1  Chemical versus Physical Change?

Many high-school chemistry texts, general science texts, and similar authorities emphasize the distinction between “chemical” change and “physical” change. (Often the first lesson of the school year revolves around this distinction.) There is one major problem with this, and a long list of lesser problems.

There is a moose on the table, i.e. a fundamental dilemma that the authorities are apparently unable to resolve, or even to face squarely.

• In an introductory chemistry class, early in the term, it would be nice to tell students roughly what a chemical reaction is. That’s a reasonable goal, but many authorities go about it in a silly way. They ask students to classify processes as “chemical” versus “physical” based on superficial, macroscopic observations. The books wantonly ignore the fact that chemical reactions are ultramicroscopic ... inherently and necessarily ultramicroscopic. As will be demonstrated below, there is not any reliable way to use macroscopic observations to classify something as chemical, physical, or both, or neither.

The books try to waltz around this fundamental dilemma. This produces all sorts of nonsense, including:

• First of all, the books wrongly claim that there exists a meaningful distinction between “physical” and “chemical” change. They point to a few extreme cases where such a distinction can be made ... while ignoring the fact that in the real world there is a vast gray area where chemistry and physics overlap. At the microscopic level, it is hard to draw a sharp distinction between chemistry and physics, and at the macroscopic level it is even harder.
• Secondly, nobody cares. If you did label this-or-that process as “chemical” or “physical”, the label would have zero practical value. This point is of course related to the previous point, because if the label did have any practical use, that would in itself form a sufficient basis for making the distinction.
• In the course of scientific work, it is common to begin with a few simple observations, then notice a pattern in the data, and then formulate a rule that (a) gives a unified description of the existing observations and (b) makes predictions about future observations.¹ So far so good.

  The problem is that all too commonly, the authorities give us rules that are grossly inconsistent with the examples they have given! Four such rules will be exhibited below.

• What’s worse is that sometimes the books ask the students to make the generalizations on their own, generalizing from a small number of examples. Perhaps this is supposed to give the students practice in “thinking” and in “doing science”.

  This, alas, violates one of the most basic rules of scientific thinking. As discussed in reference 1, one should always consider all the data, or at least a fair sampling of the data. It is outrageous to base a rule on a handful of examples that have been selected or contrived to support the rule, while ignoring other examples that conflict with the rule. (See section 2 for examples relevant to the “chemical versus physical” discussion.)

  The result is that students are being taught something that is a mockery of thinking and a travesty of science. They are learning a bunch of baloney that will have to be unlearned later. Any good teacher knows that unlearning something is an exceptionally difficult and unpleasant task.

2  Some Examples ... and Attempted Rules

Keep in mind that according to the authorities, the main point of the exercise is:

Fundamental Tenet: Naive students should be able, just by looking, to distinguish a “physical” change from a “chemical” change.

These authorities say this distinction is manifested in examples ... and also in rules that unify previous observations and predict future observations.

Let us begin with a few relatively uncontroversial examples:

Example 1:

Example 2:

Example 3:

Example 4:

Now let us see how far we can get if we attempt to form generalizations based on the available examples.

Attempted Rule A: It is superficially tempting to make a rule that says any change initiated by simple physical or mechanical means is a physical change. For instance, casual mixing is obviouly a mechanical process. Similarly cutting with scissors is a mechanical process. Opening a valve is mechanical.

This rule runs into trouble as soon as we consider example 3. The physical change is controlled by a change in temperature. If you lower the temperature, the vapor condenses into liquid. If you raise the temperature, the liquid evaporates into vapor and mixes with the air. If you raise the temperature some more, the air/vapor mixture ignites – or explodes – which is a chemical reaction.

From this example, and many others, we learn that sometimes temperature initiates a physical change, and sometimes initiates a chemical change. This already calls Attempted Rule A into doubt.

