Gases and Their Special Properties
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Chemists have long been fascinated with gases—something we all experience but seldom actually see.
Certainly gas is curious stuff: it expands to fill any container and will escape through the tiniest of holes.
Yet gas can be easily compressed, or condensed, by reducing the size of its container. If we could
actually glimpse gas molecules, we would see them whizzing through space in relentless and chaotic
motion, bashing into each other as well as into any solid or liquid in their way.
The most familiar of all gases is the mixture we call "air"—a blend of nitrogen, oxygen, carbon dioxide,
, and other trace gases. In addition to breathing air, we harness it to do work. Sealed into tires, it
cushions our rides. Compressed at high pressures, it powers pneumatic tools and propels food products
out of aerosol cans.
Gases such as methane and ethane enable us to cook our food and heat our houses. Gases such as
argon and krypton enable us to brighten our homes and businesses with incandescent bulbs and
fluorescent lamps. As students of chemistry, we learn about the behavior of gases to better understand
how they affect our lives in these and countless other ways.
What Is a Gas?
Strictly speaking, it is inaccurate to call any
a gas, because many substances change from gas to
liquid to solid form under different conditions of temperature or pressure.
Gas is a
state of matter
. Specifically, it is the least compact and most energetic of the three states of
(solid, liquid, gas). Generally speaking, when a solid absorbs
, it changes into a liquid, or
melts. This melting occurs because the molecules (or ions) of the solid have absorbed enough
break free of their rigid connections to one another. But even in liquid form, molecules continue to cling to
one another. Additional heat gives the molecules the energy they need to break free of each other
entirely. The substance then becomes a gas. (Some substances pass directly from solid to gas form, a
The substances that we commonly refer to as gases are those that exist in gaseous form under normal
conditions—that is, at typical temperatures and pressures. Most substances in this group are made up of
relatively light molecules or ions.
They include, for example, the noble gases (helium, neon, argon, krypton, xenon, and radon), all of which
naturally occur in monoatomic, or single-atom, form. All other elemental gases exist as diatomic, or two-
atom, molecules, such as hydrogen (H
), nitrogen (N
), oxygen (O
), and chlorine (Cl
). Other common
gases include carbon dioxide (CO
), nitrogen dioxide (NO
), and ammonia (NH
Sometimes we call gases