Sublimation Often volatile solids, such as the camphor synthesized in the Jones Oxidation experiment, can be purified using a technique called sublimation . The sublimation process occurs when a solid evaporates from a warm surface and condenses on a cold surface as a solid once again. This phenomenon occurs more rapidly at reduced pressure where the molecules can escape the solid phase more readily and travel greater distances (mean free path) before colliding with another molecule. This allows them to reach the cold surface quicker.   The warm surface is supplied to the sample in the filter flask by the steam bath.   The cold surface is provided by an ice-filled test tube fitted with a green rubber stopper that we call a cold finger . The pressure is reduced via a rubber tube connected to a trap that is connected to the aspirator.