The freezing of gases often differs from liquids solidifying. Gases typically undergo deposition, a direct transformation from a gaseous state to a solid state, bypassing the liquid phase. This phenomenon requires extremely low temperatures. Alternatively, a gas can first condense into a liquid and then freeze into a solid.
How Gases Transform to Solids
The transition of a gas into a solid involves a significant reduction in molecular motion. Gas molecules possess high kinetic energy, moving freely and rapidly. To transform into a solid, these molecules must slow down sufficiently for intermolecular forces to draw them together into a fixed, ordered structure.
Removing thermal energy is essential for this process. As temperature decreases, the kinetic energy of the gas particles diminishes, causing them to move less vigorously. This allows the attractive forces between molecules to become dominant. Pressure also plays a role, as increased pressure forces molecules closer together, which can aid in the formation of a solid structure when combined with low temperatures.
Freezing Temperatures of Everyday Gases
Nitrogen, which makes up about 78% of Earth’s atmosphere, freezes at approximately -210°C (-346°F). Liquid nitrogen is used as a coolant. Oxygen, comprising about 21% of the atmosphere, freezes at an even lower temperature of approximately -218.79°C (-361.82°F). Liquid oxygen is used as an oxidizer in rocket propellants and for medical purposes.
Carbon dioxide undergoes deposition directly from a gas to a solid at about -78°C (-109.3°F) at atmospheric pressure, without forming a liquid. This solid form is known as dry ice. Hydrogen, the lightest element, freezes at an exceptionally low temperature of around -259.14°C (-434.45°F). Helium has the lowest freezing point of all elements, requiring both extremely low temperatures, near absolute zero, and significant pressure to solidify, as it does not freeze at atmospheric pressure.
Where Gas Freezing Matters
Gas freezing points are crucial in several advanced fields. Cryogenics, the study of extremely low temperatures, relies on liquefying and solidifying gases. This field has applications in medicine, such as the preservation of biological samples like blood, reproductive cells, and tissues, and in cryosurgery for removing skin lesions.
Industrial processes utilize super-cooled gases. The liquefaction of natural gas (LNG), for example, involves cooling natural gas to approximately -162°C (-260°F), reducing its volume by about 600 times for transport and storage. Air separation units employ cryogenic distillation to separate atmospheric air into its components like nitrogen, oxygen, and argon, by cooling it to a liquid state and then distilling each gas based on its boiling point. In space exploration, knowledge of gas freezing points is essential for designing equipment and understanding planetary environments, such as Pluto’s nitrogen ice glaciers.
Gas Versus Liquid Fuel Freezing
The term “gas” can cause confusion when referring to fuels. Liquid fuels such as gasoline and diesel are often colloquially referred to as “gas” and have significantly different freezing behaviors. Gasoline is a mixture of various hydrocarbons, meaning it does not have a single, precise freezing point. Its freezing range typically falls between -40°C and -58°C (-40°F and -72.4°F), though some components may freeze at even lower temperatures, sometimes as low as -100°F. Diesel fuel, on the other hand, contains paraffin waxes that begin to solidify and cause the fuel to thicken or “gel” at much higher temperatures, often around -9°C to -12°C (10°F to 15.8°F). This gelling can impede fuel flow and cause engine problems. Unlike gases that primarily undergo deposition, liquid fuels freeze through a more conventional liquid-to-solid phase transition.