The Scientific Method

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Learning Goal: To understand how the scientific method can be used to search for explanations of nature. The scientific method is a procedure used to search for explanations of nature. The scientific method consists of making observations, formulating hypotheses, designing and carrying out experiments, and repeating this cycle. Observations can be either quantitative or qualitative. Quantitative observations are measurements consisting of both numbers and units, such as the observation that ice melts at . In contrast, qualitative observations are observations that do not rely on numbers or units, such as the observation that water is clear. A hypothesis is a tentative explanation of the observations. The hypothesis is not necessarily correct, but it puts the scientist's understanding of the observations into a form that can be tested through experimentation. Experiments are then performed to test the validity of the hypothesis. Experiments are observations preferably made under conditions in which the variable of interest is clearly distinguishable from any others. If the experiment shows that the hypothesis is incorrect, the hypothesis can be modified, and further experiments can be carried out to test the modified hypothesis. This cycle is repeated, continually refining the hypothesis. If a large set of observations follow a reproducible pattern, this pattern can be summarized in a law—a verbal or mathematical generalization of a phenomenon. For example, over the years people observed that every morning the sun rises in the east, and every night the sun sets in the west. These observations can be described in a law stating, "The sun always rises in the east and sets in the west." After a great deal of refinement, a hypothesis can lead to a theory. A theory is an explanation of why something happens. For example, Newton's theory of gravitation explains why objects tend to fall toward the Earth (as well as explaining the interactions between the Earth and the other planets, etc). However, theories can still be further refined or even replaced. Einstein's theory of general relativity was able to better explain certain astronomical observations related to gravity, and therefore it replaced Newton's theory of gravitation (although Newton's theory still holds true under most everyday conditions). Similarly, the geocentric theory (that the Earth is the center of the universe) was replaced by the heliocentric theory (that the Earth revolves around the sun) based on further observations and testing of predictions. Note that a scientific theory is not the same as the popular definition of a theory—namely, a "guess" or "speculation." Instead, a theory is an explanation that can hold up against repeated experimentation. It may not be perfect, but it is the best explanation possible based on available evidence. Substances have both chemical properties and physical properties. Physical properties can be classified as either intensive properties or extensive properties. An intensive property is independent of the amount of the sample. An extensive property varies with the amount of the sample (mass, volume) A chemical reaction (also known as a chemical change) produces substances that are chemically different from the starting materials. An example of a chemical reaction is the formation of water from hydrogen and oxygen gas. Ex: Solid waste decomposes in landfills and produces methane gas. Carbon dioxide is produced by the combustion of gasoline in an automobile engine. Oxygen gas changes to ozone during thunderstorms. In a physical change, a substance changes its physical appearance but not its chemical identity. An example of physical change is the formation of liquid water from solid water, a familiar process called melting. Physically, liquid water looks very different from solid water (ice) but the chemical identity, water, is the same for both. Ex: Freezing rain develops when a warm air mass overrides a cold air mass. Fog forms...
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