Final answer:
Moth balls disappearing in a closet after a couple of days is best described by sublimation, the transition from the solid to vapor phase without becoming liquid. Two observations that indicate sublimation are the gradual disappearance of mothballs and the smoke-like vapor from dry ice.
Step-by-step explanation:
The best way to describe the phenomenon of moth balls disappearing in your closet after a couple of days is sublimation. Sublimation is the transition from the solid to the vapor phase without passing through the intermediate liquid phase. This occurs because solids like mothballs have a nonzero equilibrium vapor pressure, which allows some of the solid particles to escape directly into the gas phase. An everyday example of sublimation is the disappearance of dry ice, which is solid carbon dioxide.
Two common observations indicating that some solids have vapor pressures sufficient to sublime are the gradual loss of mass from the solid, as observed with mothballs evaporating in a closet, and the creation of 'smoke' from dry ice without the presence of liquid water. Moth balls, which are typically made of naphthalene, a slightly toxic phenol, sublimate and release their odor into the environment, serving as both a pest repellent and as evidence of sublimation.