The Scientific Method

Before we begin looking at chemistry in particular, it is useful to consider the way scientists in ALL fields -- be it chemistry, physics, biology , etc -- go about pursuing scientific knowledge.

Scientific inquiry usually begins when we observe some natural phenomena that raises our curiosity as to why nature behaves as it does. We then attempt to explain the reasons behind our observations. This initial explanation is what we call a hypothesis. Formally, we can define a hypothesis as a tentative explanation of natural phenomena. The operative word here is tentative. The initial explanation (hypothesis) is based on only limited observations. These observations are likely not sufficiently rigorous to qualify as what we would normally consider scientific investigation.

A well formulated hypothesis is one which can (at least potentially) be disproved. For example, if I tell you I have a hypothesis that a civilization of green humanoids has just sprung up on a planet in a galaxy 10000 light years from earth, there is nothing one can do to prove it false, thus it is not a useful hypothesis. A useful hypothesis can be put to the test by experiment. An experiment is an observation of natural phenomena carried out in a controlled manner so that the results are reproducible and rational conclusions can be obtained.

Suppose I have just synthesized a new chemical compound that I hypothesize will stimulate plant growth. This hypothesis is testable, and therefore has a chance of being disproved. Let us apply the definition of experiment given in the last paragraph to this scenario. Suppose I take two plants and give one of them the substance I believe stimulates plant growth while the other plant receives only water. If the plant receiving the substance suspected of stimulating growth does indeed grow better (i.e., faster growth, healthier looking leaves, etc) you might be tempted to conclude that the substance does indeed stimulate plant growth.

But now suppose I let you in on the following facts:

Are you still sure my "growth substance" enhances plant growth? Or could it be that

In the above example, it is not difficult to imagine that if the experiment was repeated with different plants, or with the plants in different environments, very different results might have been obtained.

This is what is behind the requirement in the definition of experiment that the observations be carried out in a controlled manner and that the results be reproducible. Applying this to our plant growth experiment, this means we should carry out our experiment

If we carry out the experiment in such a way that the only difference in the treatment of the two plants is that one receives the "growth substance" and the other does not, we can have much more confidence in the conclusions we reach. This is an example of making the observations in a controlled manner. Furthermore, if someone else carried out this experiment using the same plant species and experimental conditions that we used, they would likely obtain similar experimental results. Hence, the results would be reproducible.

This is what sets the observations we make as part of an experiment apart from casual observations, which lack that degree of rigor.

The result of the experiment we carry out will give us some indication of the truth or falsehood of our hypothesis. If our first experiment yields results that support our hypothesis, we have some indication that it may be correct. We would not want to accept the hypothesis on the basis of just one experiment, however, so we would continue to do experiments to further test the hypothesis. With each experimental test the hypothesis passes, we can have increasing confidence in it.

We should note, however, that we can never absolutely prove a hypothesis true. No matter how many experimental tests a hypothesis survives, there is always the chance that one day, we will do an experiment that will disprove our hypothesis. Thus, while it may sound pessimistic, all we can really say about a hypothesis that has survived numerous experimental tests is that it has not been disproved so far. Such a hypothesis, that is supported by many experiments, is called a theory. Formally we shall define a theory as a tested explanation of natural phenomena. The operative word here is tested. Note that this definition is almost the same as that for hypothesis. The only difference is that the word tested appears in place of the word tentative.

Like hypotheses, theories can also be incorrect.  While it does not happen frequently, there have been cases where long accepted theories had to be discared in the face of newly discovered contradictory evidence.

Scientists sometimes summarize patterns and regularities of nature in what are called scientific laws.  These laws usually summarize the results of numerous observations and large amounts of data.  Quite often, these laws are stated mathematically.  Formally, we shall define a law as a concise statement or mathematical equation about natural phenomena.  Laws tend to be broadly based, rather than narrowly focused.  For example, consider the law of conservation of mass.  This is an important law that we shall rely on in many of the problems we solve in general chemistry.  A common wording of this law is that matter can not be created or destroyed.  An equivalent description that you may find more useful is the total mass remains constant during a chemical reaction.  Notice that this applies to any chemical reaction, not just certain ones.  That is, the law is a completely general description of a phenomenon that has been observed in every case.

Another law you will come across in this course is the ideal gas law.  Like many laws, this one is stated mathematially.  It is usually written in the following form:

PV = nRT

This equation describes how the pressure, volume and temperature of a gas are all related.  You might apply this equation to helium in a balloon, the air in a scuba diver's tank, and so on.  So again, the law is general -- it can be applied to many different gases, and under a wide range of conditions.

