Oxygen is the second most abundant gas in Earth’s atmosphere and is necessary for life. While typically a colorless, odorless gas, oxygen changes its physical state when sufficiently cooled. Removing thermal energy transforms the molecule into a liquid and, eventually, a solid. Understanding the specific temperatures for these phase changes is relevant to cryogenics and industrial processes.
The Melting Point of Oxygen
The melting point of oxygen is -218.79 degrees Celsius (-218.79 °C). When measured on the absolute temperature scale, this temperature is 54.36 Kelvin (K). This value signifies the equilibrium point where the solid and liquid phases of pure oxygen coexist under specific pressure conditions.
The melting point is a precise physical constant, representing the lowest temperature at which molecules possess enough thermal energy to overcome the intermolecular forces holding them in a fixed crystalline structure. Because oxygen molecules (O₂) are held together by relatively weak van der Waals forces, they require an extremely low temperature to solidify. Oxygen must be cooled far below the temperatures required to freeze water or carbon dioxide to achieve the solid form.
The Liquid Range and Standard Conditions
The melting point is typically measured at Standard Atmospheric Pressure, defined as 1 atmosphere (atm) or 101.325 kilopascals (kPa). This pressure approximates the average air pressure at sea level. The liquid state of oxygen exists across a narrow temperature span between its melting point and its boiling point under this standard pressure.
The boiling point of oxygen is -182.96 degrees Celsius (or 90.19 K), the temperature at which the liquid turns back into a gas. The “liquid range” for oxygen is the difference between its melting point and its boiling point. A change in external pressure will cause a shift in both the melting and boiling points, though the boiling point is far more sensitive to pressure variations.
The Appearance of Liquid and Solid Oxygen
The appearance of oxygen changes as it transitions from a gas to a liquid and then to a solid. Liquid oxygen (LOX) is a clear substance that exhibits a distinctive pale blue tint. This color results from the way light is absorbed by the oxygen molecules in their condensed state.
A characteristic of liquid oxygen is its paramagnetism, meaning it is weakly attracted to magnetic fields. A stream of liquid oxygen can be visibly suspended between the poles of a powerful magnet, a direct result of the two unpaired electrons in the oxygen molecule. When the temperature drops below the melting point, the liquid freezes into solid oxygen, which forms crystals that are a slightly deeper blue color than the liquid.