Last updated 1-15-11
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Gases: Composition, Misconceptions, Gases at Work, Gas Laws, and Density

Composition of Clean Air

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.

Here are 10 gases that make up clean air: In order of highest to lowest concentration they are Nitrogen, Oxygen, Argon, Carbon dioxide, Neon, Helium, Methane (CH4), Krypton, Hydrogen, and Xenon.  Five of them travel alone, so we call them atoms. For example, a helium balloon contains atoms of helium (He), but an oxygen tank contains molecules of oxygen (O2). When there's two or more atoms are bonded together, they are called molecules.

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 #2: Liquid nitrogen is also used to cool certain metals to the point where they become superconductors. Superconductors conduct electricity (electrons) without any resistance. In this picture a rectangular shaped magnet had been dropped onto a metal disk. As the magnet fell toward the metal disk, the magnetism in the magnet caused currents of electricity to flow inside the metal disk. The currents created in the metal disk by the falling magnet made that part of the metal disk a magnet itself. The falling magnet then feels the repulsion of the magnet it just created. The falling magnet will then stop just above the metal disk. Electricity continues to flow in the metal disk constantly creating a mirror image magnet that repels the real magnet. This will continue forever as long as the metal is cool enough to stay a superconductor.

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..

NITROGEN goes bad: Under high temperatures, like in a jet engine or car engine, nitrogen will combine with oxygen to form a class of toxic compounds called nitrogen oxides. The simplest having one nitrogen and one oxygen (NO). Others have two nitrogens and one oxygen (N2O), one nitrogen and two oxygens (NO2), and the fourth has two of each (N2O2). Their names (in order) are nitrogen oxide, dinitrogen oxide, nitrogen dioxide, and dinitrogen dioxide. Nitrogen dioxide has a brownish appearance and is often what you see in polluted cities.
Oxygen #1: 2 out of 10 atoms/molecules of air is oxygen. Oxygen is produced by plants and is consumed by animals. Animals need oxygen to "burn" the food they eat in order to get the calories (energy) they need. The way animals use oxygen to burn food is different than a fire, but it produces the same products of carbon dioxide and water. Of all of the gases that make up air, oxygen is the most reactive.

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.

