Scientific Method Section 1 2B, 2C, 2D Key Terms scientific method system hypothesis model theory s Observation includes making measurements and collecting data. Sometimes progress in science comes about through accidental discoveries. Most scientific advances, however, result from carefully planned investigations. The process researchers use to carry out their investigations is often called the scientific method. The scientific method is a logical approach to solving problems by observing and collecting data, formulating hypotheses, testing hypotheses, and formulating theories that are supported by data. Hypotheses are testable statements. Scientific theories are wellestablished and highly reliable explanations. Observation includes making measurements and collecting data. Observing is the use of the senses to obtain information. Observation often involves making measurements and collecting data. The data may be descriptive (qualitative) or numerical (quantitative) in nature. Numerical information, such as the fact that a sample of copper ore has a mass of 25.7 grams, is quantitative. Non - numerical information, such as the fact that the sky is blue, is qualitative. Experimenting involves carrying out a procedure under controlled conditions to make observations and collect data. To learn more about matter, chemists study systems. The students in Figure 1.1 are doing an experiment to test the effects absorbed water has on popcorn. A system is a specific portion of matter in a given region of space that has been selected for study during an experiment or observation. When you observe a reaction in a test tube, the test tube is the system. 2B know that scientific hypotheses are tentative and testable statements that must be capable of being supported or not supported by observational evidence. Hypotheses of durable explanatory power which have been tested over a wide variety of conditions are incorporated into theories; 2C know that scientific theories are based on natural and physical phenomena and are capable of being tested by multiple independent researchers. Unlike hypotheses, scientific theories are well-established and highly-reliable explanations, but they may be subject to change as new areas of science and new technologies are developed; 2D distinguish between scientific hypotheses and scientific theories. Figure 1.1 Observation in an Experiment Students observe whether the volume of popped corn is greater when the kernels have been soaked in water prior to popping or when they have not. 31
Figure 1.2 Formulating Hypotheses A graph of data can show relationships between variables. In this case, the graph shows data collected during an experiment to determine the effect of phosphorus compounds on plant growth. Plant Growth vs. Time 30 25 50% phosphorus critical thinking Predict Outcomes How would you finish this hypothesis: If phosphorus stimulates corn-plant growth, then...? Growth (cm) 20 15 10 25% phosphorus 10% phosphorus 5 no 0 0 5 10 15 20 25 Time (days) 2B Hypotheses are testable statements. As scientists examine and compare the data from their experiments, they attempt to find relationships and patterns in other words, they make generalizations based on the data. Generalizations are statements that apply to a range of information. To make generalizations, data are sometimes organized in tables and analyzed using statistics or other mathematical techniques, often with the aid of graphs and a computer. Scientists use generalizations about the data to formulate a hypothesis, or testable statement. The hypothesis serves as a basis for making predictions and for carrying out further experiments. Figure 1.2 shows data collected to test a hypothesis about the effects of phosphorus on plant growth. A good hypothesis, however, doesn t just predict what is expected to happen. A good hypothesis gives a possible explanation for the reason something occurs. It s not enough to say, If we add phosphorus to the soil, plant growth will increase. A good hypothesis would say, If we add phosphorus to the soil, plant growth will increase because phosphorus is essential for growth processes such as energy transfer and photosynthesis. Hypotheses often are referred to as tentative statements. However, before a scientist would publicly support a particular hypothesis, the hypothesis would need to be tested over a wide variety of conditions to ensure its validity. It s not uncommon, especially in newer fields of study, to have several competing hypotheses for a phenomenon, each one having its own supporters and detractors. 32 Chapter 2
Controls and Variables Have you ever opened a package you ve received in the mail to find a packet of what appears to be small beads or crystals? Most likely, this is a package of silica gel (Figure 1.3), which is a desiccant. Desiccants are materials that remove moisture from the air. In packaging, they help insure that the contents of the package aren t exposed to excessive amounts of moisture that could damage them. While the sender can t control the weather conditions the package will pass through on its way to its recipient, the desiccant can help ensure the moisture inside the package doesn t vary greatly. The silica gel helps control a variable. Testing a hypothesis also requires the control of variables. During testing, scientists attempt to control all external conditions that possibly could have an effect on the outcome of an experiment. Any conditions they cannot control or allow to change are known as variables. Any change observed, therefore, has a greater likelihood of being due to the effects of the variable being tested. If testing reveals that the predictions were not correct, the hypothesis on which the predictions were based must be discarded or modified. Figure 1.3 Controlling Variables Silica gel packets are placed in packages in order to absorb excess moisture that could harm the contents of the package. Rob Walls/Alamy Images; PhotoCuisine/Alamy Images Repetition and Replication Just one prediction, or even several correct predictions, does not make a hypothesis strong. It s also important that the same experiments continue to get the same results and that what one scientist claims his or her experiments show can be verified by others. These considerations