The element iodine belongs to the halogen family, a group of highly reactive non-metallic elements that includes chlorine and bromine. It is distinct from its lighter counterparts because it exists as a dark, lustrous solid under typical room conditions. The concept of a substance’s “normal boiling point” is a specific physical measurement. This temperature is defined as the point at which the substance transitions from a liquid to a gas when the surrounding atmospheric pressure is exactly one atmosphere, or 101.3 kilopascals. This standard condition allows for consistent comparison between different materials, providing a fundamental characteristic for each element.
The Specific Value for Iodine’s Boiling Point
Iodine’s normal boiling point is approximately 184.3 degrees Celsius, which translates to about 363.7 degrees Fahrenheit. This temperature represents the point where the vapor pressure of liquid iodine becomes equal to the standard atmospheric pressure. When heated under normal pressure, the intermolecular forces holding the liquid molecules together are overcome, causing a rapid phase change into a gaseous state. The vapor produced is a distinctive, intense violet color.
This boiling point is relatively high compared to lighter halogens like chlorine (a gas) or bromine (a liquid). Iodine’s larger atomic size results in stronger London dispersion forces between its diatomic molecules, requiring significantly more thermal energy to separate them into a gas. Understanding this boiling temperature is necessary for industrial processes that require iodine in its gaseous form.
The Role of Sublimation in Iodine’s Behavior
Despite having a clearly defined normal boiling point, iodine is perhaps most famous for the property of sublimation. Sublimation is the direct transition from a solid state to a gaseous state, bypassing the intermediate liquid phase entirely. When solid iodine is gently heated in an open container, it visibly produces a dense, purple vapor well below the 184.3°C boiling point, leading to the common misconception that it never exists as a liquid.
Iodine melts at standard atmospheric pressure, with a melting point of approximately 113.7 degrees Celsius. The liquid state exists in the temperature range between this melting point and the 184.3°C boiling point. Sublimation is prominent because iodine has a relatively high vapor pressure even when solid, meaning a significant amount of the solid constantly turns directly into gas at room temperature.
For a substance to skip the liquid phase at a given pressure, its triple point must lie above that pressure, as is the case with solid carbon dioxide (dry ice). Iodine’s triple point occurs at a pressure far below one atmosphere. Therefore, liquid iodine can be observed if a closed system is heated quickly between 113.7°C and 184.3°C to contain the dense, dark vapor that typically obscures the liquid.
Applications Influenced by Iodine’s Volatility
The tendency of iodine to easily form a vapor is directly utilized in several technological and industrial applications. One prominent example is the tungsten-halogen lamp, commonly known as a quartz-iodine bulb. These lamps use a small amount of iodine vapor sealed inside a quartz envelope along with a tungsten filament.
The iodine vapor participates in the halogen cycle, a reversible chemical reaction that prolongs the life of the filament. As the tungsten filament heats up to extreme temperatures, tungsten atoms evaporate and move toward the cooler glass wall. There, the evaporated tungsten combines chemically with the gaseous iodine to form a tungsten iodide compound.
This compound then circulates within the lamp’s gas until it reaches the extremely hot surface of the filament. The heat causes the tungsten iodide to break down, redepositing the tungsten back onto the filament and releasing the iodine gas to repeat the cycle. This continuous process prevents the bulb from blackening and maintains a consistent light output. Iodine’s high volatility is also employed in certain chemical purification processes, where the solid material is converted into a gas and then re-solidified to remove non-volatile impurities.