Iodine: A Sublimation Classic
When iodine crystals are gently heated, they produce striking purple vapors ā one of the most visually dramatic demonstrations of sublimation in chemistry. But why purple?
The Color of Iodine Vapor
Iodine vapor appears purple because Iā molecules absorb light in the yellow-green region of the visible spectrum (around 500-550 nm). The remaining transmitted and reflected light ā a combination of red and blue wavelengths ā appears purple to our eyes.
This absorption occurs because of electronic transitions within the iodine molecule. When photons of the right energy strike Iā, they excite electrons from bonding to antibonding molecular orbitals.
Why Does Iodine Sublime?
Iodine's triple point pressure is 0.12 atm ā below atmospheric pressure. This means at 1 atm, iodine technically melts at 113.7Ā°C before boiling. However, iodine has unusually high vapor pressure even at room temperature, so it sublimes noticeably at temperatures well below its melting point.
The relatively weak van der Waals forces between Iā molecules (compared to their molecular weight) allow surface molecules to escape into the gas phase readily.
The Reverse: Deposition
When iodine vapor contacts a cold surface, it deposits back into beautiful, shiny crystals ā demonstrating the reverse of sublimation. This is actually how pure iodine crystals are commercially refined through a process called sublimation purification.