Example 5:

Suppose we have an ordinary off-the-shelf bottle of carbonated water. If you remove the cap, gas rushes out. This seems closely analogous to example 4. But are we seeing a chemical change, or a physical change? It turns out we are seeing both. Before the bottle was opened, there was a four-way equilibrium:

• Some of the CO₂ was in the gas phase, in the head space.
• Some of the CO₂ was dissolved in the water, in the form of CO₂. The dissolved CO₂ was in equilibrium with the gaseous CO₂.
• Some of the CO₂ had reacted with the water, to form carbonic acid, i.e. H₂CO₃. The reaction CO₂ + H₂O ↔ H₂CO₃ was in nontrivial equilibrium, i.e. it did not go to completion in either direction.
• Some of the carbonic acid had ionized. The reaction H₂CO₃ ↔ H⁺ + HCO₃⁻ did not go to completion in either direction.

Now the point is that when you reduce the pressure by removing the cap, there are some obvious physical changes that take place, but there are also two chemical reactions that take place. At the new pressure, the chemicals are no longer in equilibrium, so some of the carbonic acid dissociates into water and CO₂. Also some of the carbonate ions convert to un-ionized carbonic acid molecules.

Example 6:

This is the same as example 5, but rather than merely opening the cap and letting gas escape, we attach a piston to the top of the vessel. This allows us to raise and lower the CO₂ pressure in the head space.

When we do this, we discover that the system is reversible. A higher pressure of CO₂ in the head space is associated with more CO₂ in solution, and hence more H₂CO₃ in solution, and hence more H⁺ and HCO₃⁻ ions in solution.

At this point, it should be obvious that Attempted Rule A is deader than a doornail. Indeed, the Fundamental Tenet is also untenable. There is absolutely no way that an incoming student in an introductory-level course could tell by looking whether example 5 depends on chemical changes, physical changes, or both. (Also, as mentioned in section 1, keep in mind that nobody cares.)

At this point, any sensible person would give up, since the Fundamental Tenet has been discredited. But chemistry texts rush in where angels fear to tread. Here’s another example:

Attempted Rule B: Suppose we say this: Chemical change involves breaking chemical bonds, whereas physical change does not break chemical bonds.

This attempted rule cannot withstand scrutiny. It conflicts with example 2, since cutting a piece of paper consists almost entirely of breaking chemical bonds, i.e. cutting high-molecular-weight molecules into lower-molecular-weight molecules.

Some people try to salvage Attempted Rule B by claiming that the scissors (a) doesn’t cut any covalent bonds and therefore (b) doesn’t “really” cut molecules but rather cuts “between” molecules. I am not convinced that part (a) of this claim is true. Also, part (b) violates the IUPAC definition of molecule. However, I do not wish to argue about such details. It is easier to invoke example 7 and example 8, which show that this salvage attempt cannot possibly succeed.

Example 7:

There is no doubt that nylon consists of very long macromolecules – centimeters long, sometimes many meters long, covalently bonded from end to end – which can be readily cut by scissors. Similarly the macroscopic calcite crystal in example 8 is one huge covalently-bonded macromolecule. This can be cut into lower-molecular-weight molecules by purely mechanical means.

You might imagine trying to repair Attempted Rule B by specifying how many bonds must be broken in order to pass from the physical category to the chemical category, but this wouldn’t be worth the trouble. There will always be marginal cases, as you can see from example 8.

Example 8:

Suppose we start with a good-sized crystal of calcite. The crystal is beautifully clear. Split it using a geologist’s hammer. We now have have two crystals, just smaller. This process was carried out using an obviously “physical” means, but the most-important step in the process was the breaking of a huge number of chemical bonds in the crystal. If you keep pounding on the calcite with a hammer, you can quickly reduce it to a fine white powder.

In example 8, there is no practical limit as to how finely the crystal can be crushed, and therefore no practical limit as to how many bonds can be broken.

Still, though, in example 2 and example 8, most of the chemical bonds remain unbroken; only a minority are broken by the “physical” process. You might hope this fact could be used disinguish “chemical” from “physical” processes, but alas, even that last-ditch position is indefensible, because there are processes that the authorities like to call “physical” where a majority of the chemical bonds are broken. In example 9, for instance, you could say that 5/6ths of the ionic bonds are broken. In example 10, all of the covalent bonds are broken.

