Gases and Their Special Properties

The most familiar of all gases is the mixture we call "air"—a blend of nitrogen, oxygen, carbon dioxide, ozone, 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 thing 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 matter (solid, liquid, gas). Generally speaking, when a solid absorbs heat, it changes into a liquid, or melts. This melting occurs because the molecules (or ions) of the solid have absorbed enough energy to 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.

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 naturally occurring noble gases (helium, neon, argon, krypton, xenon, and radon), all of which occur in nature in monoatomic, or single-atom, form. All other elemental gases exist as diatomic, or two-atom, molecules, such as hydrogen (H2), nitrogen (N2), oxygen (O2), and chlorine (Cl2). Other common gases include carbon dioxide (CO2), nitrogen dioxide (NO2), and ammonia (NH3).

Sometimes we call gases vapors, a term that has a more specific meaning. A vapor is a gas that readily turns into a liquid with a modest change in temperature or pressure. Air, for example, typically contains water vapor, which condenses to form the droplets of liquid water we see as clouds, mist, dew, or rain. But the nitrogen and oxygen gases that make up the bulk of air do not liquefy except under extreme conditions of very low temperature combined with high pressure. Nitrogen and oxygen gases are clearly not vapors.

Special Properties of Gases

As mentioned, gas is the least compact of the three states of matter. To illustrate: a half cup of water heated to its boiling point (212° F or 100° C) will evaporate into enough steam (gaseous water) to fill a 50-gallon (190-liter) oil drum to overflowing. Its volume increases more than 1,200 times over.

Since the water molecules themselves have not changed in size, it follows that the distances between the molecules must be much greater in a gas such as steam than in a liquid such as water. In fact, it is the great space between gas molecules that makes it easy to compress gas in a closed container.