MCQ Detailed View

Sublimation is a physical process in which a substance changes directly from the solid state to the gaseous (vapor) state without going through the intermediate liquid phase. This transformation occurs when the vapor pressure of the solid becomes equal to or greater than the atmospheric pressure at a particular temperature.

Common examples of substances that undergo sublimation include:

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  Dry ice (solid CO₂): It directly converts into carbon dioxide gas at normal atmospheric pressure, creating a fog-like effect often used in stage shows and refrigeration.

  
  


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  Iodine crystals: They turn into violet-colored vapors when heated gently.

  
  


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  Naphthalene balls: Widely used as moth repellents, they slowly disappear at room temperature by subliming into vapor.

  
  


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  Camphor: Used in medicines and as an insect repellent, it also sublimes easily.

  
  

This process is different from evaporation, which involves the liquid state. In evaporation, a liquid gradually changes to vapor at its surface below the boiling point. In sublimation, the liquid stage is completely bypassed.

The reverse process of sublimation, where a gas converts directly into solid, is called deposition (e.g., frost formation on cold surfaces).

From a physical chemistry perspective, sublimation is important because it demonstrates how matter can transition between states depending on temperature, pressure, and intermolecular forces. The phase diagram of a substance shows regions where sublimation occurs, usually at low pressures or specific temperature conditions.

Sublimation has practical applications in:

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  Purification of substances: Many solids that sublime can be purified by this method since impurities do not vaporize.

  
  


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  Freeze-drying: A process used in food preservation and pharmaceuticals where water is sublimed from frozen samples.

  
  


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  Air fresheners and insect repellents: Substances like naphthalene and camphor release a steady stream of vapors through sublimation.

  
  

Therefore, the correct answer is Sublimation, as it best describes the direct conversion of solid to vapor without passing through the liquid state.