On earth, we live under a blanket of air that presses down us and everything else surrounding us. According to weather report, the pressure of atmosphere varies day to day. Atmosphere contains several gases like nitrogen, oxygen, argon etc. Their relative percentages in air are given below.

Figure 9.2 Different gases present in atmosphere

Ref: commons.wikimedia.org/

As the height of the air column decreases in higher altitude, the atmospheric pressure decreases. Air pressure is highest at the sea level. Below sea level pressure increases due to water pressure and atmospheric pressure.

Throughout much of human history, “airs” or gases were not believed to be matter at all; their apparently weightless nature and their ability to move about freely and fill all available space, while carrying with them definite physical properties such as odor and sometimes color, conferred upon them a somewhat mysterious nature. Even the scientist Robert Boyle wrote about “The Strange Subtility, Great Efficacy and Determinate Nature of Effluviums”

It’s interesting, however, that around 550 BCE the Greek philosopher Anaximenes maintained that all matter consists of air: “It is from air that all the things that exist, have existed, or will exist come into being.“

The invention of the sensitive balance in the early seventeenth century showed once and for all that gases have weight and are therefore matter. Guericke’s invention of the air pump (which led directly to his discovery of the vacuum) launched the “pneumatic era” of chemistry long before the existence of atoms and molecules had been accepted. Indeed, the behavior of gases was soon to prove an invaluable tool in the development of the atomic theory of matter.

The study of gases allows us to understand the behavior of matter at its simplest: individual particles, acting independently, almost completely uncomplicated by interactions and interferences between each other. Later on, our knowledge of gases will serve as the pathway to our understanding of the far more complicated condensed phases (liquids and solids) in which the theory of gases will no longer give us correct answers, but it will still provide us with a useful model that will at least help us to rationalize the behavior of these more complicated states of matter.

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