OXYGEN #3: In the movie Total Recall, there was an ancient alien factory that could release the oxygen from the Martian soil. This is actually possible. Extreme heat will cause the oxygen to separate from the metals and non-metals that it was bound to. Most of Martian soil is iron oxide, which would turn to iron metal and oxygen gas at high temperatures. In the pictures above, Arnold presses the touch sensitive on "button" of the oxygen generator.  Next he gets blown out of the factory into the thin Martian air, which causes the air in his body to expand. Fortunately, the oxygen rushes out of the top of the mountain that housed the factory and they show Mars getting an atmosphere similar to Earth's. Actually, in the movie, they say the alien factory is getting oxygen from water, which is possible but a lot of hydrogen would be also created, making an explosive mixture. My idea of getting it from the red iron oxide would be safer.
ARGON: Argon is 1 out of 100 atoms/molecules of air. Argon is an inert gas that is used to pressurize light bulbs. Being inert it doesn't react with the tungsten metal, which makes the filament. The argon gas also helps keeps the tungsten metal atoms on the surface of the filament from jumping off the filament.
CARBON DIOXIDE: Carbon dioxide only makes 3 out of 10,000 atoms/molecules of air; however, it supplies the carbon that plants use to make leaves, trunks, roots, etc. It's hard to believe that the weight of giant redwood trees came from the small amount of carbon dioxide gas in the air.
NEON: 2 out 100,000 atoms/molecules of air.  Neon is a clear inert gas; however, high voltage can strip electrons off neon atoms. As the electrons fall back into place, they give off ultraviolet light (black light), which you can't see. However, the glass tubes are coated with different phosphor powders, which glow when ultraviolet light hits them.
HELIUM: 5 out of 1,000,000 atoms or molecules of air are helium. Helium is the second lightest gas (hydrogen is lightest). It is used to fill toy balloons and weather balloons. It is so inert that there is no danger of igniting nor will it combine with other elements. For this reason it is used in welding by having helium gas surround an electric arc that is melting a metal. The flow of helium gas keeps the oxygen away from the metal so that the oxygen won't oxidize (rust) the metals being welded.
METHANE: 2 out of 1,000,000 atoms/molecules of air are methane molecules. Bacteria is a big source of methane gas and this type of bacteria is found in termites (see huge termite mound), cattle (cow flatulence), rice paddies, swamps (called swamp or marsh gas), and in the sea bed. In the sea bed, methane can be found trapped in ice (see person holding ice that's on fire). Methane also comes from petroleum fields and is the natural gas you cook with. An embarrassing source of methane is from the eating of beans.
KRYPTON: 1 out of 1,000,000 atoms/molecules of air is krypton gas. You may remember krypton as the name of the planet that Superman came from. The pieces of the planet were called kryptonite. The element krypton is used in lamps in a similar way that argon is used (see above). The word krypton comes from "Krypto" meaning hidden. The gas krypton was in a way hidden because it took quite a while for scientists to discover it.
HYDROGEN: 1 out of 2,000,000 atoms/molecules of air is hydrogen. Hydrogen is the lightest element, which is why is was the preferred gas to fill blimps and dirigibles. Unfortunately, hydrogen is very flammable, which led to the catastrophic tragedy of Germany's Hindenburg airship.
XENON: 87 atoms out of 1,000,000,000 (one billion) atoms/molecules of air are xenon. Like argon and krypton, xenon is used as the gas in lamps. This one is for a car's headlamp. These are probably the ones that look more blue.
WATER VAPOR: The other gas we didn't list before is water vapor. The concentration of the other gases are pretty consistent, but water vapor varies greatly. Using a fish-eye lens, I took this picture of a double rainbow in Geneva, Switzerland. It had rained all day, but just before the sun went down, the clouds parted enough to allow the sunshine to strike the droplets of liquid water still in the air. Water in vapor form (gas) is not visible.
One Last Look at Composition of Air:  Imagine the volume of air in a typical classroom that is 30 feet by 30 feet with a 10 foot high ceiling. Also assume, we separated all the gases. Oxygen would cover the room to about 2 feet deep. Nitrogen would fill almost to the ceiling (another 8 feet minus a couple of inches). Argon gas would fill a one inch layer over the whole room. The remaining gases fill the last one inch. Carbon dioxide has about the same volume of one student. Neon is 1.5 gallons. Helium would fill a one liter bottle. Methane gas would fill someone's 1/2 liter bottle. Krypton would fill a 12 oz soda can. Hydrogen would fill about half of a 12 oz soda can. And xenon gas would have the volume of a pencil's eraser.

Misconception #1: There's a lot of air

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

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.

Air is so heavy that it can lift this Jet. Even when the jet is sitting on the ground, the air underneath the jet is lifting with several thousand pounds of force. However, the air on top of the jet and on top of the wings is pushing it down with practically the same thousands of pounds of force.

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.

Air gets heavier as it cools. For example, sometimes rain falling in a thundercloud cools the air fast and the heavy air comes crashing downward. The dropping speed is usually about 45 miles per hour but can reach 200 miles per hour endangering planes, people, and property. This is called a microburst.
Normally the air gets cooler as we go higher, but sometimes the air near the ground is colder than the air above it. Because cold air is heavier, it will stay close to the ground. This traps pollutants. This condition is common in Phoenix in the winter and results in the infamous brown cloud.
Misconception #3: Calm Air

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)


This means that if a small air leak develops in the vacuum of space, air molecules rush out over a 1,000 miles per hour. There is no atmosphere to hold back the leak.
Misconception #4: Suction

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.

Before a suction cup even attaches to a surface, there is air pressure pushing on the surface at about 15 pounds per square.
When the suction cup touches the surface, air is still between the suction cup and the surface (e.g., table top or window). Air underneath the suction cup presses in all directions, which includes pushing up at 15 psi. The suction cup can easily be pulled away at this point.

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.


Note: In a similar fashion, any time air gets squeezed out from between two objects, they will be held together by air pressure. For example, if you've ever walked through mud, you noticed how hard it is to lift your feet. That's not because mud is sticky or thick. It's because air gets squeezed out from between your shoes and the mud. Air pressure will try to hold your feet and shoes down.

