Why are sublimation and deposition unusual changes of state?
Sublimation and deposition are unusual changes of state because a solid is changing to a gas or a gas is changing to a solid without going through a liquid state.
Sublimation is the conversion of a solid directly to a gas.
Deposition is the conversion of a gas directly to a solid.
People who live in cold climates are familiar with deposition. After a clear, cold night, they often have to scrape the frost off their windshields in the morning. The little water vapour in the air at below-freezing temperatures has deposited on the glass as ice.
They don’t usually remove all the frost completely, and small streaks remain. As they drive along, however, the frost sublimates directly to water vapour, and the glass then becomes clear.