The relative density of methane is approximately 0.554 at its boiling point (-161.5 °C) compared to air at the same temperature. However, the value changes with temperature.
Understanding Relative Density
Relative density, also known as specific gravity, is the ratio of the density of a substance to the density of a reference substance. For gases, the reference substance is usually air. A relative density less than 1 indicates that the gas is less dense than air, while a relative density greater than 1 indicates that it is denser.
Temperature Dependence
As mentioned in the IPCC Reference Manual (IPCC, 1996, vol. 3, p. 1.124), the density of gases, including methane, is dependent on temperature and pressure. Therefore, the relative density also changes with temperature. Reported values for methane's relative density include:
- 0.7154 at 0°C
- 0.6666 at 20°C
These values highlight the importance of specifying the temperature when stating the relative density of a gas.
Why is Relative Density Important?
The relative density of methane is a crucial factor in understanding its behavior in the environment, particularly regarding its dispersion and potential for accumulation. Because methane is lighter than air, it tends to rise and disperse more readily in open, well-ventilated areas. However, in enclosed or poorly ventilated spaces, it can accumulate near the ceiling, posing a fire or explosion hazard.
Example:
Imagine a methane leak in a room. Because methane is lighter than air (relative density < 1), it will rise towards the ceiling. If there's no ventilation, the methane concentration will be higher at the top of the room than at the bottom. This localized concentration can create a dangerous situation if an ignition source is present.
Conclusion
The relative density of methane is approximately 0.554 at its boiling point compared to air. However, it is essential to remember that gas densities, and therefore relative densities, are temperature-dependent. Values around 0.7154 at 0°C and 0.6666 at 20°C are more commonly cited, but the temperature must always be specified for accuracy. The fact that methane is lighter than air is crucial for understanding its dispersion and potential hazards related to its accumulation.