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.

Drinking through a Straw
Even though babies know how to drink from a straw, most people, young or old, don't know how it works. Most people think the suction caused by our mouth pulls the liquid up through the straw.

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.

Normally when your mouth is closed, there's not much air (blue spheres) inside your mouth. They bounce around causing 15 psi pressure in the mouth. However, when you drop your jaw and keep your lips closed, there's more room for the air to spread out (Roll mouse over image to see this). The air molecules are now spread out over a larger volume, so fewer are now striking each square inch of the inside of the mouth; so the air pressure inside the mouth is less (perhaps about 10 psi). Outside air at 15 psi is trying to get into the mouth. It pushes on the cheeks causing them to be sunken.

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.

To the left is a microscopic image of globules of fat in milk. When it was first seen through a microscope, they though these spheres were living microbes because they kept moving. Later they realized that there were simply being knocked around by something invisible, which was water molecules in motion. (roll cursor to see short clip of the motion). This same jostling motion is seen with dust or smoke particles. They are getting knocked around by molecules of air.

Here I've made a one dimensional animation of a dust particle getting bounced back and forth. (Roll cursor over image). Remember each air molecule has about 7,000,000,000 collisions per second. Since the collisions are equal on both sides, the particle stays almost in the same location. Now imagine if a fan were to push one of the air molecules on the right side away from this region. What happens is the moving air molecules on the right side have fewer air molecules to bounce off of, so they would travel farther to the right before bouncing back. The collisions on the left side would keep on coming at their same rate. Therefore, the dust particle is going to be pushed to the right. This is how a vacuum cleaner works. Air is blown out of the yellow container shown, so there's less air in the container. Less air means less collisions. However, outside the number of air molecule collisions have not gone down, so particles are going to get knocked into the vacuum cleaner by the outside air. We should also credit gravity for keeping the outside air compressed.
Misconceptions #4 & 5: Air is uniform & Hot Air Rises
Air is indivisible, so when we look at things through the air, we normally think the air is uniform. If that were true, this balloon would not float.

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.

In this picture, I replaced some of the balloons with bowling balls to make the point that air is pushing up on bowling balls or helium filled balloons the same amount. (as long as they are the same size). Again, that's because the air under a bowling ball or balloon is more dense (compressed) than the air near the top of the bowling ball or balloon.
The balloon rises because the air pushing on the bottom of the balloon has a greater force than the downward force of the air on top of the balloon plus the balloon's own weight. In the case of the bowling ball, the red weight arrow is so large, the upward lift from the bottom air goes unnoticed.
So the real reason hot air balloons float, is NOT "Hot Air Rises." It's because the air pressure on the lower half of the balloon is greater than (the combined air pressure on the upper half plus the weight of the balloon). All the hot air does is to reduce the weight of the air in the balloon. Hot air is more spread out than cool air, so it's lighter. When the burner is on, the air inside is heated and as it expands much of it goes out the bottom of the balloon. This reduces the red arrow, which represents the total weight of the balloon.
These high tech jet fighters take off from an aircraft carrier with a boost from one of the oldest of technologies — steam.
The secret to water and steam power is that the pressure inside a closed container will get very high when water is heated. That's because more and more water molecules will go into a gas phase, which dramatically increases the pressure. Normally, the pressure is used to push a piston like in the jet fighter launch above or in a steam engine. If the pressure isn't released, the container is likely to explode (roll cursor over image for animation).
The very first engine was powered by steam. It was invented by Hero of Alexandria, Egypt. Water was placed in a sphere and then heated to boiling. Small tubes allowed the steam to release. The openings caused an imbalance of the pressure in the sphere, which spun the sphere. A similar engine is done with a thermos bottle filled with liquid nitrogen. As the liquid nitrogen warms and turns to gas, the escaping gas causes the thermos to spin.

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.


