Science: An Elementary Teacher’s Guide/Properties of Air
Properties of Air
editEarth's atmosphere is composed of air. Air is a mixture of gases, 78% nitrogen and 21% oxygen with traces of water vapor, carbon dioxide, argon, and various other components.
We usually model air as a uniform (no variation or fluctuation) gas with properties that are averaged from all the individual components. Any gas has certain properties that we can detect with our senses. The values and relations of the properties define the state of the gas. Air is odorless, colorless, and tasteless. Air is everywhere, it is Earth's main resource. Among its fascinating property are the effects of air pressure and Bernoulli's principle.
Air is an example of matter. Air is a mixture of gases consisting of approximately 4/5 Nitrogen and 1/5 Oxygen. Like all matter, air has weight and takes up space. It can also be compressed, unlike solids and liquids.
Earth is covered by a blanket of air called the atmosphere. The atmosphere begins at the Earth’s surface and extends over 600 miles into space. Let’s investigate the properties of air and the structure and dynamics of the Earth’s atmosphere!
Air is a mixture of gaseous elements and compounds. These include nitrogen, oxygen, water, argon, and carbon dioxide. Surprisingly, the element nitrogen makes up the largest proportion of Earth’s air. Oxygen makes up the second largest portion of air. Although air appears invisible to us, it has mass and takes up space like other types of matter. Because of this, air exerts pressure. The air surrounding the Earth is held in place by gravity. Gravity pulls the air toward the Earth’s surface giving the air its weight. Air pressure is the weight of a column of air pushing down on a portion of the Earth’s surface. As you move up into the atmosphere the air pressure decreases. This is because at higher altitudes there is less air pushing down from above.
Air has weight. Because the weight of air varies with pressure and temperature it has to be defined accurately. The following figures may be used. The weight of dry air (no moisture content) at 0 deg C and under a normal atmospheric pressure of 1013 mbar is 1.293 kg/m3. The weight of dry air (no moisture content) at 0 deg C and at a pressure of 1000 mbar (1 Bar) is 1.275 kg/m3.
Air is under pressure. Air is under pressure; this is caused by gravity. Air pressure at sea level is approximately 1013 mbar, which is about the same as 14.7 psi. The reason for this pressure is because there is so much air stacked up on top of it. If you were higher up, say in and aero plane, the air pressure outside the 'plane would be much lower. We know that the air pressure at 18,000 ft. (about 5500 meters) is approximately half that at sea level. At 32,000 ft. (about 10,000 meters) the air pressure is only a quarter of that at sea level. The reason for the reduction in pressure is because there is less air stacked up on top at these high altitudes.
- Air pressure is all around us as we live under a "sea of air" - a bit like a fish surrounded by a sea of water.
- The air presses on us from all sides, but we are so used to it we don't feel it. Every part of our body is pushing back (each cell is like a balloon) so we don't get squashed flat.
Air has temperature. Like most things around us, air expands when it gets hot and contracts when it gets cold. Temperature has an effect on Volume, and that Volume has an effect on Pressure.
Air has a volume. Air occupies a specific volume. This volume is inter-related with pressure and temperature. If you squeeze air into a smaller space the air gets hotter. This is easily demonstrated when you pump up a bicycle tire. The harder you pump, the hotter the air gets and the hotter the hand pump gets. Because the amount of air contained within a box will vary with temperature and pressure, it is necessary to qualify the temperature and pressure.
Air usually contains some water vapor. Air behaves a bit like a sponge, if there's any water around it will try to absorb it. Like a sponge it can only hold just so much water before it becomes saturated. Again like a sponge, if you squeeze it (compress it) the water will drip out. A dry sponge doesn't have any water in it; therefore it has a relative humidity of 0%. A soaking wet sponge can't take in any more water because it's already saturated. Therefore this sponge has a relative humidity of 100%.
Air usually has some velocity (speed). You can see this every day, leaves getting whipped up by the breeze and being blown down a road. Outside air velocity is a function of wind strength. The velocity of air in a room may be low at 0.25 m/s or much higher in a compressed air pipe.
Experiments Ideas
editExperiment 1
Magic Water Glass Trick
Fill a glass one-third with water. Cover the mouth with an index card and invert (holding the card in place) over a sink. Remove your hand from the card. Voila! The card stays in place because air is heavier than water, and the card experiences about 15 pounds of force pushing upward by the air and only about one pound of force pushing downward from the water - hence the card stays in place. (Try this trick over someone's head when you get good at it.)
Experiment 2
Fountain Bottle
Seal a 2-liter soda water bottle (half-full of water) with a lump of clay wrapped around a long straw, sealing the straw to the mouth of the bottle. Blow hard into the straw. As you blow air into the bottle, the air pressure increases. This higher pressure pushes on the water, which gets forced up and out the straw.
Experiment 3
Kissing Balloons
Blow up two balloons. Attach a piece of string to each balloon. Have each hand hold one string so that the balloons are at nose-level, 6" apart. Blow hard between the balloons and watch them move! The air pressure is lowered as you blow between the balloons (think of the air molecules as ping pong balls ... they balls don't have enough time to touch the balloon surface as they zoom by). The air surrounding the balls that's not really moving is now at a higher pressure and pushes the balloons together.