Phases of Matter V - Gases 1
Written by Translator on June 25, 2008 – 7:29 pm -Crossposted at Dailykos.com
Gases, like liquids are fluids, in that they have no definite shape, but take that shape of their container, be it a balloon, a pressure cylinder, or a gravity envelope. Unlike liquids, gases are not “condensed” in that their molecules have a much greater spacing than that of liquids or solids.
This spacing is on the order of around 1000 times more as a general rule of thumb. This makes it possible to compress gases, using energy to push the molecules closer together. The ramifications of this are huge, from air conditioning to energy transport to solar power storage.
Gases (also spelled gasses) are noncondensed materials that have a minimum amount of interaction between particles in comparison to condensed phase materials. When a liquid is subjected to pressure, it either moves or bursts its container. This the basis of hydraulics, where a liquid in a small reservoir is pressurized, forcing it into a larger one where it does work. Hydraulic jacks for cars are familiar to most folks. Gases can do this as well, but not very efficiently, because they undergo compression, and, depending on the container, can be compressed quite a bit.
Like all other “normal” matter, there are some interactions between molecules of gases, but these become important only under high pressures and low temperatures. At ambient pressures and temperatures, gases behave almost exactly like hard spheres, knocking each other about in a random fashion. Thus a balloon is filled with air and it occupies space. The energy for these collisions is supplied by the environment in the form of heat. Try this experiment: take a balloon and blow it up. If you have a ruler, measure it. Then put it in the refrigerator for a couple of hours. Working fast, take it out and measure it again. It will be smaller. Then allow it to warm up for a couple of hours, and remeasure. It will be about the same size that it was before refrigeration.
You can do the same thing in the freezer, but latex can become embrittled at very low temperatures, and might crack. No chance of that in the refrigerator, and that gives you the experimental evidence of one of the basic gas laws: V1/T1 = V2/T2. In other words, the volume times the absolute temperature at one temperature decreases with the absolute temperature, so the quotient is always the same. This is Charles’s Law, elucidated in 1787 by Jacques Charles in France.
Another gas law is that pressure and temperature are related. There is not a really safe way to demonstrate this at home quantitatively, but you can do it my feel. Empty a 2 liter soda bottle and allow it to dry inside. Screw on the cap and put it in the freezer. After a couple of hours, the bottle will have collapsed, because the volume of the air decreases with temperature. The equation is P1/T1 = P2/T2, where P1 is the initial pressure, T1 the initial temperature, P2 the next pressure, and T2 the next temperature. That means that in a constant volume, the pressure of a gas decreased with a decrease in temperature in a linear fashion. This is known as Gay-Lussac’s law, elaborated by the French chemist of that name in 1802. Now, one more gas law.
The eldest gas law is Boyle’s law, which says that there is a direct relationship between pressure and volume of a gas. He came up with this in 1662, when the grandkids of the Pilgrims were celebrating only the 41st or 42nd Thanksgiving. It says that if you take a specific quantity of a gas and suddenly expand the container, the pressure decreases. This one is also hard to do at home, but your car does it all of the time. The equation is: P1V1 = P2V2, where the letters are already defined. There is one way to do it, and it has to do with balloons again.
Find two similar balloons, and blow one up. Without letting any air out, attach the second one, neck to neck, to the first one. Superglue and a pair of latex gloves are in order. A twist of two of tape would not hurt, either. Once attached, let the air out of the filled one. If the balloons are close to similar, each will be about half filled, unless you blow the seal, wherein both collapse.
Now you know the basic three laws of gases. There is another one, called the combined gas law, and it, as the name says, combines all three of these. It goes as follows: P1V1/T1 = P2V2/T2. The symbols have the same meanings as described before.
What this means is that if you know a little about a gas in a certain situation, you can predict the properties of it in another, assuming no change of phase. This is what is cool about science: you can predict the future if you have a valid model.
There is one more mathematical model that you should know, and it is called the “Ideal Gas Law”. The title does mean that the law is ideal, but that it applies to “ideal” gases. That means gases that behave as perfect little billiard balls, neither attracting nor repelling other molecules. At low pressures and high temperatures, it works well, but as pressure increases (more molecules in a volume unit) and temperature decreases (translational energy becomes less important), gas molecules tend to stick together or repel each other, depending on the gas and the condition.
The relation for the Ideal Gas Law is PV = nRT, where P, V, and T are defined as above, and “n” is the number of gas molecules (technically, the number of gram equivalent weights of a gas) and “R” is a constant determined from experiment (that is, an empirical constant). Using this equation, one can calculate the volume, pressure, temperature as one wishes if one knows how much gas is in question and either of the other two factors, pressure or volume.
I said that we would get into air conditioning and your car, but this is already too long. I will post a second installment soon to cover that. As always, questions, comments, and criticisms are always welcome. I have said before that I learn much more than I teach here, so please have no reservations. Warmest regards, Doc.
Tags: Boyle's Law, Charles's Law, Combined Gas Law, Gases, Gasses, Gay-Lussac's Law, Ideal Gas Law, Phases of matter, Science, Teaching
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5 Comments
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Is gasoline a gas, a liquid or both?
Properly, it is a liquid, but evaporates easily. In the old days, when we had carburetors, it was mostly the vapor that went into the engine, because the intake manifold was hot enough to vaporize it. The exception was when one “kicked down” the pedal, and the accelerator pumps injected lots of raw liquid gasoline into the acceleration ports of the carburetor.
These days, there are two flavors: indirect fuel injection, where a pump sprays gasoline into a distribution port that delivers it as a vapor, or direct injection where gasoline is directly injected into the combustion chamber as a finely divided particulate liquid. Warmest regards, Doc.
My sweetie talks about Gay-Lussac’s law all the time.
I can hardly wait for the next installment. Neon & freon are more my thing! (My Dad was a HVAC specialist with LTV in the 40-70’s) I learned alot about this from him. Gases and expansion and contraction and, and, and….
And enthalpy (heat) of compression (makes stuff hotter, so you run a fan through the compression coils to cool them, outside), or enthalpy of decompression (makes stuff cooler, so you run a fan through the expansion coils into your living space to cool it, inside). Freons were used for many years because they have excellent thermal properties and are not corrosive nor toxic.
I suspect that we will get back to ammonia, chlorine, and sulfur dioxide in the near future, since none of them injure the ozone at high altitudes. Warmest regards, Doc.
Maybe you are right. Sometimes it is the very basic things that are best.