To see what really gives the missile thrust, let's look at a car that was designed to use steam propulsion. Whenever there's pressure built up inside a chamber due to steam or exploding gases, the pressure is pushing on in all directions and all sides of the chamber. The chamber (or vehicle) doesn't move because pressure is equal in all directions. But if you have an opening on one side of the chamber, the fast moving gas molecules causing the pressure have nothing to bounce off of (there's a hole there). That means the the pressure on the side opposite of the opening is not getting canceled by any pressure at the exhaust opening. So the chamber gets pushed by gas molecules hitting the chamber opposite of the opening (toward the front). (roll cursor over steam car to see animation). Realize it's the collisions of gases opposite of the exhaust opening that pushes the vehicle or missile.
Using steam for propulsion like jet propulsion isn't very efficient. The better way was have the steam pressure push on a piston. Here are the basic components of the steam engine. First there's the boiler, which is where you heat water and get a lot of pressure built up. Next you open a valve to the piston cylinder, which let's in the high pressure from the boiler. That pushes the piston up. At the top of the stroke, you open a value to the condenser, which is cool and condenses the water so it has no pressure. Outside air pressure will push the piston down because there's very little pressure left in the piston's cylinder. (Roll cursor over image to see animation)
In the early days of steam engines, they were such a novelty that people were charged admission to come look at a steam engine powered train.
Steam engines were quite the work horse in factories and on the farm. Anything that could burn could be used to turn water into steam. Once you had steam pressure you could get it to push a piston that would then turn a wheel. The large wheel (pulley) on top was fitted with a large leather belt. The belt could be used to spin another pulley that was attached to a saw, a pump, or whatever the factory or farm needed to power. It was quite versatile. The drawback was all the smoke from whatever fuel was used to heat the water. A new engine that had much less smoke exhaust was the gasoline engine.

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."

Even though the gasoline engine created much less pollution than did the steam engine, there's still a lot of pollution when there's so many cars. Here's Phoenix with its famous brown cloud caused by nitrogen oxides coming from car exhausts.
Gases that Obey the Law
There's several laws that gases obey, and I honestly don't remember what name goes to what behavior. It's more important that one understands the basic behavior of gas. One of the laws has to do with pressure versus volume. The molecules of gas are bouncing around inside this box. There's a sensor that counts the hits. The hits are proportional to the pressure. Next the box will get half its size and the hits are counted again. What you see if that the hits are doubled if the volume is halved. (Roll cursor over image for animation).
This law is actually quite logical. If the volume decreases, the pressure increases because as the volume (size) of the box gets smaller the molecules of gas don't have to travel as far to bounce into each other or the sides of the box. So they bounce into each other and the sides more often causing increased pressure.
When one value goes up and another goes down, they call that inversely proportional. In math they write it as shown on the left. This says volume is inversely proportional to pressure. If we multiply both sides by pressure (P), we get another equation that also says pressure and volume are inversely proportional to each other.
Another law for gases involves pressure versus temperature. There's a name for it, but again, I don't remember. However, this animation will reveal the concept. First the pressure is measured at 81°F (27°C or 300K). We get 1 hit per second, which is an indication of pressure. Now we heat up the gas in the box to double the temperature. We count the hits again, and it has gone up to 2 hits per second, which means the pressure doubled. (Roll cursor over image for animation).

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.

