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1.2: Science- Reproducible, Testable, Tentative, Predictive, and Explanatory

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  • Learning Objectives

    • Describe the differences between hypothesis and theory as scientific terms.
    • Describe the difference between a theory and scientific law.
    • Identify the components of the scientific method.

    Although many have taken science classes throughout their course of studies, incorrect or misleading ideas about some of the most important and basic principles in science are still commonplace. Most students have heard of hypotheses, theories, and laws, but what do these terms really mean? Before you read this section, consider what you have learned about these terms previously, and what they mean to you. When reading, notice if any of the text contradicts what you previously thought. What do you read that supports what you thought?

    What is a Fact?

    A fact is a basic statement established by experiment or observation. All facts are true under the specific conditions of the observation.

    What is a Hypothesis?

    One of the most common terms used in science classes is a "hypothesis". The word can have many different definitions, dependent on the context in which it is being used:

    • An educated guess: a scientific hypothesis provides a suggested solution based on evidence.
    • Prediction: if you have ever carried out a science experiment, you probably made this type of hypothesis, in which you predicted the outcome of your experiment.
    • Tentative or proposed explanation: hypotheses can be suggestions about why something is observed. In order for a hypothesis to be scientific, a scientist must be able to test the explanation to see if it works, and if it is able to correctly predict what will happen in a situation. For example, "if my hypothesis is correct, I should see _____ result when I perform _____ test."

    A hypothesis is tentative; it can be easily changed.

    What is a Theory?

    The United States National Academy of Sciences describes a theory as:

    "Some scientific explanations are so well established that no new evidence is likely to alter them. The explanation becomes a scientific theory. In everyday language a theory means a hunch or speculation. Not so in science. In science, the word theory refers to a comprehensive explanation of an important feature of nature supported by facts gathered over time. Theories also allow scientists to make predictions about as yet unobserved phenomena."

    "A scientific theory is a well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experimentation. Such fact-supported theories are not "guesses," but reliable accounts of the real world. The theory of biological evolution is more than "just a theory." It is as factual an explanation of the universe as the atomic theory of matter (stating that everything is made of atoms) or the germ theory of disease (which states that many diseases are caused by germs). Our understanding of gravity is still a work in progress. But the phenomenon of gravity, like evolution, is an accepted fact."

    Note some key features of theories that are important to understand from this description:

    • Theories are explanations of natural phenomenon. They aren't predictions (although we may use theories to make predictions). They are explanations of why something is observed.
    • Theories aren't likely to change. They have a lot of support and are able to explain many observations satisfactorily. Theories can, indeed, be facts. Theories can change in some instances, but it is a long and difficult process. In order for a theory to change, there must be many observations or evidence that the theory cannot explain.
    • Theories are not guesses. The phrase "just a theory" has no room in science. To be a scientific theory carries a lot of weight—it is not just one person's idea about something

    Theories aren't likely to change.

    What is a Law?

    Scientific laws are similar to scientific theories in that they are principles that can be used to predict the behavior of the natural world. Both scientific laws and scientific theories are typically well-supported by observations and/or experimental evidence. Usually, scientific laws refer to rules for how nature will behave under certain conditions, frequently written as an equation. Scientific theories are overarching explanations of how nature works, and why it exhibits certain characteristics. As a comparison, theories explain why we observe what we do, and laws describe what happens.

    For example, around the year 1800, Jacques Charles and other scientists were working with gases to, among other reasons, improve the design of the hot air balloon. These scientists found, after numerous tests, that certain patterns existed in their observations of gas behavior. If the temperature of the gas increased, the volume of the gas increased. This is known as a natural law. A law is a relationship that exists between variables in a group of data. Laws describe the patterns we see in large amounts of data, but do not describe why the patterns exist.

    Laws vs Theories

    A common misconception is that scientific theories are rudimentary ideas that will eventually graduate into scientific laws when enough data and evidence has been accumulated. A theory does not change into a scientific law with the accumulation of new or better evidence. Remember, theories are explanations; laws are patterns seen in large amounts of data, frequently written as an equation. A theory will always remain a theory, a law will always remain a law.

    Video \(\PageIndex{1}\) What is the difference between scientific law and theory?

    The Scientific Method

    Scientists search for answers to questions and solutions to problems by using a procedure called the scientific method. This procedure consists of making observations, formulating hypotheses, and designing experiments, which in turn lead to additional observations, hypotheses, and experiments in repeated cycles (Figure \(\PageIndex{1}\)).

    Figure \(\PageIndex{1}\) The steps in the scientific method.

    Step 1: Make observations.

    Observations can be qualitative or quantitative. Qualitative observations describe properties or occurrences in ways that do not rely on numbers. Examples of qualitative observations include the following: "the outside air temperature is cooler during the winter season," "table salt is a crystalline solid," "sulfur crystals are yellow," and "dissolving a penny in dilute nitric acid forms a blue solution and a brown gas." Quantitative observations are measurements, which by definition consist of both a number and a unit. Examples of quantitative observations include the following: "the melting point of crystalline sulfur is 115.21° Celsius," and "35.9 grams of table salt—the chemical name of which is sodium chloride—dissolve in 100 grams of water at 20° Celsius." For the question of the dinosaurs’ extinction, the initial observation was quantitative: iridium concentrations in sediments dating to 66 million years ago were 20–160 times higher than normal.

