Solidified carbon dioxide
Explanation
Dry ice is the common name for the solid form of carbon dioxide ($CO_2$). It is primarily used as a cooling agent for preserving frozen foods and medical supplies where mechanical cooling is unavailable.
Detailed Analysis:
- Composition: Dry ice consists entirely of carbon dioxide that has been compressed and cooled until it solidifies.
- Sublimation: At atmospheric pressure, dry ice sublimates at $-78.5^\circ\text{C}$ ($-109.3^\circ\text{F}$). This means it transitions directly from a solid state to a gaseous state without passing through a liquid phase.
- Nomenclature: It is termed "dry" ice because it does not melt into a liquid, unlike water ice, which melts into liquid water.
- Other Options:
- Solidified water: This is commonly known as "ice."
- Mixture of ice and common salt: This creates a freezing mixture used to lower temperatures below $0^\circ\text{C}$, but it is not dry ice.
- Solidified nitrogen: This is distinct from dry ice and exists only at extremely low temperatures (below $-210^\circ\text{C}$).
Key Takeaway:
Dry ice is solidified carbon dioxide ($CO_2$). Its defining characteristic is sublimation, where it turns directly into gas without leaving any liquid residue.