Earlier I had the animation that showed liquid water becoming water vapor with these extra water molecules increasing the pressure. This demonstrates that pressure is directly proportional to the number of gas molecules. (Roll cursor over image for animation).
Our formula is now factoring in the influence of the number of gas atoms or molecules that is contained in the volume. In many cases the gas is held in a container that can't stretch, so when more molecules are added, the pressure has got to increase. (Roll cursor over image for animation).
In this animation, I will again reinforce the idea that when volume goes down the pressure will increase, and then the opposite, which is when the volume can get larger and large, the pressure will go very low. Again, this makes sense because the collisions of the gases with themselves and the sides will get less and less as the volume increases (roll cursor over image for animation)
By using a conversion factor, "R", we can set these values equal to each other. This law has a name I remember. It's called the "Ideal Gas Law" and explains how an ideal gas behaves. This means a gas who atoms or molecules don't stick to each other. They just bounce off like balls.
In this formula, we will measure pressure (P) in atmospheres (atm), which is the pressure at sea level. Volume (V) should be in liters (L) . "n" represents the count of the gas atoms or molecules in moles (mol). "R" is a constant that converts these units. To do that it has a value of 0.0821 atm•L/mol•K. "K" is the temperature measured in degrees of Kelvin.
Simple algebra can be used to solve for each of these values. That way you can solve for pressure, volume, moles, or temperature by knowing the other units. "R" is a constant so it doesn't really need to be solved.
Car talk is an entertaining radio show that talks about car issues, but these car experts have a lot of chemistry under their belt. One caller had a problem with the pistons that hold up the hatchback to their car. To explain it, they mention the ideal gas law. Click on link to hear this 3 minute dialog.
Let's do a problem using the ideal gas law equation. The question is we have a gas inside a one liter volume and the pressure is 1 atmosphere. There's one mole of gas in the one liter, so what must be the temperature? It must be cold because in earlier tutorials, I said one mole of gas is 22.4 liters. However, I was assuming it was at one atmosphere of pressure (14.7 psi=pounds per sq. inch) and 0°C. Here the pressure is the same, but it must be much colder for the volume to shrink from 22.4 liters down to just one liter.
To solve for temperature, we divide both sides by nR. Then we plug in the values.
Now we have 1 atmosphere for pressure, 1 liter for pressure, 1 mole for quantity, and the 0.0821 conversion constant. Normally, I'd write all of the units, but for now let's keep it uncluttered. So the math is pretty easy. Multiply 1 times 1 and divide that by 1 x 0.0821. The answer turns out to be 12.2 K . That's close to absolute zero. That's -261°C or -478°F.     Like I said, we expected it to be cold because the gas is in a volume over 22 times smaller than normal.
Let's do another problem that has to do with dry ice evaporating inside a close container. As the dry ice changes to vapor, pressure will increase dramatically inside of the container. This technique is used to make dry ice bombs out of 2 liter soda bottles. Fortunately, the bottle is plastic so the plastic shrapnel isn't too dangerous, but the bottle cap can do harm. (Roll cursor over image if you want to see an injury to the forehead when a bottle cap blew off. Warning: it's a bit graphic). That guy probably will think twice about doing that again.


Here's the question: What pressure could be reached when ¼ lb of dry ice is placed in this 2 liter bottle? Temperature is 86 °F (30 °C).
Since pressure is asked for, the ideal gas law is solved for P. Now we see that we need moles (n), temperature (T), and volume. The problem says it's a 2 liter container. It doesn't give moles, but we can convert pounds to grams, and then grams to moles. The temperature of 30°C has to be convert to Kelvin by adding 273. That gives us 303K. When we plug those 3 values into our formula, we get the answer of 32.3 atmospheres. If we want the answer is psi, just multiply by 14.7 psi/ATM, which gives us 475 psi. That's a lot of pressure. Most air compressor tanks only go up to 120 psi. A heavy duty car tire will blow out at around 70 psi. So this bottle is definitely going to blow up.

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.

By hooking a vacuum pump to a container and pumping out the air (making a vacuum) you can get air pressure to push the liquid up into the container. Remember a vacuum cannot "suck" things into it. The lifting force only comes from the difference in air pressure. The more air pumped out of the container, the bigger the difference. Even if all air is pumped out, water will only rise 34 feet because at 34 feet water weighs 14.7 lbs. per sq. inch equaling the air pressure.
For those of you going into a medical field, you will undoubtedly see a device for measure blood pressure, which is measured in millimeters of mercury. One of the more expense types actually uses mercury in a tube and the height of the mercury is how you read the pressure. These devices are called either sphygmomanometers or sphygometers.
Gas Density

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.

Air is mostly N2. So the grams per mole is 14.01 x 2=28g. Since O2 is a little heavier, the density of air is actually 28.8 grams per mole. Let's see if fluorine gas sinks or rises in the air. Fluorine (F2) is 38.0 grams per mole (19.0 x 2). So it will lay close to the ground, which means if there's a fluorine leak, run out of the area on your tiptoes. Cl2 is 35.45x2=70.9g/mole, so it is very heavy. HF is (1.008+19=20.0). It will rise and be near the ceiling. So to escape, crawl out of the room. All of these toxic gases can be done the same way.
Congratulations on getting through this long tutorial. The quiz, I promise, will be a lot shorter.
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Since Apr 26, 2008