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, as shown in Figure P.3.1.
Step 1: Make observations
Observations can be qualitative or quantitative. Qualitative observations describe properties or occurrences in ways that do not rely on numbers. For example,
- "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"
are all qualitative observations.
In contrast, quantitative observations are measurements, which, by definition, consist of both a number and a unit. For example,
- "the melting point of crystalline sulfur is 115.21 degrees Celsius," and
- "35.9 grams of table salt dissolves in 100 grams of water at 20 degrees Celsius"
are both quantitative observations.
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, which, as previously-defined, is a tentative prediction that explains the observation(s). The hypothesis may not be correct, but its purpose is to frame the scientist's understanding of the system that is being studied into a form that can be tested. For example, the observation that alternating periods of light and darkness correspond to observed movements of the sun, moon, clouds, and shadows is consistent with the following hypotheses.
- The Earth rotates on its axis every 24 hours, alternately exposing one side to the sun, or
- The sun revolves around the Earth every 24 hours.
Suitable experiments can be designed to choose between these two alternatives.
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 in which, ideally, only 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. If experimental evidence suggests that a hypothesis is true, the original prediction can be honed into a theory or a law. However, if a hypothesis is proven to be incorrect, it must be modified and further studied.
Step 5: Development into a law and/or theory
Upon the collection and analysis of additional experimental data, a scientist may begin to think that the results are sufficiently reproducible to merit being summarized in a law, which, as previously-defined, is a verbal or mathematical description of a phenomenon. Whereas a law states only what happens, a theory attempts to explain why nature behaves as it does.
Because scientists can enter the cycle shown in Figure P.3.1 at any point, the applying the scientific method to different topics can take many different forms. For example, a scientist may start with a hypothesis that is formed by reading about work done by others in the field, rather than by making direct observations.