Vapor Distribution in a Vacuum
Date: Summer 2013
When something vaporizes in a vacuum, the vapors seem to distribute all around, and even rise. What is the force that is causing this, since there is not buoyancy?
At finite temperature, the molecules in the system have kinetic energy.
- Isaac Tamblyn
The vapor molecules all possess translational kinetic energy because their temperature is greater than absolute zero. That movement disperses them throughout the available volume.
Richard E. Barrans Jr., Ph.D., M.Ed.
The individual molecules of a vapor are moving extremely fast in all different directions. These molecules frequently crash into each other. This is what provides the pressure in the container before the vapor is released into the vacuum. Once released, the molecules are still very fast, still crashing into each other. This causes the molecules to spread out. Because single molecules have very little mass, the forces from the collisions have much more effect than does gravity. The molecules do not ever stop moving, because there is nothing to slow them down.
Solid and liquid molecules move around just as fast as do vapor molecules, but the molecules of solids and liquids are joined together, preventing the material from spreading out. This makes it possible to see the effects of crashing into neighbor molecules visible only in vapors.
Dr. Ken Mellendorf
Illinois Central College
The force that kicks a molecule out of the liquid or solid that is vaporizing
is what causes that molecule to move away from the source. It keeps moving
because it does not run into anything, because it is in a vacuum.
It is pressure that causes gas molecules to distribute out and result in uniform pressure. Gaseous molecules are able to move freely and redistribute until the pressure in a container is uniform.
Hope this helps,
Every material has what is called a "vapor pressure" which is the pressure at which an equal amount of gas is ejected from the material as is deposited over time. What happens is that every gas is comprised of free molecules in motion. These molecules are flying in all directions bouncing off one another, walls, and anything in their way. The average energy of all this random motion is related to the temperature of the gas. Thus, when you heat something to vaporization, you are increasing the energy of this motion to such an extent that they break free from the solid or liquid in which they were bound. If we think of a surface vaporizing, we can imagine the molecules traveling in all directions in a random fashion. Those traveling towards the surface will be stopped, or they will bounce away from the surface. Those pointed away from the surface will travel until they hit another molecule or simply continue to travel away. The net result of all this random motion is that the gas will fill whatever vacuum you have. If you have a container holding the vacuum, you will start to see a pressure rise as the molecules start hitting the container walls. Some of the molecules will bounce back to the original surface. The pressure of this gas will rise until the net molecules ejected from their source (the vaporized surface) equals those bouncing back and returning. The pressure stabilizes, and we call this the vapor pressure of the material.
The appearance of the gas to "rise" is simply due to the fact that the gas molecules are traveling in every direction including away from the ground.
Kyle J. Bunch, PhD, PE
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Update: November 2011