Gases: Composition, Misconceptions, Gases at Work, Gas Laws, and Density

In the fall of 2002 I got to go to Switzerland. On the morning of November 6th, I had to leave the village of Zermatt, but wanted to take one more look at the Matterhorn. The air was absolutely clear. Cleaner than I have ever seen air anywhere. The Matterhorn jumped out at you. By the way, Zermatt does not allow automobiles. People walk or ride electric vehicles. The train that goes there and the trolley that takes you to the ski lifts also run off of electricity.

NITROGEN (N2) #1: 8 out of 10 atoms (or molecules) of air are made of nitrogen. Nitrogen gas can't be seen but when it is cooled to -320 °F (-195°C) it turns to a liquid, which you can see. Being this cold makes it handy for many things. Shown here, it is being used to quickly freeze cream and sugar to make ice cream.

NITROGEN #3: Oxygen reacts with many things including paint. Therefore, expensive works of art and other rare relics are stored in pure nitrogen. Nitrogen is fairly inert and doesn't react (combine) with most other substances, so it is used to protect items and keep oxygen away..

OXYGEN #2: Oxygen is everywhere. As a gas it makes up 20% of the air. Some oxygen gas is dissolved in water, and some oxygen gas is in the pores in the ground.  However, there's a lot more oxygen in this picture that we don't normally consider. The weight of the water in the river is 90% oxygen atoms. Remember water is H₂0.  The one oxygen in H₂0 weighs 8 times has much as the two hydrogen atoms combined.  The trees and grass are made of carbon, oxygen, hydrogen, and nitrogen, with oxygen accounting for about half of a plant's weight.  The rocks, the soil, and the mountains contain metals and silicon that is bound to oxygen, which makes up about 1/2 of their weight. Think about it; one half of a mountain's weight is from oxygen. The only thing in the picture that doesn't contain oxygen are the metal rails.

To us the atmosphere seems very thick, but compared to the Earth, it is only a thin skin of gas. Seeing how thin it should help us realize how easily it could be filled with pollutants.

Besides giving us the oxygen we need to breathe, it also shields us from harmful ultraviolet radiation and small meteors. The atmosphere also helps hold in the warmth of the sun and spreads the warmth more evenly over the Earth's surface.  It also carries water from the oceans to the land by way of clouds and rain.

Misconception #2: Air is light weight.

We don't feel the weight of air nor do balloons seem heavy.  It is true that for the same volume, air is lighter than liquid or solids.  But there are many miles of air above us pushing down with incredible weight.

This is how the plane can fly. The top drawing shows the wing not moving in the air. Because the top of the wing is curved, there is more surface area and more air hitting it. The 7 balls represent the number of air molecules hitting it. Underneath the wing there are only 6. The top 7 don't push down more than the bottom 6 because the 7 are hitting at an angle which decreases their downward push. For example, the top #3 molecule has the same force (pressure) as the bottom #3, but the top #3 hits at an angle reducing it's downward force.

But when the wing is moving, the air splits at 0 and rejoins at 7. So the top doesn't have 7 hitting it anymore, just 6. The six on the top of the curved wing hit at angles which reduces their downward force. The 6 under the wing hit the wing with a push that goes straight up. So the bottom 6 out-push the top 6 and the plane is lifted up.

Since air is invisible it's easy to think of it just being still because we've heard the phrases, "calm air" and "the air was still". However, even when air is not blowing, it is far from being still.

In a helium balloon, the helium atoms are traveling an average of 3,000 miles per hour! In air, the oxygen and nitrogen molecules* are traveling about 1,000 miles per hour! At these incredible speeds, one atom will collide with other atoms and objects near them 7 billion times a second!

(*Note: Oxygen atoms travel in pairs as does nitrogen atoms. Two or more atoms combined is called molecules. So we say a gas of oxygen molecules and nitrogen molecules rather than saying oxygen atoms and nitrogen atoms)

Suction Cups:

The way suction cups work is usually very much misunderstood. Contrary to popular believe, suction is not what holds suction cups to the surface.

However, if the suction cup is pressed all the way down to expel all of the air, there is no longer any pressure pushing upward on the suction cup and the outside air pressure is now pressing down on the suction cup. This is what holds the suction cup in place.