Example 9:

Suppose we start out with a good-sized crystal of halite, which has empirical formula NaCl. The crystal is a single molecule, i.e. a macromolecule, ionically bonded. We can agree that the empirical formula is NaCl, and the unit cell formula is NaCl, but actual molecular formula is Na_xCl_x, to a high degree of approximation, for some large value of x. Values on the order of x=10²⁰ or even larger are commonly encountered.

Now suppose we heat the crystal so that some vapor is formed. Under a wide range of conditions of temperature and pressure, the vapor will contain lots of NaCl molecules, that is, plain old Na₁Cl₁ molecules. This process is reversible, in the sense that if you lower the temperature, it is possible to regrow the crystal at the expense of the vapor.

In the vapor, each Na atom is bonded to exactly one Cl atom, and vice versa. In the crystal, each of them was bonded to its six nearest neighbors. Forming the vapor involves breaking the vast majority of bonds.

Example 10:

This is the same as example 9, but this time we use a covalently-bonded crystal such as silicon. The crystal is one big molecule. Each atom is covalently bonded to each of its four nearest neighbors. The vapor (under suitable conditions) consists of isolated silicon atoms – no bonds at all.

Believe it or not, there are people who still try to defend Attempted Rule B. They say it is important, because molecules are the “stable particles of matter” and therefore deserve special consideration.

Alas, that argument is a non-starter, as illustrated by example 11.

Example 11:

Suppose we have a container of ordinary water sitting on the shelf. We just let it sit there. Conditions of temperature, pressure, etc. do not change. By any reasonable definition, no physical change is occurring, and certainly no chemical change.

However, note that at standard temperature and pressure, about 18 ppb of the water is auto-ionized, i.e. dissociated into H⁺ and OH⁻ ions. (The ions are, of course, solvated.)

If you add heavy water (D₂O) to ordinary water, you will very quickly wind up with a lot of HDO molecules.

Water molecules are not “stable particles of matter”. Let’s be clear about this: water molecules are not stable in aqueous solution.

So let’s give up on Attempted Rule B.

At this point, you may be convinced that trying to distinguish physics from chemistry is a pointless exercise. If so, you can stop reading now. However, experience shows that some people are not yet convinced.

Attempted Rule C: Some people say a chemical process (unlike a physical process) produces a product with different properties than either ingredient.

This rule is, alas, quite unreliable. There are many counterexamples, including example 8. The powdered calcite has several properties not shared by the original large crystal. For starters, it is a different color. Other counterexamples include example 12, example 13, and example 14.

Example 12:

Suppose we take some copper and alloy it with a little bit of tin. Most people consider this a purely physical mixture. That is to say, there is no sign of any intermetallic compound being formed. However, the product – called bronze – has properties are definitely not what you would predict just by averaging the ingredients:

• It has a lower melting point than either ingredient.
• It has a different color.
• It has different thermal-expansion properties.
• It has greater hardness than either ingredient.
• It has wildly different electrical conductivity, especially at low temperatures.

Example 13:

As a more prosaic example, consider what happens if you mix the yellow dye from an ink-jet printer with the cyan dye. The mixing process is not a chemical process; basically it is just a mixture, rather like example 1.

However, the mixture has a different color than either ingredient: it’s green.

Example 14:

Perhaps an even better example is the following: Start by asking students what color is the stuff in an ordinary bottle of yellow food coloring. Many of them will assume that it is yellow. A few of them may know, based on experience, that the right answer is otherwise. The stuff in the bottle is red. Really, really red. It is only when you greatly dilute it that it becomes yellow.

If you do the experiment, many students will tell you that diluting the yellow food coloring must involve some sort of “chemical change” because they’ve been taught that color change implies chemical change. That is, alas, dead wrong in this case.

To understand what is really going on, dilute the yellow food coloring to a moderate degree and put it in a white bowl with sloping sides. You will observe that in thin layers, the liquid is perceived as yellow, while in moderately thick layers it is perceived as orange, and in yet thicker layers it is perceived as red.

