By Bob Feurer
A column of air is pressing down on all of us. Since gravity pulls most of the atoms and molecules toward the Earth the atoms are closer together at the bottom of the column than at the top, "outer space", as we sometimes call it. Even though any air molecule or atom does not weigh much by itself, together all of these atoms and molecules add up to be a lot of mass.
Investigating Air Pressure
Let's take a look at the four major gases that comprise the atmosphere, nitrogen, oxygen water and argon. Nitrogen is 80% of air, oxygen-20%, water 0 to 4% and argon is 1%. You see these add up to more than 100%. These figures are rounded and the amount of each type of molecule or atom is not always the same in air. The other gases are present in tiny amounts!
Nitrogen is a diatomic atom as mentioned earlier. Oxygen is also diatomic, water is a molecule and argon is an atom. The formula for each is N2, O2, H2O, and Ar, respectively.
Use your periodic chart of the elements to determine how many atomic mass units each molecule or atom of each of these gases weighs.
- Nitrogen (N2)=
- Oxygen (O2)=
- Water (H2O)=
- Argon (Ar)=
We are going to create a model of this column of air and discover one of the reasons why low and high pressure systems exist and why low pressure is often associated with bad weather.
Equipment needed: To do this you, your partner and you or your team will need a couple of sheets of cm square or larger graph paper, a stapler and a balance, preferably an electronic or precise triple beam that measures mass accurately to the hundredth of a gram.
Procedure: Treat each square on the graph paper as one atomic mass unit. How many squares would it take to make a model of nitrogen atom?
Of a nitrogen molecule?
Using the scissors, graph paper and stapler, what would make the best model of a nitrogen molecule. (Hint: what would be "most"like the true condition of N2?)
You need to create eight nitrogen molecules, two oxygen molecules, four water molecules and one argon atom before you begin this activity. Don't use more than two staples on any model!
Our make believe column of air has room for only 11 atoms or molecules of air. So, begin by creating a "dry"column of air, that is no water molecules. Weigh these 11 air parts on the balance and record the mass below in the data table.
Now begin adding water to the column. Remember only a total of 11 atoms or molecules can exist in your column so if you add a water molecule an atom ar molecule of one of the other compounds or elements must be removed! (Atoms/molecules all take up space and two can't exist in the same space!) Weigh your slightly wet column of air and record the mass below.
Repeat this procedure until all four water molecules have been added, one at a time.
mass of model air column
- "dry column of air"
- 1st "wet" column
- 2nd "wet" column
- 3rd "wet" column
- 4th "wet" column
Which column would create a "high" pressure area? Why?
Which column would create the "lowest" pressure area? Why?
How would the outcome of your model above have changed if you had used 800 nitrogen molecules, 200 oxygen molecules, 400 water molecules and 100 argon molecules?
Describe below as specifically as you can what kinds of weather is usually described as "bad" weather?
What thing(s) do they all seem to have in common?
What do the bad weather and your lowest pressure model air column have in common?
What would a low pressure area on a weather map usually be an indicator of?
Low Pressure can also be created by temperature differences without any changes in the numbers of atoms/molecules of air. Sketch an example or explain anecdotally (in words) how temperature can affect air pressure! Do the sketches or the anecdote, not both!
Make a circle about 3" in diameter and sketch a sample of air molecules/atoms that are cold. Use oxygen, nitrogen, carbon dioxide and argon. The circle is just a sample line for the gases. It cannot contain the particles.
Make another circle about 3" in diameter, the same atoms/molecules you sketched above have increased their temperature. Sketch them as they would appear heated up. As prior, the circle is just a sample line and cannot keep the atoms/molecules from moving outside the circle.
Calculate the mass of the cold and hot air molecules in your examples above. Does high pressure occur with cold or hot temperatures? Why?
If you chose not to do sketches, write your explanation below!