In our discussion of gases we talked about which elements in the periodic table existed as gases at room temperature.

The elements which are gases include, hydrogen, helium, oxygen, nitrogen, fluorine, chlorine and all the noble gases. We took a few moments to look at several of these gaseous elements. I'll expect you to know the standard state phase of every element in the periodic table. Now you might be thinking, holy cow, are you nuts, but remember 2 of the elements are liquids and 10 of the elements are gases. So it is not so bad. Also I'll expect you to know the color of each of the gaseous elements.

Hydrogen is the most abundant element in the universe. Hydrogen is a colorless, odorless, tasteless gas. If boils at -252. degrees C and freezes at -259.1 degrees C. Water is a good source of hydrogen, as is natural gas. The formula for hydrogen is, H₂.

Helium was the 1st noble gas to be discovered. It was identified on the sun before it was found on the earth. It discovery was made by a French astronomer Pierre Jules Casar Janssen in 1868. Helium is colorless, odorless, tasteless gas at room temperature. It boils as -268.6 degrees C. It is the only element that cannot be converted to a solid by cooling alone. (Note: the pressure must be increased to 26 atmospheres before helium solidifies at -272 degrees C.) The formula for helium is the same as its symbol, He.

Oxygen, O₂, is the most abundant element in the earths crust. O₂ was discovered in the late 18th century by the English chemist Joseph Priestly. A colorless gas, at room temperature it condenses to a blue liquid at -183 degrees C and freezes to a pale blue solid at -218 degrees C. Obtained by the liquification of air.

Although nitrogen is the most abundant element in the atmosphere it is not particularly abundant on earth because there are few compounds containing nitrogen which are solids. Nitrogen is a colorless, odorless, tasteless gas. It boils at -196 degrees C and freezes at -210 degrees C. It is obtained from liquification of air. We saw liquid nitrogen in class when I poured it on the balloon containing carbon dioxide and helium.

Fluorine is a pale yellow gas with a boiling point of –188 degrees C. It freezes at –218 degrees C. Fluorine is an extremely reactive gas and very difficult to work with in its a elemental state. I cannot show you a sample of fluorine beause of its reactivity. In fact, it is so reactive it even forms compounds with noble gas elements like xenon. We will talk about this interesting behavior later. Fluorine is commonly found in many minerals including fluorspar, cryolite, and fluorapatite.

Argon was identified by the physicist Lord Rayleigh and Sir William Ramsay. Their individual experiments identified the colorless, odorless, tasteless gas in 1894. It was the first nobel gas isolated on earth.

Later in 1898 Ramsay and an assistant Morris Travers isolated neon, from a sample of impure oxygen. They were also able to show that air contained two other element which they identified Krypton and Xenon.

Radon is obtained as a disintegration product of radium. It is a radioactive gas. Radon-222 decays by alpha particle emission to a variety of solid radioisotopes. Two of these isotopes are polorium-218 and polorium-214 are also alpha emitted. As solids these remain in the lung. Radon, as a gas, is inhaled and then exhaled.

Chlorine is a pale greenish–yellow gas, with a boiling point of –101 degrees C and a melting point of –34 degrees C. Chlorine is the most important of the halogens.

Under appropriate conditions substances which are normally liquids or solids can exist in the gas phase. In such cases the gaseous phase of the substance is called vapor.

After looking at several of the gaseous elements using the Periodic Table Videodisc we explored the behavior of gases by pouring liquid nitrogen over a balloon filled with carbon dioxide gas. Specifically we were interested in what happened to the volume of the gas in the balloon as we lower the temperature. As I poured the liquid nitrogen over the balloon we observed the balloon getting smaller, until it got completely flat as though no gas was present at all. When I picked up the balloon and started shaking it we could hear the sound of a solid inside the balloonn. As I held the balloon it began to return to its original shape. As I continued to shake the balloon the sound due to the solid inside diminished as the balloon got larger. Finally when we could not hear the solid rattling around inside the balloon it had returned to it original size.

This experiment demonstrated the relationship of temperature and volume for any gas. As the temperature was lowered the volume of the gas also decreased. Here is a QuickTime Movie (this is a large file so if you are using a modem wait and view this file on campus) of what happens when a balloon containing carbon dioxide gas is placed into liquid nitrogen. Here are a few slides from the movie. There is a direct relationship between the temperature and the volume of a gas. Here is a QuickTime Movie of what happens when a balloon containing helium is placed in liquid nitrogen.

