The transformation of a gas directly into a solid requires a significant reduction in molecular energy, which translates to extremely low temperatures. Unlike everyday substances that freeze at relatively warm temperatures, like water at 0 °C, gases must be cooled far below what most people experience. This process bypasses the liquid state for many common gases at standard atmospheric pressure. The resulting solid forms of these atmospheric components exist only in environments shielded from everyday warmth.
Defining the Transition for Gases
The term “freezing” typically describes the phase change from a liquid to a solid. However, for many gases, the transition directly from the gaseous state to the solid state is more accurately called deposition. The reverse process, the transition from solid directly to gas, is known as sublimation. This direct phase change occurs when the pressure is too low for the substance to exist as a liquid.
The conditions under which a substance can exist in all three phases—solid, liquid, and gas—in equilibrium are defined by its triple point. This specific temperature and pressure combination is fundamental to understanding gas solidification. If a gas is cooled at a pressure below its triple point pressure, it will move straight from gas to solid (deposition) without ever forming a liquid. The triple point is therefore a demarcation line that determines the nature of the phase change.
Freezing Points of Common Atmospheric Gases
The temperatures required to solidify atmospheric gases vary widely, with most occurring far below the freezing point of water. Nitrogen, the most abundant gas in the atmosphere, solidifies at approximately 63 Kelvin (K), which is equivalent to -210 degrees Celsius (°C). Oxygen, the second most common atmospheric gas, requires even colder temperatures, solidifying at 54.36 K, or about -218.79 °C. Argon, a noble gas present in the air, solidifies at a slightly warmer triple point temperature of 83.8 K.
Carbon dioxide (CO₂) exhibits unique behavior at standard atmospheric pressure, bypassing the liquid phase entirely and subliming directly to a gas from a solid at 194.7 K, or -78.5 °C. The triple point of carbon dioxide, where a liquid can form, is at a much warmer -56.4 °C, but this requires a pressure of at least 5.13 atmospheres. This difference means that at the air pressure we breathe, carbon dioxide only exists as a solid or a gas.
How Pressure and Purity Influence Solidification Temperature
The temperature at which a gas solidifies is not a fixed number but is strongly influenced by external conditions, particularly pressure. Generally, increasing the external pressure on a gas tends to raise the temperature required for it to solidify. This phenomenon can be mapped on a phase diagram, which shows how the solid-gas boundary shifts with changes in pressure and temperature.
Higher pressure forces the gas molecules closer together, making it easier for the weak intermolecular forces to lock them into a solid structure even at slightly higher temperatures. Conversely, reducing the pressure lowers the solidification temperature. For a substance to exist as a liquid, the pressure must be above its triple point pressure.
The presence of impurities in a gas stream also affects the solidification point, typically by lowering it. When a substance is not perfectly pure, foreign molecules interfere with the formation of a uniform solid crystal lattice, meaning a lower temperature is necessary to achieve full solidification. In industrial processes, moisture or water vapor is a common impurity that can freeze out prematurely, causing blockages long before the main gas component begins to deposit as a solid.
Real-World Instances of Gas Solidification
The solidification of gases has several practical and natural occurrences. The most well-known example is the creation of dry ice, which is the solid form of carbon dioxide. Dry ice is produced by cooling and compressing CO₂ gas, and its -78.5 °C sublimation temperature makes it an excellent, residue-free cooling agent.
The extremely low boiling point of liquid nitrogen, at -196 °C, is used for rapid cooling and freezing in scientific and medical applications. While liquid nitrogen is used in its liquid state, the process relies on temperatures near the solidification points of other gases. For instance, if liquid nitrogen is left exposed, it can condense oxygen from the surrounding air, as oxygen solidifies at a lower temperature than nitrogen’s boiling point.
Beyond Earth, gas solidification is a widespread process in the cosmos. The surfaces of distant planets and moons, like Mars, are covered in polar caps made of frozen carbon dioxide and water ice. In interstellar space, the low temperatures allow various gases, including methane and carbon monoxide, to solidify into interstellar ices on dust grains. These frozen gases represent a fundamental state of matter in some of the coldest regions of the universe.