are known as repetition and replication. Repetition is the expectation that an experiment will give the same results when it is performed under the same conditions. Sometimes an experiment can give inaccurate or unusual results. This could happen for many unknown and unforeseeable reasons. It does not necessarily mean the scientist has not done the experiment well. However, repeating the experiment provides a check on its accuracy and prevents scientists from jumping to unwarranted conclusions. Results from multiple trials ensure that the scientist is using the most accurate and complete data. Replication is the idea that experiments should be reproducible by other scientists. Any scientist in the world should be able to perform the same experiment and get similar results. Replication is a major reason that scientists publish detailed notes on how they performed their trials. Figure 1.4 Recipes exist for much the same purpose (Figure 1.4). Providing exact amounts of ingredients and specific mixing and heating instructions greatly increases the chances that a cake made today tastes like one made last week. The chance that a cake made by one baker will taste similar to one made by another also is increased. Replication Exact measurements and instructions allow recipes to be replicated many times. 33
2B, 2C, 2D Scientific theories are well-established and highly reliable explanations. A hypothesis is an explanation for why a certain outcome should be expected. Hypotheses can be supported by evidence, but they cannot be proven to be true. They can only be disproved when they fail to account for what is observed. Remember that it is possible that several hypotheses could account for the same observation equally well. However, when a hypothesis or group of related hypotheses has survived repeated testing over a reasonable period of time, it can form the basis for a scientific theory. A theory is a broad generalization that explains a body of facts or phenomena. Theories also encompass a wider area, explaining a greater number of observations in more-general terms. Figure 1.5 The Collision Theory For centuries, the mortar and pestle have been used to grind up substances to encourage a faster reaction, even before chemists had formulated the collision theory. Theories in Chemistry Only about a hundred years ago, there was still some confusion over how to explain the kinetics of some chemical reactions, especially those occurring at low pressures. Some scientists believed these only could be caused by background radiation that energized the molecules. This radiation hypothesis had a relatively short life in the history of chemistry. It was quickly abandoned for the collision theory, which states that reactions occur when reacting atoms and molecules collide in the right orientation and with sufficient energy. The collision theory explains a great deal about chemical reactions. For example, it explains why substances that have been ground up, as in Figure 1.5, react more vigorously than those that haven t been. The greater surface area means more of the reactant molecules can come into contact (collide) with one another, providing a greater opportunity for them to react. Heating also increases a reaction s rate because the increased energy results in faster-moving molecules. Faster-moving molecules produce increased collisions with greater energy and so a greater chance for reaction. As theories change, discarded theories can still provide insight into scientific phenomena. For example, there is some evidence that suggests that radiation causes some reactions at low pressures, where collisions between molecules are not so frequent. However, experiments have yet to provide conclusive evidence that radiation plays a role in reactions, so this idea is not currently widely accepted. Even widely accepted theories do not necessarily account for all observations. Theories represent merely the best, most-consistent explanations scientists have found to explain a variety of observations. Charlie Winters/Houghton Mifflin Harcourt 34 Chapter 2
Figure 1.6 The Scientific Method The scientific method is not a single, fixed process. Scientists may repeat steps many times before there is sufficient evidence to formulate a theory. You can see that each stage represents a number of different activities. STAGES IN THE SCIENTIFIC METHOD OBSERVING collecting data measuring experimenting FORMULATING HYPOTHESES analyzing data classifying inferring TESTING experimenting collecting data measuring THEORIZING constructing models PUBLISH RESULTS Not supported revise or reject hypothesis Results confirmed by other scientists validate theory Theories and Models After formulating what they feel to be strong theories or even hypotheses, scientists typically try to explain the phenomena they are studying by constructing a model. A model in science is more than a physical object; it is often an explanation of how phenomena occur and how data or events are related. Models may be visual, verbal, or mathematical. One important model in chemistry is the atomic model of matter, which states that matter is composed of tiny particles called atoms. Figure 1.6 attempts to summarize the steps of the scientific method from observing phenomena to hypothesizing to theorizing. Don t be fooled by what s depicted as a very linear process. Most scientists will agree that their actual work is rarely this simple. Hypotheses are formed and rejected, and scientists can spend months, even years, following a hypothesis that eventually is proven false. The important thing to take away from the above is the constant questioning. Scientists are always asking questions, always testing, and always attempting to gather more support for their ideas, regardless of how strong they may seem. Section 1 Formative ASSESSMENT Reviewing s 1. What is the scientific method? 2. How do hypotheses and theories differ? 2D 3. Once scientists have agreed upon a theory, will that theory ever change? Are all scientific theories perfect? 2c 4. How are models related to theories and hypotheses? Critical Thinking 5. INTERPRETING CONCEPTS Suppose you had to test how well two types of soap work. Describe your experiment by using the terms control, variable, reproducible, and repetition. 35