In this experiment solid iodine is vaporized without passing through a liquid state. The process of solid phase conversion directly to the gas phase is called sublimation. Each substance at given pressure and temperature possess one of the three main states: solid, liquid or gaseous. Temperature or pressure change can lead to appropriate phase transition.
The dependency of the substance state on temperature and pressure could be represented by a phase diagram. A phase diagram is type of chart that describes the conditions at which distinct phases can occur in equilibrium. A typical phase diagram has “y-like” shape. On a phase diagram one can find a special point – triple point – at which solid, vapor and liquid can coexist in thermodynamic equilibrium.
Sublimation takes place at temperatures and pressures below the substance triple point. When iodine crystals are heated, the temperature increases and we shift along the abscissa axis from the area of the phase diagram corresponding to solid state to the area corresponding to gas.
Cooling causes the reverse transition. When iodine vapors reach the bottom of the flask with ice-cold water they recrystallize. The reverse process of direct solid phase formation from vapors is called desublimation or decomposition.