Had we recorded some data we could plot the temperature versus volume for the gas. I have obtained some data from a different experiment where I was able to carefully measure both the temperature and the volume of a gas. Here is the animation showing the plot of temperature versus volume for a gas. (ATTENTION: This is an early animation of mine and you will need to click the mouse to advance through the animation. So if the animation stops and appears to be doing nothing, just click the mouse once. Be sure the mouse cursor is in the animation window when you click it.)

To view the animation of this experiment your browser must have the Shockwave plug-in from MacroMedia. If you do not have the plug-in installed on your computer get it before trying to run the animation.

In the next experiment with a gas we explored how the volume of a gas depended on pressure. Here is a movie of the experiment like we did in class. We used an apparatus which consisted of a syringe containing air connected to a pressure transducer to measure the pressure. The pressure transducer measured the pressure in pounds per square inch (lb in^-2). In class we began the experiment with 30 mL of air and the pressure reading was 14.7 lb in^-2. I then began to reduce the volume of the air in the syringe by pushing on the syringe plunger. We continued to collect data (pressure and volume readings). The data we collected is summarized in the table below;

Overall we can see as the volume decreases the pressure increases. This is an inverse relationship. Here is an animation showing the plot of pressure versus volume for a gas. The data plotted in this animation is different from the data collected in class, but the relationship is the same. The units on pressure are not lb in^-2 but atm . An atm is an atmosphere and the relationship between lb in^-2 and atm is

14.7 lb in^-2 = 1 atm

(ATTENTION: This is an early animation of mine and you will need to click the mouse to advance through the animation. So if the animation stops and appears to be doing nothing, just click the mouse once. Be sure the mouse cursor is in the animation window when you click it.)

To view the animation of this experiment your browser must have the Shockwave plug-in from MacroMedia. If you do not have the plug-in installed on your computer get it before trying to run the animation.

We'll learn about plotting this data in the in-class (lab) problem set during the week of September 7, 1999.

What is pressure?

Pressure is a measure of the force of an object, or collection of particles, on a given area. Another way of saying the same thing, is pressure is the force exerted on an object divided by the area over which the force is distributed. Mathematically, we would write this relationship in the following way;

We need to make a distinction between pressure and force. Something familiar to all of us is going down to the gas station to pump up a bicycle tire. Tires require between 40 psi (pounds per square inch) to 130 psi. We know adding more air to the tire increases the pressure. The number of psi increase. The units (psi) describe the weight exerted by the gas divided by the area over which the weight is distributed. We also understand that P is proportional to the number of moles of gas in a container.

The difference between force and pressure can be described in terms of a person walking on a frozen lake. Up right, the person's weight (mass x gravity) is distributed over a small area (area of shoes) so the pressure exerted by the person is high. If the ice is thin it may break as a result of the person standing on it. By lying down flat on the ice the individual's weight is distributed over a greater area and the pressure decreases.

Atmospheric pressure is a term that should also be familiar to you. The local weather report includes a map of the US with indicated regions of high and low pressure. This suggests that pressure is not constant. If we read the newspaper or watch a weather report the pressure is stated and it changes each day. You also know that at high altitudes (show an airplane) the pressure is lower than on the ground.

We can measure the pressure exerted by the atmosphere by filling a hollow glass tube with mercury, (a tube longer than 76. cm) and while plugging the bottom of the tube inverting it into a pan containing mercury. (Note: the mercury barometer was first developed by Evangelista Torricelli in 1643) Such a device is called a barometer (). The atmospheric pressure is obtained by measuring the distance between the surface of the mercury in the reservoir and the top of the mercury in the tube. Well you might ask, why didn't the mercury run completely out of the tube when it was inverted into the pan? The answer is the pressure exerted by the atmosphere supports the column of mercury. Another way to describe this is the weight of mercury in a column 760 mm high is equal to the weight of the air above the surface of the mercury in the pan.

We can demonstrate the presents of atmospheric pressure by the following simple experiment (QuickTime movie, 1.6 M file)(this version is displayed in a larger frame size/2.4 M file). If a soda can is partially filled with water and the water heated to boiling, nearly all of the air is swept from inside the can. If the top of the can is quickly sealed and the water in the vapor phase rapidly cooled a large difference in pressure is obtained. This can be accomplished by inverting the can and immersing it in a container of water at room temperature. We will discuss aspects of this experiment later in the lecture. Area of the pop can is 2(pi)r(h + r) = 2(3.14)(1.125)(4.75+1.125) = 32 in². Pressure is

so the weight of the atmosphere on the can is 14.7(32) = 470 lbs. Inside the can the weight of any gas is close to zero since there is very little gas in the can. Normally when a can of soda is open the pressure inside and outside the can are the same, and nothing happens to the can walls. But if we creat a large difference in pressure inside and outside the can we see what happens.