Laws and theories complement each other.  A law is a description of how nature behaves, and a theory offers an explanation why.  As we saw earlier, the law of conservation of mass states that the total mass remains constant during a chemical reaction.  However, the law itself does not explain why this phenomenon is observed -- it merly notes that the phenomenon exists.  Now consider the atomic theory.  This theory proposes that all matter is made up of extremely tiny particles called atoms.  There are several postulates associated with this theory.  The ones we need to consider now are the postulates that every atom has a characteristic mass, and that a chemical reaction is simply a rearrangement of atoms.  If these postulates are correct, then it makes sense that the total mass should remain constant during a chemcal reaction.  In the chemical reaction, we change the way the atoms are grouped, but we don't change their characteristic mass.  All the atoms that were present at the beginning of the reaction are still present at the end, and vice versa.  Therefore, the atomic theory offers an explanation for the widely observed phenomenon that has been summarized as the law of conservation of mass.  Similarly, the kinetic-molecular theory of gases is a set of postulates that -- upon careful consideration -- very nicely explain why the pressure, volume and temperature of a gas should be related as described in the ideal gas law. Even though a scientific law is an summary of patterns in nature that have actually been observed, a scientific law can still be "wrong", in the sense that the actual pattern in nature may be more complicated than the law suggests.  For example, on close scrutany, no real gas is described perfectly by the ideal gas law.  The ideal gas law is derived from a set of logical assumptions about gases -- but not all of them are true.  An ideal gas is an imaginary gas in which the molecules do not experience any attractive forces, and in which the molecles occupy no volume.  No real gas satisfies these conditions, but making these assumptions makes it far easier to come up with an equation to describe gas behavior.  The equation we come up with on the basis of these assumptions (PV=nRT) gives a good apporixmation of gas behavior in most cases, but for exacting work involving gases (particularly at low temperatures or high pressures) alternative equations must be used.  When we deal with gases in this course, we will normally use the ideal gas law, trusting that it is a good enough approximation for our purposes.

Even the law of conservation of mass -- regarded as correct by just about everyone -- is not quite correct, according to Einstein's theory of relativity.  It is from Einstein's theory that we get the well known equation

E = m c2

In this equation, E is energy, m is mass and c is the speed of light.  The speed of light is extremely large -- and squaring this quantity makes it even larger.  The equation says that energy and mass are equivalent, and the energy corresponding to a given mass is found by multiplying the mass by the square of the speed of light.  Since the square of the speed of light is a fantastically large number, even a small quantity of matter multiplies out to a large amount of energy.  Conversely, energies of ordinary size equate to extremely small masses.  Einstein's famous equation can be rewritten in the following form:

m = E / c2

Notice that the energy has to be divided by an extremely large quantity to calclate the mass.  That is why energies of ordinary size equate to such small masses.

Later in this course, you will learn about thermochemistry.  You will see that few reactions are "heat neutral" -- most reactions either give off heat or absorb heat.  For now, let's consider a reaction that gives off heat (such a reaction is said to be exothermic).  According to Einstein's theory, a chemical reaction that is losing energy should also be losing mass, since energy and mass are related.  However, the energy released by any exothermic chemical reaction equates to such a small mass that that is is impossible to detect any loss in mass.  So the law of conservation of mass -- while theoretically incorrect -- is completely correct within the limit of our precision of measurement.

To summarize, hypotheses, theories and laws can only be disproved, never proved.  However, if numerous experiments support our ideas about nature, we proceed as if those ideas are correct. To be scientific, a hypothesis or theory must be falsifyable -- that is, it must be possible to make predictions which can then be tested by doing experiments.

After reading all of the above, you might be tempted to conclude that the scientific method was very carefully and methodically worked out by scientists -- perhaps by a committee of scientists who got together to adress the question "how should we study science?".  But in fact, it's really just human nature.  If you will reflect on your own life, you can probably come up with some problems you have had to solve where you followed a procedure much like that described here.

As an example, suppose you find one morning that your car won't start (observation).  Almost without making a conscientious effort to do so, you begin to wonder if your battery is dead (hypothesis).  You then make some prediction from this hypothesis, like "my horn probably won't honk" or "My headlights won't burn".  You then check to see if you can honk the horn or get the headlignts to burn (experiment) and based on your findings, you either conclude that your battery is indeed dead, or you decide your battery is ok and continue looking for some other cause (formulating new hypothesis if necessary).  You just followed the scientific method!

Formally, we will define the scientific method as the general process of acquiring, testing and refining information about natural phenomena.


Read section 1.2, "The Scientific Approach to Knowledge" (pages 5-7 in your textbook by Nivaldo Tro).

Assignments related to this topic:

  1. Do the homework assignment titled "The Scientific Method" found at the Mastering Chemistry Website
Note: Homework URL:

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Page Updated Sunday January 13, 2008.