A gas is a state of matter that has no fixed volume or shape. In other words, a gas takes the shape and volume of its container. Gases condense into liquids or ionize into plasma. The particles in a gas are more distant from each other than in liquids. Gas particles may be elements or compounds. A gas may be either a pure substance (e.g., oxygen, helium, carbon dioxide) or a mixture (e.g., air, natural gas).
Examples of Gases
Here are examples of gases:
- Acetylene
- Air
- Argon
- Carbon dioxide
- Carbon monoxide
- Helium
- Natural gas
- Neon
- Nitrogen
- Oxygen
- Ozone
- Propane
- Water vapor
Elements That Are Gases at Room Temperature
Several elements are gases at room temperature and ordinary pressure. Hydrogen, nitrogen, oxygen, fluorine, and chlorine form homonuclear diatomic gases. Oxygen also forms the homonuclear triatomic gas, ozone. All of the noble gases (helium, neon, argon, krypton, xenon, radon) are monatomic gases, except for oganesson. Although oganesson (element 118) resides in the noble gas group, it’s likely a solid at room temperature.
- Hydrogen (H2)
- Helium (He)
- Nitrogen (N2)
- Oxygen (O2)
- Ozone (O3)
- Fluorine (Fl2)
- Neon (Ne)
- Chlorine (Br2)
- Argon (Ar)
- Krypton (Kr)
- Xenon (Xe)
- Radon (Rn)
Gas vs Vapor
Sometimes the term “vapor” is used to describe a gas. Usually, vapor refers to the gaseous form of a substance that’s normally in another state. For example, water is normally a liquid at room temperature and pressure. Its gaseous form is called water vapor.
Gases change into plasma, liquids, and gases. Learn about the different transitions.
Difference Between Ideal Gas and Real Gas
Chemistry and physics make a distinction between ideal gases and real gases. An ideal gas behaves according to the kinetic-molecular theory of gases and behaves according to the ideal gas law. Essentially, this means particles of an ideal gas take up no volume, are not attracted to each other, and interact with perfectly elastic collisions. No real gas is ideal, but under ordinary temperatures and pressures, their behavior is close enough to ideal that you can use the ideal gas law to approximate their behavior. However, at low temperature or high pressure, real gases deviate too much from ideal behavior. This is because high pressure force gas molecules together and their particle size matters. At low temperatures, particles lack enough kinetic energy to behave ideally. They experience attraction to each other and don’t bounce off each other with perfectly elastic collisions.
References
- Haynes, W. M., ed. (2016). CRC Handbook of Chemistry and Physics (96th ed.). Boca Raton, Florida: CRC Press/Taylor and Francis. ISBN 978-1482260960.
- John, James (1984). Gas Dynamics. Allyn and Bacon. ISBN 978-0-205-08014-4.