Hypothesis, Theory & Law in Science

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  • 0:01 Definitions
  • 4:17 Comparing and Contrasting
  • 5:43 Lesson Summary
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Lesson Transcript
Instructor: Elizabeth (Nikki) Wyman

Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.

In this lesson, learn how to define hypothesis, theory, and law in science. Then, discover the ways that these ideas are related to each other and how they help us to explain the universe around us.


We construct the world around us by continually making observations about what we see. An observation is a phenomenon that can be witnessed and recorded. A set of observations can be used to make a hypothesis, which is a possible explanation for the observations made, but note a hypothesis is just a possible explanation.

Sometimes, we get new evidence from an experiment or new observations that contradict our hypothesis. An experiment is a procedure carefully done to examine the validity of a hypothesis. In fact, scientists seek to test their hypotheses by making extensive observations or conducting many experiments. The idea is to prove a hypothesis by trying to disprove it first. A hypothesis can be changed or reformulated over a series of observations or experiments. Once a hypothesis holds true, it is accepted as fact.

Over the course of time, a collection of hypotheses can be used to generate either a scientific law or theory. A scientific law is a statement that summarizes a collection of observations or results from experiments. Scientific laws are always true under the same conditions and therefore can be used to make predictions.

In fact, our cartoon friend here could use some of the Laws of Inheritance to better understand why some cats are gray, while other cats are orange, black, white, or even calico! In case you've never heard of them, the Laws of Inheritance were developed by the Austrian monk Gregor Mendel to explain inheritance patterns initially observed in pea plants. Collectively, the laws explain how genes are passed from parents to their offspring.

Because Mendel's explanations were true for a variety of organisms, they became a set of laws. A scientific law may also be referred to as a principle. Other examples of scientific laws or principles include:

  • The Law of Conservation of Mass
  • Coulomb's Law
  • Newton's Law of Universal Gravitation
  • The Ideal Gas Law
  • Bernoulli's Principle

A theory, or model, is also based on a set of hypotheses. Unlike a law, however, a theory describes and explains why a natural phenomenon occurs. In science, theories explain reality well and are generally accepted as truth. Theories are based on information from many different areas of study.

Our cartoon friend might find the theory of evolution quite helpful when trying to understand where cats came from or why different colored fur might be beneficial in helping cats to survive. To briefly summarize, the theory of evolution explains that organisms slowly change over time due to genetic mutations. Organisms that have beneficial mutations, like the ability to camouflage well, are more likely to survive and pass their traits on to their offspring.

Theories let scientists make testable predictions. Theories can also be changed or modified if new evidence is found! Examples of theories include:

  • Cell Theory
  • Theory of Relativity
  • Atomic Theory
  • Plate Tectonics Theory

To better understand the relationship between our three main ideas - hypothesis, law, and theory - let's compare them to an old, strong structure that has withstood the test of time: the Parthenon in Greece. The Parthenon has three fundamental parts. The foundation is the ground structure on which the entire structure is built. The columns are tall pillars built upon the foundation. Together, all of the columns support the weight of the roof, which is used for protection.

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