Why does solid Iodine crystals on heating turn into vapor instead of melting?« Back to Questions List

Chemical compounds generally exist in three states – solid, liquid and gas. You may be familiar with processes like melting, evaporation, boiling, freezing, condensation etc. All these processes involve phase transitions that occur in an orderly pattern either solid – liquid or liquid – gas or gas – liquid.​


The process of transition of a substance directly from its solid phase to gaseous phase without going through the liquid state is called sublimation. Iodine crystals, dry ice, naphthalene (moth balls) and arsenic tend to sublimate. Sublimation occurs at a very low super cold temperature and low pressure. You can send something frozen you can pack it in dry ice. It will remain frozen till it reaches its destination with no messy liquid around. We cannot touch dry ice with bare hands as it will be too cold and might affect the skin.


iodine 1

Iodine is a dark grey crystalline solid. Iodine is very slightly soluble in water and freely soluble in organic solvents. Its melting point is 114 degree C and boiling point is 184 degree C.



iodine 2


The properties of molecules within a compound are responsible for their structure, melting point, boiling point etc. There are many forces acting between the molecules of a compound. They could be physical, chemical or electrostatic in nature. They could be attractive or repulsive forces. The total sum of these attractive or repulsive forces between the molecules (intermolecular forces) is collectively called as van der walls force.




The iodine atoms within one molecule are pulled closely to each other by a covalent bond. Between the molecules, there exist weak van der waals forces. When Iodine crystals are heated, the van der waals are easily overcome and the molecule breaks into gas phase. They sublime and form a purple colored dense vapor. The sublimation of iodine is used to stain chromatography plates and in the detection of finger prints in forensic science.


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