Characteristics of Gases

Although different gasses may differ widely in their chemical properties, they share many physical properties

Characteristics of Gases

The air we breathe:

78% N₂ (fairly inert)
21% O₂ (reactive)

Some common gases:

Formula	Name	Characteristics
H₂	Hydrogen	Flammable, lighter than air
He	Helium	Colorless, nonflammable, lighter than air
HCN	Hydrogen cyanide	Toxic, has been used historically to shorten people's lives
HCl	Hydrogen chloride	Toxic, corrosive
H₂S	Hydrogen sulfide	Toxic, smells like rotten eggs
CO	Carbon monoxide	Toxic, Jack Kervorkian's favorite gas
CO₂	Carbon dioxide	Colorless, odorless, not toxic, but unsupportive of respiration
CH₄	Methane	Colorless, odorless, flammable, occasional byproduct of the digestive system
N₂O	Nitrous oxide	Colorless, sweet odor, makes you feel "funny"
NO₂	Nitrogen dioxide	Toxic, red-brown, irritating odor
NH₃	Ammonia	Colorless, pungent odor
SO₂	Sulfur dioxide	Colorless, irritating odor

All of the above gasses

Are composed of non-metals
Have simple formulas, and therefore low molecular masses

Substances that are liquids or solids under standard conditions can usually also exist in the gaseous state, where they are commonly referred to as vapors

Some general characteristics of gasses which distinguish them from liquids or solids:

gasses expand spontaneously to fill their container (the volume of a gas equals the volume of its container)
gasses can be readily compressed (and its volume will decrease)
gasses form homogenous mixtures with each other regardless of the identities or relative properties of the component gases (e.g. water and gasoline vapors will form a homogenous mixture, whereas the liquids will not)
The individual molecules are relatively far apart

In air, the molecules take up about 0.1% of the total volume (the rest is empty space)
Each molecule, therefore, behaves as though it were isolated (as a result, each gas has similar characteristics)

1996 Michael Blaber