Humidity and Freezing Temperature
Date: Fall 2012
We are studying properties of water. This week we were talking about evaporation and water vapor. One of my students asked, "If there is water in the air all the time in the form of water vapor, does it freeze when the temperature goes below 32 degrees F?" I did not know how to answer. We hypothesized that maybe it did not because the molecules were moving too quickly or that maybe because of their small size. I told him I would look into it, but cannot find an answer on the Internet. Do you know?
Most people would agree this is a perplexing question. You were correct for your hypothesis. There is water in the air virtually all the time because of its motion, but the vapor pressure of it is very low at low temperatures.
Your explanation was correct. Ice has water molecules in motion with a kinetic energy sufficient to partition into the air and thus have a vapor pressure. Let's look at some temperature and vapor pressure numbers for water:
32 F, 4.6 mmHg; 70 F, 21 mmHg; 98.6 F, 47 mmHg and 212 F; 760 mmHg.
Here, you will notice that at freezing, there is only 1/4 of the water in the air as there is at room temperature. However, there is still some water present at freezing. Most is ice, but some is left because of motion.
You will also notice the frozen water has 1/10th the vapor pressure as we see at body temperature. Hence, we must constantly replenish our internal water when exercising in the cold. Essentially, we blow off a lot of water. For all warm blooded animals, cold is a water sink that must be constantly refilled. As for the boiling water, you may envision that the air pressure(760 mmHg) and water pressure are the same, the water is now a freely expandable gas.
The low vapor pressure of frozen water is partly the reason why ice cubes will mysteriously shrink in our auto-clean freezers. The frozen air of the freezer siphons away the water in the air so that it does not accumulate on the cooling coils.
You were wonderful in working through a complex problem with your students!
This note does not improve your explanation, but did locate some facts for you. Hope it helps. Peter E. Hughes, Ph.D. Physical Astrochemistry, MNHRO, Milford, NH
There is a misleading, but common, assertion here. Specifically, … “there is water in the air all the time in the form of water vapor.” The amount of water vapor in the atmosphere depends upon the temperature, and to a good approximation (ignoring the small solubility of various components of air (N2, O2, CO2, etc.). The partial pressure of water and/or ice depends ONLY on the temperature. The partial pressure of water in the atmosphere can be looked up using tabulated data from any of a number of chemical/physical handbooks. This is usually tabulated as partial pressures, but can be converted to molar density using the ideal gas law: PV = nRT, or rearranged (n/V) = P/RT. The water vapor does NOT completely disappear when the temperature decreases below the freezing point. Living in MN, you must have observed that ice (solid water) evaporates easily even below 0 C. It only depends upon the motion of water molecules in a very indirect way. That is not the major effect.
Thanks for the question. There is always water in the air, specifically water molecules. However, as the temperature gets colder, there are less water molecules because they condense out of the vapor phase to form frost, dew, and clouds. The concept of vapor pressure measures how much water is in the atmosphere. It is a technical concept and we usually do not teach it until high school chemistry. You are correct in that the water molecules are moving too quickly to "freeze out."
I hope this helps.
Technically speaking, a gas remains a gas in all temperatures.
Only liquids "freeze" (turn to the solid form).
However, if speaking about water droplets (the liquid form of water) in the air, they typically freeze well below the freezing point of water because they are normally a mixture of water and a contaminant upon which they form. Water
droplets form on a "particle" which serves as a condensation nucleus.
As the particle is dissolved in the water, it becomes a mixture and thus
changes the properties of the water droplet. The water-particle mixture has a freezing temperature lower than that of water. Thus, liquid "water" drops in the air can often remain in the liquid state to a few degrees or more below the
freezing temperature of water.
David R. Cook
Atmospheric and Climate Research Program
Environmental Science Division
Argonne National Laboratory
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