Chemical vs. Physical Change:

Frustration

We regularly interact with chemistry teachers, and always with an eye to identifying their pain points – concepts that they struggle to teach and their students struggle to grasp. With these pain points in mind, we look for ways to use Atomsmith to help resolve these frustrations.

It would be difficult to think of a topic of frustration that comes up more often than chemical vs. physical change. Much of this frustration stems from how this concept is taught at introductory levels.

You know the typical examples:

These phenomenological (macroscopic) examples are dropped on students long before they learn anything meaningful about chemical bonds or chemical compounds. And these macroscopic examples are fraught with misconception.

So much for Johnstone’s triangle!

What’s the end result of this scenario? Students can’t make heads or tails of the subject. And teachers struggle to build their own self-consistent mental model and, as a result, say things such as:

Take a moment to ponder that…

Chemistry is the science of matter and how it changes.

A fundamental property of matter is that some of its forms are stable and some are not.

Some changes of matter result in the creation of, the destruction of, or the interconversion between these stable forms. There’s even a special term for such changes: chemical reactions!

If we were to skip these most fundamental ideas of chemistry, that would leave a gaping hole in the subject.

In this post, we introduce a simple and completely self-consistent definition of chemical vs. physical change, a definition that depends only on understanding chemical bonding.

Pauling’s Chemical Bonds

Linus Pauling is well known for his revolutionary applications of quantum mechanics to chemical bonding. From hybrid orbitals to valence bond theory, he literally wrote the book on chemical bonds. (He also won the Nobel Prize in Chemistry for this work.)

What is less well known is that Pauling introduced his work on chemical bonds by first defining them in a deceptively simple, but profound way:

Forget, for a moment, everything you know about chemical bonds: electron sharing, electrostatic interactions, “seas of electrons”, etc. Just clear your mind and let Pauling’s definition sink in.

Stability is the key: chemical bonds produce stable chemical structures.

Pauling’s definition is reflexive – chemical bonds define chemical structures and chemical structures define chemical bonds.

It’s important to notice that there is nothing in this definition that depends on all of the details that we teach about chemical bonds (…electron sharing, electrostatic interactions, “seas of electrons”, etc).

So what does this definition of the chemical bond have to do with chemical change?

Everything.

Now that we understand what a stable chemical structure is (e.g., a molecule, or an ionic or elemental lattice), the definition of a chemical change should become obvious.

A chemical change is any process that :

And because Pauling’s definition of a chemical structure does not depend on any specific details of electron sharing (or exchange), electrostatic interactions, or “seas of electrons” – neither does chemical change. Think of it as analogous to a thermodynamic state function. All that matters is that a chemical change is any process that produces (or destroys) a stable chemical structure – the details of how it was produced or destroyed are irrelevant.

To complete our definitions, what is a physical change?

Answer: Anything else.

For example, a physical change may produce a new set of intermolecular forces (IMF). These, by definition, are not strong enough to produce a stable chemical structure, and therefore cannot effect chemical change.

The Dissociation of NaCl in H₂O

By far the most common topic that arises in the debate on chemical vs. physical change:

Is the dissociation of an ionic compound in water a chemical or a physical change?

Below, we use Atomsmith’s Live Lab to shine light on the particle-level changes that occur during this process. In the following section, we will address several common responses to this question.

The Live Lab simulates the motions of collections of atoms, molecules, or ions under varying conditions including particles that interact via IMFs.

Click the blue arrows to scroll through the four simulations (labeled Steps 1 - 4), above. To start a simulation, click on it. Following are descriptions of the four steps:

1. We place four Cl^- and four Na⁺ in the box. The ions’ initial positions are separate and random. The ions are close enough to interact via Coulomb’s law, but are not (yet) close enough to have formed a stable chemical structure.

2. A simulation at 300 K: watch the 4 ions approach each other and form an ionic lattice – a stable chemical structure.

   By our definition, this formation of a stable chemical structure is a chemical change.

3. Next, we use the Live Lab’s Box Builder to build a layer of water molecules around the new mini-crystal.

   Then we simulate the water molecules dissociating the crystal.

   By our definition, this destruction of the crystal is a chemical change.

   And because new ion-dipole and dispersion interactions form between the ions and water molecules, a physical change also occurs.

4. Finally, by slightly raising the temperature, we simulate the evaporation of the NaCl solution.

   As the IMFs are overcome, the water molecules escape to the gaseous state. This is a physical change.

   As the water molecules evaporate, the Na and Cl ions again aggregate to reform a crystal. This is a chemical change.

Now let’s return to the question from above: Is the dissociation of an ionic compound in water a chemical or a physical change?

Looking at the changes in Step 3 and applying our definition of chemical change, the answer is clear: both!

Discussion

Finally, let’s use our definition of chemical change to evaluate the usual talking points that arise in the context of the dissociation of ionic compounds in water.

• This cannot involve a chemical change because no electrons are exchanged. In other words, only covalent bonds are “real bonds”.

  Response: Pauling’s definition could not be more clear. Stable chemical structures don’t require sharing of electrons. So, ionic bonds are chemical bonds. Therefore chemical change does not require the exchange of electrons.

  In case you are ever tempted to say that ionic bonds are not “real bonds”, remember that ionic bonds are, in general, just as “strong” as covalent bonds.

  We have even heard the suggestion that perhaps we should stop teaching double-replacement, precipitation reactions as chemical reactions. For all of the reasons outlined in this post, this would be a mistake.

• No new chemical structures are formed when the compound dissociates. Therefore it is not a chemical change.

  Response: In the beginning, a stable chemical structure is there. Then it is not. The destruction of a chemical structure is part of the definition of chemical change.

• The evaporation of water is a physical change. Therefore, the reverse – the dissociation of an ionic compound in water – is not a chemical change.

  Response: As we saw in Steps 3 and 4, above, both chemical and physical changes occur.

• NaCl (s) and NaCl (aq) taste the same. Therefore it is not a chemical change.

  Response: This is one of those phenomenological descriptions that should be eliminated. Plus – you never really have tasted NaCl (s). You have tasted only NaCl (s) dissolved in saliva – which is NaCl (aq).

• What if we mechanically cleave NaCl (s)? Is that a chemical or physical change?

  Response: This is an interesting question that a teacher from Texas once asked us during a discussion of chemical vs. physical change. Here are two ways to think about the answer:

  1. If we are talking about cleaving a “salt” crystal, the process will leave us with two salt crystals. We would call that a physical change.
  2. Alternatively, Pauling knew that, although we teach that it is, no crystal is actually infinite. This is probably one of the reasons that he talked about NaCl molecules. If we were to cleave a crystal of Na₂₀₀Cl₂₀₀ precisely in half, we would be left with two Na₁₀₀Cl₁₀₀ crystals. Those are different stable chemical structures from the starting structure. So, strictly speaking, that’s a chemical change.

  This is one of those cases in chemistry where semantics makes a difference. We need to be very clear about what we are really asking.

Any time you consider whether a process involves a chemical change, simply ask yourself the following questions:

If the answer to any of these questions is “yes”, you know that a chemical change occured.

And if you still wish to present common phenomenological descriptions of chemical vs. physical change to your students, do so only after they have learned about chemical bonds and chemical structures, and how the two determine chemical change.    Remember:

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