ie-Chemistry P3
Gas Pressure
the collective behavior of many particles moving independently


Pressure is a phenomena that we can often detect directly with our senses.  When we push against something, we can feel it push back.  Physicists define it mathematically as pressure ≡ force / area.  So if a particular force is spread out over a larger area, the pressure is less.  Anyone who has walked on snow, soft mud or loose sand probably realizes that when the material isn't capable of resisting the pressure we apply, we tend to sink in.  But if we spread our weight (force due to gravity) over a broader area such as with skis or snow shoes, we do not sink in as much.  Conversely, someone wearing shoes with very tiny spiked heals applies greater foot pressure and therefore may damage walking surfaces made of weaker materials.  Such spiked heals have been a challenge to aircraft designers who wish to minimize the weight of an airplane, including its floor.  And if you've ever flown, you may have noticed locations on the wings labeled No Step for similar reason.

Any material is capable of exerting at least a little pressure on us.  The small pressure exerted on us by the air is something we have gotten so used to that we generally are unaware of it.  But we notice if it changes a bit such as when the wind exerts more pressure on one side of us than another.  Still, when something like air pressure becomes so common and monotonous to us, our nervous system learns to ignore it so that we can concentrate on more important information from our senses.

As with many human experiences, there is often a vast difference from having an experience and understanding the experience.  This investigation and the following one will attempt to improve your understanding of pressure.


Empedocles (of Agrigentum 492 to about 400 B.C.) combined aspects of several previous Greek thinkers.  Empedocles suggested that all material objects consist of combinations of four fundamental elements:  Fire, air, water, and earth, driven by two forces:  love (attraction) and strife (repulsion).  (Fire and the other elements were related to what we observe in our lives, but each element was thought to be more pure that anything which occurs naturally in our cosmos.  The solids around us are predominately composed of earth.  The liquids are mostly water.  And the gases are air.)  Each of the elements had their natural place in the cosmos:  The element earth, the most dense, belonged at the center of the Cosmos which was presumed to be the center of our Earth (which the Greeks were deciding must have the shape of a sphere).  Water, then air, and finally fire belonged to successive positions above and outside the core earth.  Although many situations on Earth had elements out of place, it was natural for all elements to try to return to their proper location.  Thus the object on fire in the following experiment should naturally (i.e., without further need of explanation) rise to be above the air!

LavoisiersTwo thousand years later, Galileo Galilei (b1564, d1642) vehemently argued that the Earth is not the center of the cosmos, but as the astronomer Copernicus (born 1473 as Nicholas Koppernigk, died 1543) proposed, the Earth is one of more than a handful (7 = the number of days in a week) of planets orbiting the Sun.  While Galileo had no evidence that the Earth actually moves about the Sun, he did present several bits of evidence which suggested the Greek theory must be wrong.  In 1789 the concept of Four Elements was finally replace by Antoine Lavoisier (of Paris, b7143, d1794, portrait at right with wife and assistant Marie→) who suggested there were many more fundamental chemical elements, (none of which had natural places).   So the explanation for fire rising to its natural place became false and that observed phenomena needed a replacement explanation.

The alchemists of the medieval ages had difficulty collecting and working with the element called air.  By the time of Paracelsus (short name for Philippus Aureolus Theophrastus Bomblast von Hohenheim, b1493 near Zurich, d1541), he and his followers had taken to calling air by the term chaos, which in Greek meant unknowable.  (Poor enunciation of that term led to our current general terms gas and gaseous.)  Eventually following the invention of a vacuum pump which could evaluate a chamber and methods of collecting and transferring gases, some of the last of the alchemists, still believing in variations of the Four Element theory, discovered troubling evidence for several distinct kinds of air.  By the time Lavoisier wrote his "Traite elementaire de Chimie..." proposing his new definition of element, it was clear to him that the Greek element air could not explain the several distinct gases.  It was equally obvious to him that air could not be an element at all.  Rather, under his new definition of element, several of the distinct variations of air are each a distinct element (Nitrogen, Oxygen, Hydrogen) which under common room conditions exists as a gas.  The whole concept of solid/liquid/gas changed from being fundamental elements to just being a variable state of matter.  For Lavoisier gas was merely one of three physical states of matter, along with liquid and solid, which substances could successively exhibit by sufficient cooling, or in reverse by heating.  For example warming solid ice turns it to liquid water and with addition of more heat, gaseous steam; cooling can turn steam to liquid water and eventually solid ice.

RumfordPhysicists such as James Watt (b1736, d1819) and Benjamin Thompson (b1753 in Massachusetts, became Count Rumford, d1814, cartoon at right→) tried to understand and improve the early and highly inefficient steam engines.  They began to think of the physical state of a substance to be a consequence of the amount of heat (i.e., a form of energy) in that substance.  Rumford wrote, heat (what the Greek's called fire) cannot be a material substance, but must be motion.

So it gradually became understood that a gas is a collection of loose, independent atoms or molecules, moving around randomly.  When these freely moving particles strike the container walls, or any object in the gas, they bounce, creating a small push.  With an enormous number of randomly moving particles in a sample of gas, the billions of tiny pushes average out to be a smooth pressure on the container walls and any object contained in the gas.  If we force more independent particles into roughly the same volume, chances are that more of them will hit any particular area in a given amount of time.  (This is just like you are more likely to run into other people in a crowded city than in a wilderness.) Thus with more particles in the volume, there should be more pushes and thus more pressure.


Make a balloon rocket.

  1. Obtain a rubber balloon.

  2. Blow into the balloon the inflate it with air with more pressure than what is outside the balloon.

  3. Release the ballon and observe.

  4. Try to explain why it happened.

  5. Record your procedure and observations in your journal along with any analysis and conclusions which seemed appropriate.

  6. Communicating technical information such as observations and findings is a skill used by scientists but useful for most others.  If you need course credit, use your observations in your journal to construct a formal report.


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content created 11 July 2009
revised 18 October 2009
by D Trapp
Mac made