Sublimation and deposition are unusual changes of state because they involve the transformation of a substance directly between the solid and gas phases or vice versa, without passing through the liquid phase.
Typically, when a substance undergoes a change of state, such as melting, it transitions from a solid to a liquid before vaporizing into a gas. Similarly, when a gas condenses, it usually forms a liquid before solidifying into a solid.
In sublimation, a substance transitions directly from a solid to a gas, skipping the liquid phase. This is unusual because it goes against the typical pattern observed during changes of state.
Deposition, on the other hand, is the process where a gas directly transforms into a solid, without passing through the liquid phase. Again, this is unusual because it defies the usual sequence of state changes.
These rare changes of state occur under specific conditions of temperature and pressure, and are observed in certain substances such as dry ice (solid carbon dioxide) sublimating into carbon dioxide gas, or frost forming on a cold surface as water vapor directly solidifies.
Why are sublimation and deposition unusual changes of state?
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