Chemistry is the study of matter—what it consists of, what its properties are, and how it changes. Being able to describe the ingredients in a cake and how they change when the cake is baked is called chemistry. Matter is anything that has mass and takes up space—that is, anything that is physically real. Some things are easily identified as matter—this book, for example. Others are not so obvious. Because we move so easily through air, we sometimes forget that it, too, is matter.

Chemistry is one branch of science. Science is the process by which we learn about the natural universe by observing, testing, and then generating models that explain our observations. Because the physical universe is so vast, there are many different branches of science (Figure 1.1). Thus, chemistry is the study of matter, biology is the study of living things, and geology is the study of rocks and the earth. Mathematics is the language of science, and we will use it to communicate some of the ideas of chemistry.

Figure 1.1 The Relationships between Some of the Major Branches of Science

Although we divide science into different fields, there is much overlap among them. For example, some biologists and chemists work in both fields so much that their work is called biochemistry. Similarly, geology and chemistry overlap in the field called geochemistry. Figure 1.1 shows how many of the individual fields of science are related.

Figure 1.2 The General Steps of the Scientific Method

The steps may not be as clear-cut in real life as described here, but most scientific work follows this general outline.

1. Propose a hypothesis. A scientist generates a testable idea, or hypothesis, to try to answer a question or explain how the natural universe works. Some people use the word theory in place of hypothesis, but the word hypothesis is the proper word in science. For scientific applications, the word theory is a general statement that describes a large set of observations and data. A theory represents the highest level of scientific understanding.
2. Test the hypothesis. A scientist evaluates the hypothesis by devising and carrying out experiments to test it. If the hypothesis passes the test, it may be a proper answer to the question. If the hypothesis does not pass the test, it may not be a good answer.
3. Refine the hypothesis if necessary. Depending on the results of experiments, a scientist may want to modify the hypothesis and then test it again. Sometimes the results show the original hypothesis to be completely wrong, in which case a scientist will have to devise a new hypothesis.

Not all scientific investigations are simple enough to be separated into these three discrete steps. But these steps represent the general method by which scientists learn about our natural universe.

The knowledge gained through scientific inquiry is often summarized in one of three ways:

1. Scientific Law.  A scientific law, often summarized by a mathematical equation, describes a phenomenon that has been reliably observed many times and thus can be used to predict results.  An example is using the law of conservation of mass, which states that mass is not created or destroyed in chemical reactions,  to predict the amount of products based on the amount of reactants used.
2. Scientific Theory. A scientific theory, often summarized by several statements, explains what underlies scientific laws.  For example, one point of atomic theory is that matter is made up of small discrete particles that maintain their identities as they undergo chemical changes, and this underlies that law of conservation of mass.
3. Scientific Model. Many scientific phenomena are too small, too large, or too complicated to observe directly, so scientists use physical, computer-based, and mathematical models to summarize their ideas and to allow further testing.  For example, we cannot see molecules, but we can build physical or virtual models of molecules in order to evaluate properties such as shape and polarity.