    Step 2: Formulate a hypothesis.

    After deciding to learn more about an observation or a set of observations, scientists generally begin an investigation by forming a hypothesis, a tentative explanation for the observation(s). The hypothesis may not be correct, but it puts the scientist’s understanding of the system being studied into a form that can be tested. For example, the observation that we experience alternating periods of light and darkness which correspond to observed movements of the sun, moon, clouds, and shadows, is consistent with either of two hypotheses:

    1. Earth rotates on its axis every 24 hours, alternately exposing one side to the sun.
    2. The sun revolves around Earth every 24 hours.

    Suitable experiments can be designed to choose between these two alternatives. In the case of disappearance of the dinosaurs, the hypothesis was that the impact of a large extraterrestrial object caused their extinction. Unfortunately (or perhaps fortunately), this hypothesis does not lend itself to direct testing by any obvious experiment, but scientists can collect additional data that either supports or refutes it.

    Step 3: Design and perform experiments.

    After a hypothesis has been formed, scientists conduct experiments to test its validity. Experiments are systematic observations or measurements, preferably made under controlled conditions—that is, under conditions in which a single variable changes.

    Step 4: Accept or modify the hypothesis.

    A properly designed and executed experiment enables a scientist to determine whether the original hypothesis is valid. In the case of validity, the scientist can proceed to step 5. In other cases, experiments may demonstrate that the hypothesis is incorrect or that it must be modified, thus requiring further experimentation.

    Step 5: Development of a law and/or theory.

    More experimental data are then collected and analyzed, at which point a scientist may begin to think that the results are sufficiently reproducible (i.e., dependable) to merit being summarized in a law—a verbal or mathematical description of a phenomenon that allows for general predictions. A law simply states what happens; it does not address the question of why.

    One example of a law, the law of definite proportions (discovered by the French scientist Joseph Proust [1754–1826]), states that a chemical substance always contains the same proportions of elements by mass. Thus, sodium chloride (table salt) always contains the same proportion by mass of sodium to chlorine—in this case, 39.34% sodium and 60.66% chlorine by mass. Sucrose (table sugar) is always 42.11% carbon, 6.48% hydrogen, and 51.41% oxygen by mass.

    Whereas a law states only what happens, a theory attempts to explain why nature behaves as it does. Laws are unlikely to change greatly over time, unless a major experimental error is discovered. A theory, in contrast, is incomplete and imperfect; it evolves with time to explain new facts as they are discovered.

    Because scientists can enter the cycle shown in Figure \(\PageIndex{1}\) at any point, the actual application of the scientific method to different topics can take many different forms. For example, a scientist may start with a hypothesis formed by reading about work done by others in the field, rather than by making direct observations.

    Example \(\PageIndex{1}\)

    Classify each statement as a law, theory, experiment, hypothesis, or observation.

    1. Ice always floats on liquid water.
    2. Birds evolved from dinosaurs.
    3. Hot air is less dense than cold air, probably because the components of hot air are moving more rapidly.
    4. When 10 g of ice was added to 100 mL of water at 25°C, the temperature of the water decreased to 15.5°C after the ice melted.
    5. The ingredients of Ivory soap were analyzed to see whether it really is 99.44% pure, as advertised.


    1. This is a general statement of a relationship between the properties of liquid and solid water, so it is a law.
    2. This is a possible explanation for the origin of birds, so it is a hypothesis.
    3. This is a statement that tries to explain the relationship between the temperature and the density of air based on fundamental principles, so it is a theory.
    4. The temperature is measured before and after a change is made in a system, so these are observations.
    5. This is an analysis designed to test a hypothesis (in this case, the manufacturer’s claim of purity), so it is an experiment.

    Exercise \(\PageIndex{1}\)

    Classify each statement as a law, theory, experiment, hypothesis, qualitative observation, or quantitative observation.

    1. Measured amounts of acid were added to a Rolaids tablet to see whether it really “consumes 47 times its weight in excess stomach acid.”
    2. Heat always flows from hot objects to cooler ones, not in the opposite direction.
    3. The universe was formed by a massive explosion that propelled matter into a vacuum.
    4. Michael Jordan is the greatest pure shooter ever to play professional basketball.
    5. Limestone is relatively insoluble in water, but dissolves readily in dilute acid with the evolution of a gas.
    Answer 1:
    Answer 2:
    Answer 3:
    Answer 4:
    Answer 5:


    • A hypothesis is a tentative explanation that can be tested by further investigation.
    • A theory is a well-supported explanation of observations.
    • A scientific law is a statement that summarizes the relationship between variables.
    • An experiment is a controlled method of testing a hypothesis.
    • The scientific method is a method of investigation involving experimentation and observation to acquire new knowledge, solve problems, and answer questions. The key steps in the scientific method are:
      • Step 1: Make observations.
      • Step 2: Formulate a hypothesis.
      • Step 3: Test the hypothesis through experimentation.
      • Step 4: Accept or modify the hypothesis.
      • Step 5: Development of a law and/or theory.

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