physics. The Scientific Revolution outshines everything since the rise of Christianity and reduces the Renaissance and Reformation to the rank of mere episodes…

The beginning of the 17th century is known for the drastic changes that occurred in the European approach to science during that period and is known as the Scientific Revolution. This term refers to a completely new era of academic thought in which medieval philosophy was abandoned in favor of innovative methods offered by Galileo and Newton.

Science is best defined as a careful, disciplined, logical search for knowledge about any and all aspects of the universe, obtained by examination of the best available evidence and always subject to correction and improvement upon discovery of better evidence. What's left is magic. And it doesn't work.

—James Randi

What is an Experiment?[edit | edit source]

The scientific method requires that observations be made. Sometimes, the phenomenon we wish to observe does not occur in nature or if it does, it is inconvenient for us to observe. Therefore, it is more successful for us to cause the phenomenon to occur at a time and place of our choosing. When we arrange for the phenomenon to occur at our convenience, we can have all our measuring instruments present and handy to help us make observations, and we can control other variables. Causing a phenomenon to occur when and where we want it and under the conditions we want is called an experiment. When scientists conduct experiments, they are usually seeking new information or trying to verify someone else's data. Classroom experiments often demonstrate and verify information that is already known but new to the student. When doing an experiment, it is important to set up the experiment so that relationships can be seen clearly. This requires what are called experimental controls.

Experimental Controls[edit | edit source]

Figure 1.12

Suppose a scientist, while walking along the beach on a very cold day following a rainstorm, observed two pools of water in bowl shaped rocks near each other. One of the pools was partially covered with ice while the other pool had no ice on it. The unfrozen pool seemed to be formed from seawater splashing up on the rock from the surf, but the other pool was too high for sea water to splash in, so it was more likely to have been formed from rainwater.

The scientist wondered why one pool was partially frozen and not the other since both pools were at the same temperature. By tasting the water (not a good idea), the scientist determined that the unfrozen pool tasted saltier than the partially frozen one. The scientist thought perhaps salt water had a lower freezing point that fresh water and she decided to go home and try an experiment to see if this were true. So far, the scientist has identified a question, gathered a small amount of data, and suggested a hypothesis. In order to test this hypothesis, the scientist will conduct an experiment during which she can make accurate observations.

For the experiment, the scientist prepared two identical containers of fresh water and added some salt to one of them (Figure 1.12). A thermometer was placed in each liquid and these were put in a freezer. The scientist then observed the conditions and temperatures of the two liquids at regular intervals.

The Temperature and Condition of Fresh Water in a Freezer
Time (minutes) 0 5 10 15 20 25 30
Temp., °C 25 20 15 10 5 0 −5
Condition Liquid Liquid Liquid Liquid Liquid Frozen Frozen
The Temperature and Condition of Salt Water in a Freezer
Time (minutes) 0 5 10 15 20 25 30
Temp., °C 25 20 15 10 5 0 −5
Condition Liquid Liquid Liquid Liquid Liquid Liquid Frozen

The scientist found support for the hypothesis from this experiment; fresh water freezes at a higher temperature than salt water. Much more support would be needed before the scientist would be confident of this hypothesis. Perhaps she would ask other scientists to verify the work.

In the scientist's experiment, it was necessary that she freeze the salt water and fresh water under exactly the same conditions. Why? The scientist was testing whether or not the presence of salt in water would alter its freezing point. It is known that changing air pressure will alter the freezing point of water. In order to conclude that the presence of the salt was what caused the change in freezing point, all other conditions had to be identical. The presence of the salt is called the experimental variable because it is the only thing allowed to change in the two trials. The fresh water part of the experiment is called the experimental control. In an experiment, there may be only one variable and the purpose of the control is to guarantee that there is only one variable. The "control" is identical to the "test" except for the experimental variable. Unless experiments are controlled, the results are not valid.

Suppose you wish to determine which brand of microwave popcorn leaves the fewest unpopped kernels. You will need a supply of various brands of microwave popcorn to test and you will need a microwave oven. If you used different brands of microwave ovens with different brands of popcorn, the percentage of unpopped kernels could be caused by the different brands of popcorn, but it could also be caused by the different brands of ovens. Under such circumstances, the experimenter would not be able to conclude confidently whether the popcorn or the oven caused the difference. To eliminate this problem, you must use the same microwave oven for every test. By using the same microwave oven, you control the number of variables in the experiment.

What if you allowed the different samples of popcorn to be cooked at different temperatures? What if you allowed longer heating periods? In order to reasonably conclude that the change in one variable was caused by the change in another specific variable, there must be no other variables in the experiment. All other variables must be kept constant.