So there is no sticking or attraction force that makes the suction cup stick to the surface. It is held there by air pressure. It just seems that the suction cup has some unseen stickiness holding it there.

Sometimes suction cups are used for lifting or pulling. Let's say the surface is that of a table. When a person pulls on the middle of a suction cup, they have to overcome the pressure of the 80 miles of air above the suction cup, which is 15 pounds per square inch. When they do, the suction cup lifts a little creating a vacuum gap. A vacuum is nothing, so it has no force. So it no longer pushes down on the surface. However, underneath that surface, air pressure is still pushing up at 15 psi, so it will lift the table (provided the table doesn't weigh too much). For example, if the vacuum gap covers 3 square inches, then 3 square inches under the table will push with 45 lbs (3x15 psi.) To lift a heavier table, you need a bigger suction cup that can make a vacuum gap with a bigger area.

One clue to understanding this is to notice what happens to the cheeks of people drinking through a straw. This is especially noticeable if the drink is thick like a shake. You see that their cheeks are pushed in.

This is caused by the air pressure outside their cheeks being higher than the air pressure in their mouth.

If you have a straw in your mouth, then air pressure pushing on your drink has more force than the force from the air in your mouth. The outside air pressure pushes onto the surface of the drink. This pressure pushes liquid up through the straw to your mouth. When you don't want to drink anymore, you will move your jaw upwards causing the air in the mouth to be more crowed, which increases the air pressure in the mouth to equal that of the outside air pressure. The liquid will stop flowing.

The air around us is being compressed by all of the air above us. The closer the air is to the ground the more weight is on it, and the more compressed (or dense) it is. That means the air at the top of an object is not as dense as the air under objects. In fact the air around us is constantly trying to lift everything because the air below objects press a bit harder than the air above the object.

A common misconception is that exhaust, whether it be steam or flames, is the reason for the thrust and the propulsion. It looks that way because that's where all the action is. However, how can flames outside the missile actually do any pushing on the missile? Usually people think that the flames are pushing on the air and that gives the missile propulsion. But missiles work just as well in space where there's nothing to push on.

Gasoline Engine: The word "gas" is an alteration of Latin, chaos. It's well chosen because when gas burns, the molecules are in chaos.

Here's an article from 1897 talking about the new gasoline engine.
"The gas engine is one of the wonders of the 19th century. Now, within three years of the 20th century, it is a novel machine, eagerly sought by many people. It is thought by persons who have not studied its principles that it is a steam-engine, using gas or gasoline as fuel for the purpose of making steam. This is erroneous. Gas and gasoline in specific proportion with air are explosive material."

"The expansive force derived from explosion of these materials in the cylinder is the force that is substituted for the expansive force of steam. Hence, owing to the economy of this method as a means of deriving power, the steam engine and boiler are fast disappearing, and the Gas Engine is taking their place for small power."

The above animation shows that pressure is directly proportional to temperature. In other words, if temperature goes up, pressure also goes up. If temperature goes down, pressure must go down.

When this is combined with the knowledge that pressure is inversely proportional to volume, we get the bottom relationship. That says the product of pressure multiplied by volume is directly proportional to the temperature.

So far we mentioned pressure in atmospheres and in pounds per square inch (psi). I'm sure you've heard of other units. The easiest way to measure air pressure was to pour mercury into a tube that was about a yard tall. Then put the thumb on the opening and turn it upside down with the opening submerged in a bowl of mercury. You would think gravity would just pull the mercury down; however, air pressure in the bowl pushes on the surface of the mercury in the bowl which then pushes in all directions including upward to hold the mercury up. At sea level the air pressure holds the mercury 760 millimeters (29.9) inches above the surface of the mercury in the bowl.

Instead of 760 mm, some people call it 760 torr after a scientist named Evangelista Torricelli.

If water is used in the tube, air pressure can hold a column of water that is 34 feet high no matter how large of a diameter.

Gas Density is important to emergency responders because they need to know if the toxic gas is going to be low to the ground, up near the ceiling, or mixed with the air. Knowing the density tells them that.

I've mentioned that a lot of these non-metal form compounds that are toxic. It's smart to know if they are heavier or lighter than air. The Periodic Table will help you calculate the density.