Section 1–3 Atomic processes

(Evaporation of water / Dissolution of salt / Equilibrium)

In this section, Feynman describes two atomic processes: water evaporating in air and salt dissolving in water. In addition, he explains the meaning of equilibrium and discusses liquid-vapor equilibrium during the evaporation of water and solid-liquid equilibrium during the dissolution of salt.

1. Water evaporating in air:

“From time to time, one on the surface happens to be hit a little harder than usual, and gets knocked away (Feynman et al., 1963, section 1.3 Atomic processes).”

Feynman mentions that air consists of oxygen (two oxygen atoms stuck together to form one oxygen molecule), nitrogen (two nitrogen atoms stuck together to form one nitrogen molecule), as well as lesser amounts of carbon dioxide, argon, and others. Next, there are also water vapor molecules above the liquid water. More important, the main process for the evaporation of water is that a water molecule on the surface of water happens to be hit slightly harder than usual, and gets ejected from the surface to the air. In short, it is similar to a sufficiently energetic ball hits another less energetic ball. However, there is a problem of representation in Figure 1–5 (Water evaporating in air): the dynamic process of evaporation cannot be clearly shown by the static figure.

Essentially, water evaporates when water molecules gain sufficient kinetic energy from random collisions such that they can break away from the attractions of previously nearby water molecules. Furthermore, water molecules that leave tend to have more kinetic energy than the average water molecules, whereas water molecules that stay tend to have lesser kinetic energy. Thus, the liquid gradually cools due to evaporation because the remaining water molecules have lesser average motion than what they had earlier. Nevertheless, the real process is slightly more complicated when it involves other impurities such as nitrogen molecules that “join in” or “dive in,” and “get lost” among the water molecules.

On the other hand, Feynman provides a real life example that may happen to divers. There could be some elaborations because he simply states that “the air molecules leave more rapidly than they come in, and in doing so will make bubbles.” As a suggestion, we can explain that it is about the decompression sickness and the process is similar to what happens when you open a can of coke. Simply phrased, the pressure surrounding the drink is decreased, which causes the carbon dioxide gas to come out of the liquid coke in the form of bubbles. Similarly, if excessive nitrogen bubbles are formed in your blood, they can damage blood vessels and even obstruct the flow of blood. It is the formation of nitrogen bubbles that causes decompression sickness or nitrogen narcosis.

2. Salt dissolving in water:

“If there is almost no salt in the water, more atoms leave than return, and the salt dissolves. If, on the other hand, there are too many “salt atoms,” more return than leave, and the salt is crystallizing (Feynman et al., 1963, section 1.3 Atomic processes).”

According to Feynman, salt is a solid that has an organized arrangement of “salt atoms,” or to be more precise, it is made of sodium ions and chlorine ions. He elaborates that an ion is an atom which either has a few extra electrons or has lost a few electrons. In a salt crystal, chlorine ions (chlorine atoms with an extra electron) and sodium ions (sodium atoms with one electron missing) are stick together by electrical attraction. Interestingly, they are separated in the water due to the attractions of the negative oxygen atoms and positive hydrogen atoms for the ions. The hydrogen ends of the water molecules are more likely nearer to the chlorine ions, while the oxygen ends are likely nearer to the sodium ions. This is because the sodium ion is positive and the oxygen end of the water is negative, and they attract electrically.

There is also a problem of representation in Figure 1–6 (Salt dissolving into water). Feynman asks whether we can tell from this figure that the salt is dissolving into water or it is crystallizing out of the water. It is unclear whether the figure represents the dissolution of salt because some of the salt atoms are leaving the crystal and the other salt atoms are rejoining it at the same time. If there are almost no salt atoms in the water, more salt atoms are likely to leave (dissolve) than to return (crystallize) and the salt is dissolving. If there are too many salt atoms in the water, more salt atoms are likely to return than to leave and the salt is crystallizing.

Feynman ends the section by saying that it is very difficult to predict which way the process will go, that is, whether more or less salt will dissolve. He could have related this process to another real life example. For instance, there could be an elaboration on the impurities in common salt or the salt in Dead sea. Better still, Feynman should discuss the following anecdote: Mrs. Eisenhart asked him, “Would you like cream or lemon in your tea, Mr. Feynman?” Interestingly, Feynman’s answer was “I'll have both, thank you.” However, her response was “Heh-heh-heh-heh-heh. Surely you're joking, Mr. Feynman (Feynman, 1997, p. 60).” The atomic processes involved in mixing (or dissolving) cream and lemon with tea should be more complicated. But, another question is “Would you drink Feynman’s tea?”

3. Equilibrium (similarities and differences):

“By equilibrium we mean that situation in which the rate at which atoms are leaving just matches the rate at which they are coming back (Feynman et al., 1963, section 1.3 Atomic processes).”

There are different kinds of equilibrium in the two atomic processes: liquid-vapor equilibrium during the evaporation of water and solid-liquid equilibrium during the dissolution of salt. When water is evaporating in air, it reaches a phase equilibrium in which the rate of evaporation and the rate of condensation are the same. (Feynman’s use of the term equilibrium is vague because phase equilibrium is closely related to thermal equilibrium, mechanical equilibrium, and chemical equilibrium.) When a salt crystal is dissolving in water, it reaches a solubility equilibrium in which a chemical compound such as salt in the solid state is in chemical equilibrium with a solution such as water. During this equilibrium, the rate of dissolution is equal to the rate of precipitation.

Feynman explains that the word equilibrium simply means the situation in which the rate of atoms moving away is equal to the rate of atoms coming back. Although both processes appear to be in static equilibrium according to our eyes, they are actually in dynamic equilibrium under a powerful microscope. In short, this equilibrium is macroscopically static and microscopically dynamic. Importantly, we have used the term atoms instead of molecules during the dissolution of salt. Feynman clarifies that the concept of a molecule of a substance is only approximate and exists only for certain substances. More appropriately, he describes salt in terms of an arrangement of sodium and chlorine ions in a cubic pattern and explains that it is not natural to group them as “molecules of salt.”

Questions for discussion:

1. Explain the evaporation of water in terms of atoms or molecules.

2. Explain the dissolution of salt in terms of atoms or ions.

3. Compare the two atomic processes: liquid-vapor equilibrium during the evaporation of water and solid-liquid equilibrium during the dissolution of salt.

The moral of the lesson: Phase equilibrium and solubility equilibrium are reversible atomic processes that are macroscopically static, but microscopically dynamic.

References:

1. Feynman, R. P. (1997). Surely, you’re Joking, Mr. Feynman. New York: Norton.

2. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley.