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Name: Alden
Status: other
Age: 60s
Location: N/A
Country: N/A
Date: 2001


Question:
What is the name of this natural process?


Replies:
How are the water molecules spaced within the air, do they form a regular geometrical arrangement of points over an area?


The solubility of water vapor in air has absolutely nothing to do with the molecular structure or chemical properties of air. The water molecules simply bounce around in exactly the same manner as the other molecules in the air - principally nitrogen, oxygen, carbon dioxide, and argon. For the most part, the molecules of the different gases and vapors in air are randomly distributed and continuously intermixing. Sometimes, some of the molecules will stick together in clusters for a while, but only a minor fraction at ordinary temperatures and pressures.

Incidentally, the water molecule is the LIGHTEST of all those molecules listed.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois


Last inquiry first: Water molecules that are in the vapor (gaseous) state are more or less randomly distributed in air. They do not associated with one another to any appreciable extent. Of course, air over a body of water is likely to contain more water vapor than air over a desert, but so long as there is air movement, the water molecules will be randomly distributed. In fact, that is the equilibrium condition. A region containing a higher concentration of water molecules will tend to lose water molecules to a region containing a lower concentration. This is due to the random collisions that tend to "spread" molecules, any molecules equally in all directions unless there is some external factor(s) inhibiting this diffusion process.

The amount of water vapor in the air does not depend upon the air (except to a minor extent due to the solubility of atmospheric gases in liquid water) but depends upon the vapor pressure of the liquid water. The vapor pressure of water depends only on the temperature (again, assuming the water is free of solutes). When the amount of water vapor in the air above a pool of liquid water reaches the amount corresponding to the vapor pressure, we say the air is saturated or that the relative humidity is 100% (different words, same meaning).

Transpiration (the movement of water vapor due to the flow of air), convection (the movement of water vapor due to a difference in temperature (hot air with its water tends to rise because it is less dense), and diffusion (the movement of water vapor molecules due to random collisions with other molecules) are the mechanisms for the "mixing" of water molecules.

The equilibrium condition for a mixture of any gases is equal concentration of each component gas. The reason that "heavy" gases appear to "sink" or "hover" closer to the ground is the fact that convection and diffusion are usually slow processes, so what we are observing is a non-equilibrium condition. The force of gravity on an individual molecule is extremely small because of the small mass of the individual molecules.

When we talk about a system (air + water vapor in this case) not being "at equilibrium", understand we are not talking about the "speed" at which the processes occur, but about the ultimate condition that the system. There are many cases where non-equilibrium states may last a long time even though they are not at equilibrium -- diamond with respect to graphite at room temperature, wood in the presence of air with respect to water and CO2 at room temperature are two such examples. Equilibrium refers to the ultimate "possibility / impossibility" of a process, not the "speed" with which the process occurs.

Vince Calder


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