Remark #1: It should be obvious that there is no “chemical change” involved in going from a thin layer to a thick layer.

Remark #2: There are good reasons, excellent physical reasons why the thin layer should have a different perceived color from the thick layer. The physics of light absorption is nonlinear. See reference 2.

Saving the strangest for last, we have this gem:

Attempted Rule D: Some people say that chemical changes are irreversible, while physical changes are reversible. (I’m not making this up, some people really say that!)

This rule is supported by example 1, but contradicted by a host of other examples.

It may be that the first few reactions that you saw in high-school chemistry class were irreversible, but in the real world, many reactions proceed both forwards and backwards. Example 6 is one illustration; example 15 is another.

Example 15:

Consider the electrochemical reactions in a storage battery. By the physical process of turning the crank on a dynamo, you can run the reactions forward. By turning the crank the other way, you can run the reactions backward.

The other half of Attempted Rule D is wrong, too; there are plenty of so-called “physical” processes that are, in practice, irreversible. Crushing a calcite crystal to powder (example 8) is irreversible; there is no easy way to un-crush it. Similarly, cutting paper with scissors (example 2) is irreversible; there is no way to use the scissors “backwards” so as to un-cut the paper. Other examples abound.

3  Meaningful versus Meaningless Classifications

The preceding discussion should make it clear that there is no useful distinction between chemical change and physical change ... except possibly in a few extreme cases.

Any students who are dissatisfied with the usual “textbook” discussion of this topic should be congratulated. It shows they have some critical-thinking ability.

Too much of intro-level chemistry is devoted to rote learning of notions that are not really correct, and would be worthless even if they were correct.

We should concentrate on meaningful rather than meaningless classifications.

++ I am perfectly willing to study the geology of eastern Colorado, and to classify the rocks according to their observable properties.

++ I am perfectly willing to study the geology of western Kansas, and to classify the rocks according to their observable properties.

- - I am not willing to force students to pretend there is a distinction between the geology of eastern Colorado and the geology of western Kansas.

Let me explain the meaning of this metaphor:

4  The Right Way Forward

Please let’s stop foisting ideas of physical versus chemical change onto the students. Those ideas are worse than worthless. The less said about them, the better.

The only reason why anyone would reasonably be interested in such ideas would be if they lived in the 19th century, before there was any useful understanding of atoms.

The 19th century has been over for a while now. Wake up and smell the atoms. Anything you could possibly explain in terms of a physics-versus-chemistry distinction can be explained infinitely more clearly by saying what the atoms and molecules are doing. Especially in an introductory course, students should see the best evidence, not the most ancient evidence. See reference 3.

There is a saying, “You can’t beat something with nothing”. The question therefore arises, if we aren’t going to spend the first week of class talking about “chemical” changes versus “physical” changes, what should we talk about instead?

The answer is simple: Talk about real things. Talk about protons, neutrons, and electrons, and how they form atoms (reference 4). Talk about atoms and bonds and molecules. See reference 5 for suggestions on how to introduce such ideas at the high-school or even pre-high-school level. Talk about energy and entropy (reference 6). Talk about dimensional analysis and scaling laws (reference 7 and reference 8). Talk about things that really matter.

5  References

1.

John Denker, “Scientific Methods” ./scientific-methods.htm

2.

John Denker, “Spectral Data for FD&C Food Coloring Dyes” ../imaging/dye-spectra.htm

3.

John Denker, “Students Need the Best Evidence, Not the Most Ancient Evidence” ./best-evidence.htm

4.

John Denker, “Introduction to Atoms” ./atom-intro.htm

5.

Lynn Melton, “Chemistry Chapter Zero” http://www.chemchapterzero.com/

6.

John Denker, “The Laws of Thermodynamics” ./thermo-laws.htm

7.

John Denker, “Dimensional Analysis” ./dimensional-analysis.htm

8.

John Denker, “Scaling Laws” ./scaling.htm

1

This should not be called “the” scientific method, since it is only one of the many methods available to scientists. See reference 1.