What is the color of chlorine gas?

• A. Colorless
• B. Pale yellow
• C. Brown
• D. Greenish

Answer: (D)

Chlorine gas has a characteristic color because halogen molecules absorb certain wavelengths of visible light, so the light that remains gives the gas a visible hue. Chlorine is Cl2, a diatomic molecule, and its electronic transitions absorb more strongly in the blue region of the spectrum. What we see is the remaining light shifted toward the yellow-green end, so the gas appears greenish-yellow in typical conditions. The description greenish best matches how the gas looks in ordinary lab observations, whereas colorless would imply no absorption in the visible range, brown is more typical of heavier halogens like iodine or bromine, and pale yellow is less representative for chlorine’s usual appearance.