Introduction

As mentioned before, in this lesson we will focus on a theory about atoms that will help explain the nature of elements and compounds. Before we do that, let's discuss what a theory is and how it relates to other aspects of scientific knowledge. Because theories are always tentative and open to revision, we need to consider the criteria used for judging theories to determine their value. Also in this section, we will review the classification of elements and compounds with regard to how they fit in with the other categories of materials.

Aspects of Scientific Knowledge

Let's start with facts. Of course, you know what a fact is. You should also know that something is not a fact just because someone says it is. Because the word "fact" is so loosely used in common speech, I will generally refer to observations and data, rather than to facts.

Observations, of course, are made by seeing, touching, smelling, hearing and measuring things. Data is the information collected by these observations. For our purposes, consider "observation" to be the act of determining what actually exists and "data" to be the record of those observations. Also, keep in mind that it won't usually be necessary to keep that distinction in mind.

A scientific law is a summary of a great many observations that can be generalized into a fairly broad statement. The intent of a scientific law is to describe nature, rather than explain it. For example, the Law of Constant Composition says that the composition of compounds does not vary from one pure sample to another, but it does not explain why.

Inferences are distinct from observations because inferences are made by drawing conclusions from observations. One special type of inference is hypothesis.

A person makes a hypothesis by speculating about why or how certain observations are related. A hypothesis is a tentative explanation. Scientifically, a hypothesis forms the basis for additional controlled observations which serve to test the hypothesis. In a sense it is a prediction that can be tested. After a hypothesis has held up under rigorous testing, it can evolve into a theory.

A theory is a fairly well established and significant explanation for a broad range of phenomena.

A model is somewhat like a hypothesis or a theory that focuses on how something functions. A model is quite often pictorial, mathematical or mechanical, but it does not have to be.

In this lesson we talk about atoms. When scientists and philosophers first came up with the idea of atoms, they made a hypothesis. When scientists used the idea of atoms to explain why compounds followed the Law of Constant Composition, they created atomic theory. When scientists tried to picture what those atoms must look like (or be like), they developed atomic models.

The Scientific Method

The Law of Constant Composition

The Law of Constant Composition: The composition of a compound is constant; it consists of elements combined in a fixed ratio.

Many measurements of the composition of materials were made in the early years of modern chemistry. Experimenters noticed that some materials had the property that their composition was always the same, no matter where they came from or who made the measurements. We now call these materials compounds. Due to their "constant composition," compounds have well-defined physical and chemical properties, such as their boiling points, melting points, chemical reactivities, and so on.

In fact, this is one of the operational definitions of a compound. If the composition of a material does not vary from one sample to the next, this is evidence that we are dealing with a compound and not a mixture. Of course, we are assuming that the material contains at least two different elements.

For example, if you take water and break it down into the elements that make it up, the mass of one (oxygen) will always be exactly eight times the mass of the other (hydrogen). For example, if a water sample contains 2.5 g of hydrogen, it will always contain 20 g of oxygen. It is impossible to have water with "extra" oxygen or hydrogen.

On the other hand, if you have a solution of sugar water, there are all kinds of ratios of sugar to water that it could have. A sample with 100 g of water in it could have 0.03 g of sugar in it, or 3.0 g, or 30 g - essentially any ratio is possible, so long as it was below the maximum sugar that will dissolve in water. These solutions do not obey the law of constant composition, because they are not compounds. We will discuss this distinction much more in a later section.

Criteria for Judging Theories

So how do we tell a good hypothesis or theory or model from a bad one? We test it. In fact, testability is one of the requirements of a scientific theory. If a hypothesis or theory fails a test, it is usually discarded or modified. I say "usually" because sometimes a theory is useful even though it is known to be faulty. For example, it might still explain more than any other theory, or it might be easier to remember. So what do we use to test a theory or model?

Essentially there are four criteria that we use to judge a theory or model. The first is that the theory must explain established observations. That is, it needs to be able to explain what is already known. Second, it needs to be able to explain new observations as they come up. If someone discovers something and that new discovery fits into the theory, then it is a good theory. On the other hand, if something is discovered that the theory cannot explain, then the theory is obviously wanting. The third thing is that the theory should be able to predict new observations. It should be able to tell you something new. You should be able to use the theory to say, "Now if this is true, then we should be able to find out something else "or" if this is true, then such and such should work; let's try it and see if it does." Fourth, in addition to all of that, it should be as simple as possible. Another way of saying that is that the theory should be simple if possible. Simple theories are preferred to more complex ones. If you can establish a theory that sets up four or five different rules or properties of materials that explain everything, that's much better than if you have to set up seven or eight or ten rules and then add a dozen different exceptions to those rules and so forth. So, if the theory is simple, that is a big plus.

A good theory should be able to explain established observations, explain new observations, predict new observations, and do so as simply as possible. These criteria should be applied to any theory. Specifically, in this lesson, they will be applied to Dalton's Theory of Atoms.

Inference vs. Observation

It is important to distinguish between observations and inferences. The colors and color changes, the temperature and temperature changes, the smells that you may come across in this lesson and throughout this course are directly observed and they can be classified as observations. When you do something with that observation, like draw a conclusion or offer an explanation or decide that a chemical reaction occurred, then you are making an inference. The inference may or may not be a correct one. Correctness is not what makes the difference between observation and inference.

An observation is the awareness of some condition; inference is the result of a mental process which attempts to explain or catalog or speculate about that observation. So far we have had several examples of observations (and measurements as well), but we have not really talked about inferences. A few examples might help to illustrate the point.

There are times when observations and inferences are very much intertwined with one another and then it can be very difficult to make the distinction. This is because observation and inference both are mental processes. An example of this is a mirage or an optical illusion.

If you have ever seen the light shimmering off the road or countryside out in the desert, it looks like water. That is an inference. What you are observing is the reflection of the light, and you are inferring that it is reflecting off water.

If you have ever seen the light shimmering off the road or countryside out in the desert, it looks like water. That is an inference. What you are observing is the reflection of the light, and you are inferring that it is reflecting off water.

So, observations and inferences are sometimes a bit hard to distinguish from one another. But for the most part, if you are careful about making the distinction, at least in this course, you won't have too much trouble figuring out what is an observation and what is an inference. Making inferences involves knowing how to look beyond what you actually observe, and to know that you are doing it. Remember that the point here is not that observations are correct and inferences are incorrect, but rather that there is a difference and that you need to know what that difference is.

Study Check: Inference vs. Observation

Identify which portions of the following statements are observations and which are inferences. Check your answers below.

1. When paper burns, that is a chemical reaction because the paper becomes brittle and then powdery while it changes color to black and then to white (or light grey). Also, heat and light are given off.

2. There are some flashing red and blue lights up ahead, so I better slow down.

3. Distilled water is a transparent, colorless, homogeneous liquid.

Answers to Study Check on Inference

Portions of the following statements are marked as observations and inferences.

1. When paper burns, that is a chemical reaction because the paper becomes brittle and then powdery while it changes color to black and then to white (or light grey). Also, heat and light are given off.

2. There are some flashing red and blue lights up ahead, so I better slow down.

3. Distilled water is a transparent, colorless, homogeneous liquid. (All observation, no inference.)