States of Matter:

By the Book

Atoms and molecules are always moving – unless they are in textbooks!

Particles in all states of matter are in constant motion. And the details of their movement depends on the state. However, with even the most powerful optical microscope, these particles and their motions are invisible. And textbook and whiteboard descriptions and drawings of particles in different states are only that – simply words and 2D, static pictures.

Listed below are some of the characteristics that we commonly associate with solids, liquids, and gases.

Commonly Taught Characteristics of Solids, Liquids, and Gases
State:	Solid	Liquid	Gas
Volume:	fixed	fixed	variable
Shape:	fixed	fills container	fills container
Inter-Particle Distance:	close	close	distant
Speed of Particles:	slow	faster	fastest
Movement of Particles:	rigid	flow easily	free
Particle Model:

Students are very capable of grasping and reproducing these ideas, as demonstrated by the figure below.

[Posted by a chemistry teacher on Twitter. Used by permission.]

Individually, there is nothing wrong with any of the above descriptors of the states of matter. But even collectively, they add up to a missed opportunity:

They describe, but do not explain the differences between the states.

Hold On. Then Let Go.

What’s missing are any descriptors that address what holds the particles together… and how they let go. These are, of course, the attractive forces between the particles and the details of how these forces are overcome at different temperatures.

One could, of course, simply draw vectors between the particles to indicate these forces. But, as you’ll soon see, the story is more complicated and static drawings simply aren’t up to the task.

What if you could simply show these interactions to your students (or better yet, let them do the experiment themselves) and let them deduce, for themselves, how they work and how they lead to different states of matter?

You can.

Click to view each of the simulations of a cluster of water molecules at three different temperatures. These temperatures correspond to the solid, liquid, and gas states.

Hydrogen bonds are highlighted as green lines. Observe the behavior of the hydrogen bonds in each state.

You should notice that in the:

• solid state, the hydrogen bonds are static
• liquid state, the hydrogen bonds repeatedly break and re-form – they are dynamic
• gas state, the hydrogen bonds are simply absent

As we all know, the most important attractive force that determines water’s state is the hydrogen bond. But this concept of the dynamic nature of the attractive forces applies equally to any substance that undergoes a solid → liquid → gas transition.

How should we interpret this correlation between the attractive forces and the states? Let’s add the following row to the above table:

State:	Solid	Liquid	Gas
Potential Energy of Interaction:

All interactions between particles at the atomic level can be represented by a potential energy curve (function) such as in the table above. The vertical axis represents the potential energy of the interaction and the horizontal axis represents the distance (r) between the particles. (The bond dissociation energy curve is an example that should be familiar.)

These curves all share common features. On the left-hand side, the curve is repulsive and the potential energy goes to infinity as the particles overlap. Somewhere near the middle of the curve, the “bottom of the well” represents the distance between particles at which the potential energy is at a minimum. This minimum could represent a covalent bond length or the ideal hydrogen bond distance, as in the simulations above. To the right of the bottom of the well, the potential energy rises until a distance is reached at which the particles no longer interact.

In the solid state curve, above, at low temperatures, the particles oscillate about the bottom of the well. This is illustrated in the Water Ice simulation above; the green hydrogen bonds form and they do not break.

As temperature rises, increased kinetic energy causes the particles to sample a higher section of the curve. In the liquid state, this means that sometimes the particles are in the attractive part of the curve, and sometimes they are on the right-hand side of the curve – where they are no longer constrained by attractive forces and they become separated. In the liquid simulation, this is illustrated by the green lines forming, disappearing, and re-forming dynamically. The dynamic hydrogen bonding (as well as other IMFs) is still sufficiently attractive to hold the water molecules together.

As temperature rises further, the system samples levels of the potential energy curve at which the particles do not re-enter the attractive section of the curve. This is the gas state – there are no longer any hydrogen bonds.

Of course, we could just describe this behavior, in words, by adding the following two rows to the table.

Which of the two methods do you think your students will better understand and remember?

• Words
• Hydrogen bonds that they SEE appear and disappear between water molecules at different temperatures (and, better yet, that they “FEEL”‡)

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