Sublimation is the transformation of a substance directly from the solid to the vapor state without its becoming liquid. All solids will sublime below their triple point (at which solid, liquid, and vapor are all in equilibrium), but in only a few cases – including dry ice, iodine, naphthalene, and sulfur – is this at a high enough temperature and rate to be useful for purification. Only those molecules with above-average energy are able to overcome the cohesive forces holding the liquid together and escape from the surface. Freeze-drying is by sublimation. See also condensation; evaporation.