Errors in the Use of the Scientific Method[edit | edit source]

The scientific method requires the observation of nature and correspondence between the suggested explanation and the observations. That is, the hypothesis must explain all the observations. Therefore, the scientific method can only work properly when the data (observation list) is not biased. There are several ways in which a biased set of data can be produced. It is always possible for anyone to make an error in observation. A balance can be misread or numbers can be transposed when written down. That is one of the reasons that experiments are run several times and the observations made over and over again. It is also possible that an unrecognized error is present and produces the same error in every experiment. For example, a scientist may be attempting to test normal rainwater, but unknown to him a nearby factory is sending soluble substances out of their smoke stack and the material is contaminating the scientist’s samples. In such a case, the scientist's samples would yield false observations for normal rainwater. Other scientists reproducing the experiment would collect uncontaminated samples and find different results. Multiple testing of the experiment would determine which set of data was flawed. Failing to apply appropriate experimental controls would certainly bias data.

There are also dishonest mistakes that occur when the experimenter collects only supporting data and excludes contradictory observations. There have even been scientists who faked observations to provide support for his/her hypothesis. The attractions of fame and fortune can be hard to resist. For all these reasons, the scientific method requires that experimental results be published and the experiment be repeated by other scientists.

Lesson Summary[edit | edit source]

  • Before the development of the scientific method, mankind made only slight achievements in the areas of transportation, communication and medicine.
  • Use of the scientific method allowed mankind to make significant achievements in transportation, communication, and medicine.
  • The scientific method has been much more successful than the methods of superstition, opinion, and authority.
  • The steps in the scientific method are:
    1. Identify the problem
    2. Gather data (make observations)
    3. Suggest a hypothesis
    4. Test the hypothesis (experiment)
    5. Continue testing or reject the hypothesis and make a new one
  • Experimental controls are used to make sure that the only variables in an experiment are the ones being tested.

Review Questions[edit | edit source]

Figure 1.13

Use the following paragraph to answer the first two questions. In 1928, Sir Alexander Fleming was studying Staphylococcus bacteria growing in culture dishes. He noticed that a mold called Penicillium was also growing in some of the dishes. In Figure 1.13, Petri dish A represents a dish containing only Staphylococcus bacteria. The red dots in dish B represent Penicillium colonies. Fleming noticed that a clear area existed around the mold because all the bacteria grown in this area had died. In the culture dishes without the mold, no clear areas were present. Fleming suggested that the mold was producing a chemical that killed the bacteria. He decided to isolate this substance and test it to see if it would kill bacteria. Fleming grew some Penicillium mold in a nutrient broth. After the mold grew in the broth, he removed all the mold from the broth and added the broth to a culture of bacteria. All the bacteria died.

  1. Which of the following statements is a reasonable expression of Fleming’s hypothesis?
    (a) Nutrient broth kills bacteria.
    (b) There are clear areas around the Penicillium mold where Staphylococcus doesn't grow.
    (c) Mold kills bacteria.
    (d) Penicillium mold produces a substance that kills Staphylococcus.
    (e) Without mold in the culture dish, there were no clear areas in the bacteria.
  2. Fleming grew Penicillium in broth, then removed the Penicillium and poured the broth into culture dishes containing bacteria to see if the broth would kill the bacteria. What step in the scientific method does this represent?
    (a) Collecting and organizing data
    (b) Making a hypothesis
    (c) Testing a hypothesis by experiment
    (d) Rejecting the old hypothesis and making a new one
    (e) None of these

A scientific investigation is NOT valid unless every step in the scientific method is present and carried out in the exact order listed in this chapter.

(a) True
(b) False

Which of the following words is closest to the same meaning as hypothesis?

(a) fact
(b) law
(c) formula
(d) suggestion
(e) conclusion

Why do scientists sometimes discard theories?

(a) the steps in the scientific method were not followed in order
(b) public opinion disagrees with the theory
(c) the theory is opposed by the church
(d) contradictory observations are found
(e) congress voted against it

Gary noticed that two plants which his mother planted on the same day that were the same size when planted were different in size after three weeks. Since the larger plant was in the full sun all day and the smaller plant was in the shade of a tree most of the day, Gary believed the sunshine was responsible for the difference in the plant sizes. In order to test this, Gary bought ten small plants of the size and type. He made sure they had the same size and type of pot. He also made sure they have the same amount and type of soil. Then Gary built a frame to hold a canvas roof over five of the plants while the other five were nearby but out in the sun. Gary was careful to make sure that each plant received exactly the same amount of water and plant food every day.

