Correct Option
The correct option isIt does not wet the surface on melting because it converts directly into gas.
Explanation
Solid Carbon Dioxide ($CO_2$) is commonly referred to as "dry ice." This substance is formed by compressing and cooling carbon dioxide gas until it solidifies. The nomenclature is derived from its specific physical behavior during phase transition at standard atmospheric pressure.
Detailed Analysis:
- It is colder than normal ice. is incorrect: Although dry ice is significantly colder than water ice (sublimating at approximately -78.5°C), the term "dry" refers to the absence of a liquid phase, not its temperature relative to normal ice.
- It does not wet the surface on melting because it converts directly into gas. is correct: Under normal atmospheric pressure (1 atm), solid carbon dioxide undergoes sublimation. This means it transitions directly from a solid state to a gaseous state without passing through an intermediate liquid state. Consequently, it does not wet surfaces or leave any liquid residue upon heating, which justifies the name "dry ice."
- It is free from water molecules. is incorrect: While dry ice is chemically distinct from water ($H_2O$) and contains no water molecules, the name is specifically descriptive of its sublimation property (lack of wetness), not merely its chemical composition.
- It is used to dry moisture from air. is incorrect: Dry ice is primarily used as a refrigerant due to its low temperature. While it can freeze moisture, the name is not derived from a function as a desiccant (drying agent) for air.
Key Takeaway:
"Dry ice" is solid Carbon Dioxide ($CO_2$) which sublimates directly into gas at 1 atmospheric pressure, bypassing the liquid state and leaving no wet residue.