Oxygen (O₂), the colorless, odorless gas that makes up about one-fifth of Earth’s atmosphere, is typically thought of in its gaseous state. Like all elements, oxygen can exist as a solid if its temperature is lowered significantly. The melting point is the specific temperature at which a substance transitions from a solid to a liquid state. Finding this temperature requires measuring temperatures on an absolute scale used in cryogenics.
The Melting Point of Oxygen and the Kelvin Scale
The precise melting point of oxygen is 54.36 Kelvin (K). The Kelvin scale is the standard unit for scientific temperature measurement, particularly in cryogenics and thermodynamics. This absolute thermodynamic scale sets its zero point, 0 K, at absolute zero, the theoretical temperature where all particle motion ceases.
Because the Kelvin scale starts at absolute zero, it does not use negative numbers, simplifying many scientific calculations. At 54.36 K, oxygen molecules slow down enough to arrange themselves into a solid, crystalline structure. This pale blue solid requires extreme refrigeration to maintain.
Converting Oxygen’s Melting Point to Familiar Scales
While 54.36 K is the scientific standard, translating this value to Celsius and Fahrenheit helps illustrate how cold this temperature truly is. The relationship between Kelvin and Celsius is straightforward: the temperature in Celsius is found by subtracting 273.15 from the Kelvin temperature (K = °C + 273.15). This conversion reveals that oxygen’s melting point is an incredibly cold -218.79 degrees Celsius (°C).
Converting this value to the Fahrenheit scale shows the melting point is equivalent to -361.82 degrees Fahrenheit (°F). These extremely low temperatures highlight that oxygen requires deep-freeze conditions to exist as a solid. This is why it is almost exclusively encountered as a gas in everyday environments.
Boiling Point: Oxygen’s Other Critical Temperature
To fully understand oxygen’s phase changes, its boiling point must also be considered, as this marks the transition from liquid to gas. The boiling point of oxygen is 90.19 K, which is 35.83 degrees warmer than its melting point. At this temperature, liquid oxygen absorbs enough energy to overcome intermolecular forces and vaporize into a gas.
In the Celsius scale, the boiling point is -182.96 °C, which is far below any temperature naturally experienced on Earth’s surface. This shows that oxygen exists in its liquid state only within a narrow temperature window, specifically between 54.36 K and 90.19 K. Any temperature above 90.19 K at standard pressure will find oxygen in its gaseous state.