  1. Which of the following is a reasonable statement of Gary’s hypothesis?
    (a) Different plants have different characteristics.
    (b) Plants that get more sunshine grow larger than plants that get less sunshine.
    (c) Plants that grow in the shade grow larger.
    (d) Plants that don’t receive water will die.
    (e) Plants that receive the same amount of water and plant food will grow the same amount.
  2. What scientific reason might Gary have for insisting that the container size for the all plants be the same?
    (a) Gary wanted to determine if the size of the container would affect the plant growth.
    (b) Gary wanted to make sure the size of the container did not affect differential plant growth in his experiment.
    (c) Gary want to control how much plant food his plants received.
    (d) Gary wanted his garden to look organized.
    (e) There is no possible scientific reason for having the same size containers.
  3. What scientific reason might Gary have for insisting that all plants receive the same amount of water everyday?
    (a) Gary wanted to test the effect of shade on plant growth and therefore, he wanted to have no variables other than the amount of sunshine on the plants.
    (b) Gary wanted to test the effect of the amount of water on plant growth.
    (c) Gary's hypothesis was that water quality was affecting plant growth.
    (d) Gary was conserving water.
    (e) There is no possible scientific reason for having the same amount of water for each plant every day.
  4. What was the variable being tested in Gary's experiment?
    (a) the amount of water
    (b) the amount of plant food
    (c) the amount of soil
    (d) the amount of sunshine
    (e) the type of soil
  5. Which of the following factors may be varying in Gary’s experimental setup that he did not control?
    (a) individual plant variation
    (b) soil temperature due to different colors of containers
    (c) water loss due to evaporation from the soil
    (d) the effect of insects which may attack one set of plants but not the other
    (e) All of the above are possible factors that Gary did not control

When a mosquito sucks blood from its host, it penetrates the skin with its sharp beak and injects an anti-coagulant so the blood will not clot. It then sucks some blood and removes its beak. If the mosquito carries disease-causing microorganisms, it injects these into its host along with the anti-coagulant. It was assumed for a long time that the virus of typhus was injected by the louse when sucking blood in a manner similar to the mosquito. But apparently this is not so. The infection is not in the saliva of the louse, but in the feces. The disease is thought to be spread when the louse feces come in contact with scratches or bite wounds in the host's skin. A test of this was carried out in 1922 when two workers fed infected lice on a monkey taking great care that no louse feces came into contact with the monkey. After two weeks, the monkey had NOT become ill with typhus. The workers then injected the monkey with typhus and it became ill within a few days. Why did the workers inject the monkey with typhus near the end of the experiment?

(a) to prove that the lice carried the typhus virus
(b) to prove the monkey was similar to man
(c) to prove that the monkey was not immune to typhus
(d) to prove that mosquitoes were not carriers of typhus
(e) the workers were mean

Eijkman fed a group of chickens exclusively on rice whose seed coat had been removed (polished rice or white rice). The chickens all developed polyneuritis (a disease of chickens) and died. He fed another group of chickens unpolished rice (rice that still had its seed coat). Not a single one of them contracted polyneuritis. He then gathered the polishings from rice (the seed coats that had been removed) and fed the polishings to other chickens that were sick with polyneuritis. In a short time, the birds all recovered. Eijkman had accurately traced the cause of polyneuritis to a faulty diet. For the first time in history, a food deficiency disease had been produced and cured experimentally. Which of the following is a reasonable statement of Eijkman’s hypothesis?

(a) Polyneuritis is a fatal disease for chickens.
(b) White rice carries a virus for the disease polyneuritis.
(c) Unpolished rice does not carry the polyneuritis virus.
(d) The rice seed coat contains a nutrient that provides protection for chickens against polyneuritis.
(e) None of these is a reasonable statement of Eijkman's hypothesis.

The three questions below relate to the following paragraphs.

Scientist A noticed that in a certain forest area, the only animals inhabiting the region were giraffes. He also noticed that the only food available for the animals was on fairly tall trees and as the summer progressed, the animals ate the leaves high and higher on the trees. The scientist suggested that these animals were originally like all other animals but generations of animals stretching their necks to reach higher up the trees for food, caused the species to grow very long necks.

Scientist B conducted experiments and observed that stretching muscles does NOT cause bones to grow longer nor change the DNA of animals so that longer muscles would be passed on to the next generation. Scientist B, therefore, discarded Scientist A's suggested answer as to why all the animals living in the area had long necks. Scientist B suggested instead that originally many different types of animals including giraffes had lived in the region but only the giraffes could survive when the only food was high in the trees, and so all the other species had left the area.

  1. Which of the following statements is an interpretation, rather than an observation?
    A. The only animals living in the area were giraffes.
    B. The only available food was on tall trees.
    C. Animals which constantly stretch their necks will grow longer necks.
    D. A, B, and C are all interpretations.
    E. A, B, and C are all observations.
  2. Scientist A's hypothesis was that
    A. the only animals living in the area were giraffes.
    B. the only available food was on tall trees.
    C. animals which constantly stretch their necks will grow longer necks.
    D. the animals which possess the best characteristics for living in an area, will be the predominant species.
    E. None of the above are reasonable statements of Scientist A's hypothesis.
  3. Scientist A's hypothesis being discarded is
    A. evidence that the scientific method doesn’t always work.
    B. a result achieved without use of the scientific method.
    C: an example of what happened before the scientific method was invented.
    D. an example of the normal functioning of the scientific method.
    E. an unusual case.

