The question of which gas is purple has two distinct answers: one involves a substance in its natural gaseous state, and the other involves a gas altered by energy. Most gases, including the air we breathe, are transparent and colorless under normal conditions because their molecules do not interact with visible light. However, a few gases possess a visible hue, confirming that a purple gas does exist.
The Primary Answer: Iodine Vapor
The gas that is distinctly and naturally purple is iodine vapor, the gaseous form of the element iodine (\(\text{I}_2\)). Iodine is unique because it bypasses the liquid phase when heated at standard atmospheric pressure, transitioning directly from a grayish-black solid to a gas in a process called sublimation. This sublimation occurs readily when solid iodine is gently warmed, filling the container with an intense violet or reddish-purple cloud.
The deep purple color is an inherent property of the diatomic iodine molecule, governed by its electronic structure. The molecule’s electrons absorb specific wavelengths of light in the yellow and green regions of the visible spectrum. When white light passes through the vapor, the yellow and green components are removed by absorption, leaving the human eye to perceive the complementary color, violet-purple. This absorption promotes an electron to a higher energy state.
The intensity of the purple color is directly related to the concentration of the vapor. A small amount of solid iodine can produce a vivid color because the molecular structure is well-suited to absorbing visible light.
Color Generated by Ionization
A different kind of purple is generated when electrical energy is applied to colorless gases. This purple is produced by gas ionization, where a high voltage excites atoms, creating a plasma state. The color seen is not the color of the gas itself, but the light emitted as the excited electrons fall back down to their lower energy levels.
Noble gases, such as argon, are colorless in their ordinary state but emit a characteristic light when excited electrically, such as inside a discharge tube or plasma globe. Argon gas typically glows with a pale blue-violet or purple light when a current is passed through it. Nitrogen gas will similarly glow a pinkish-purple when ionized under low pressure. This emitted light is known as an emission spectrum, and the specific wavelengths released are unique to the element being excited.
Understanding Gas Color
Most common gases are colorless due to the physics of light interaction with simple molecular structures. Gases like oxygen and nitrogen are small molecules whose electronic energy levels are spaced far apart. They require high-energy photons, typically found only in the ultraviolet (UV) region, to cause an electronic transition.
Since the human eye is not sensitive to UV light, these gases appear transparent because they do not absorb visible wavelengths. For a gas to have a visible color, its molecules must be complex enough to have energy level spacings that correspond to the energy of visible light. The iodine molecule meets this requirement, allowing it to absorb lower-energy visible photons, while simpler molecules do not.
Real-World Encounters and Safety
The purple color of iodine vapor is most commonly encountered in chemistry laboratories, where it is used to demonstrate phase changes or as a stain in chromatography. It is also used in modern forensics to reveal latent fingerprints, as the vapor adheres to organic residues. However, iodine vapor is highly irritating and toxic to the respiratory system and eyes, even at low concentrations. The permissible exposure limit is very low, requiring careful handling in well-ventilated spaces.
The purple glow from ionized gases is found in everyday technology, most notably in plasma televisions, welding torches, and decorative lighting such as neon signs. While the plasma state involves high energy, the gas is safely contained within sealed glass tubes. Argon is often used as a protective, non-reactive shield gas in industrial welding processes.