Characteristics of Colorless Gases

Separation

• Colorless gases contain particles that are separated by a long distance. The color of any object, solid, liquid or gas depends upon the ability of the object's surface to interact with electromagnetic light radiation. An object takes on the color of the radiation that successfully escapes its surface. In the case of colorless gases, the particles may be separated so much that even particles reflecting a color will appear colorless to the human eye. In contrast, liquid and solid particles are much more tightly packed, allowing them to reflect or absorb denser quantities of light.

Limited energy emission

• According to Steven and Susan Zumdahl in their book "Chemistry," gases may produce color when they contain an element releasing energy with "a colored emission spectrum." The visible light spectrum extends from around 400 nm (red) to 700 nm (violet). Colorless gases lack an unexcited element or molecule with an energy emission that corresponds to this range. Some colorless gases will emit light outside of this range while others do not emit any energy under standard conditions. In addition, some colorless gases can acquire a color when energy is introduced. For example, neon is a colorless gas under standard conditions. However, when introduced to energy, neon will emit a bright red-orange light.

Size

• Colorless gas molecules tend to be smaller than the molecules of gases with a discernible color. Their bonds are also weaker and less numerous. The small size of the gas molecule and its constituent bonds inhibit the ability of the molecule to absorb or reflect light.

Density

• Colorless gases tend to have lower densities than colored gases. Zumdahl and Zumdahl define density as "the mass of substance per unit volume of the substance." Oxygen gas is a colorless gas with a density of 1.429 g/l. Ozone is a light blue gas with a density of 2.144 g/l. Both gases are composed of oxygen molecules, but ozone's greater density contributes to its discernible color.