When a theory has been known for a long time, it becomes a law.

(a) True
(b) False

During Pasteur's time, anthrax was a widespread and disastrous disease for livestock. Many people whose livlihood was raising livestock lost large portions of their herds to this disease. Around 1876, a horse doctor in eastern France named Louvrier, claimed to have intvented a cure for anthrax. The influential men of the community supported Louvrier's claim to have cured hundreds of cows of anthrax. Pasteur went to Louvrier's hometown to evaluate the cure. The cure was explained to Pasteur as a multi-step process during which: 1) the cow was rubbed vigorously to make her as hot as possible; 2) long gashes were cut into the cows skin and turpentine was poured into the cuts; 3) an inch-thick coating of cow manure mixed with hot vinegar was plastered onto the cow and the cow was completely wrapped in a cloth. Since some cows recover from anthrax with no treatment, performing the cure on a single cow would not be conclusive, so Pasteur proposed an experiment to test Louvrier's cure. Four healthy cows were to be injected with anthrax microbes, and after the cows became ill, Louvrier would pick two of the cows (A and B) and perform his cure on them while the other two cows (C and D) would be left untreated. The experiment was performed and after a few days, one of the untreated cows died and one of them got better. Of the cows treated by Louvrier's cure, one cow died and one got better. In this experiment, what was the purpose of infecting cows C and D?

(a) So that Louvrier would have more than two cows to choose from.
(b) To make sure the injection actually contained anthrax.
(c) To serve as experimental controls (a comparison of treated to untreated cows).
(d) To kill as many cows as possible.

A hypothesis is

(a) a description of a consistent pattern in observations.
(b) an observation that remains constant.
(c) a theory that has been proven.
(d) a tentative explanation for a phenomenon.

A number of people became ill after eating oysters in a restaurant. Which of the following statements is a hypothesis about this occurrence?

(a) Everyone who ate oysters got sick.
(b) People got sick whether the oysters they ate were raw or cooked.
(c) Symptoms included nausea and dizziness.
(d) The cook felt really bad about it.
(e) Bacteria in the oysters may have caused the illness.

Which statement best describes the reason for using experimental controls?

(a) Experimental controls eliminate the need for large sample sizes.
(b) Experimental controls eliminate the need for statistical tests.
(c) Experimental controls reduce the number of measurements needed.
(d) Experimental controls allow comparison between groups that are different in only one independent variable.

A student decides to set up an experiment to determine the relationship between the growth rate of plants and the presence of detergent in the soil. He sets up 10 seed pots. In five of the seed pots, he mixes a precise amount of detergent with the soil and the other five seed pots have no detergent in the soil. The 5 seed pots with detergent are placed in the sun and the five seed pots with no detergent are placed in the shade. All 10 seed pots receive the same amount of water and the same number and type of seeds. He grows the plants for two months and charts the growth every two days. What is wrong with his experiment?

(a) The student has too few pots.
(b) The student has two independent variables.
(c) The student has two dependent variables.
(d) The student has no experimental control on the soil.

A scientist plants two rows of corn for experimentation. She puts fertilizer on row 1 but does not put fertilizer on row 2. Both rows receive the same amount of sun and water. She checks the growth of the corn over the course of five months. What is acting as the control in this experiment?

(a) Corn without fertilizer.
(b) Corn with fertilizer.
(c) Amount of water.
(d) Height of corn plants.

If you have a control group for your experiment, which of the following is true?

(a) There can be more than one difference between the control group and the test group, but not more three differences or else the experiment is invalid.
(b) The control group and the test group may have many differences between them.
(c) The control group must be identical to the test group except for one variable.
(d) None of these are true.

If the hypothesis is rejected by the experiment, then:

(a) the experiment may have been a success.
(b) the experiment was a failure.
(c) the experiment was poorly designed.
(d) the experiment didn't follow the scientific method.

A well-substantiated explanation of an aspect of the natural world is a:

(a) theory.
(b) law.
(c) hypothesis.
(d) None of these.

Vocabulary[edit | edit source]

The act of conducting a test to create observations that will prove or disprove the hypothesis.
A proposal intended to explain a set of observations.
A relationship that exists between specific observations.
scientific method
A method of investigation involving observation to generate and test hypotheses and theories.
A hypothesis that has been supported with repeated testing.
In science, something with results that can be obtained by multiple scientists in different times and in different places, usually an experiment.
Something that can be proved false, usually a hypothesis.

The Science of Chemistry · Chemistry in History

This material was adapted from the original CK-12 book that can be found here. This work is licensed under the Creative Commons Attribution-Share Alike 3.0 United States License