# Could an LPG gas ever become lighter than air?

Both of the LPG gases, propane ($$\ce{C3H8}$$) and butane ($$\ce{C4H10}$$) are heavier than air. Propane is $$1.55$$ times heavier and butane is $$2.08$$ times heavier. Is same-temperature air always lighter than the LPG gases? If not, at what temperature(s) would a change(s) take place?

The reason I am asking this is because I recently opened a barbecue gas canister to drain it empty. But I stopped the draining not too long after, when I read on the net that LPG gas was heavier than air and would therefore not raise into the sky. The thought then came to me that maybe there is a difference between summer and winter here. That the gas may diffuse easier during the warmer part of the year.

• This is why many indoor garages forbid the entrance of LPG-powered vehicles. Jan 30, 2022 at 15:07
• Every useful word avoided in the initial question formulation is paid later by several words posted by responders and the question poster during clarification. :-) Jan 30, 2022 at 15:08
• Density is not the relevant concept for your actual question. Gases mix eventually, so the propane and butane would escape the open container no matter what their density (within reason, i.e. given the densities and the gravity we have - the situation would be different in an ultracentrifuge or on the surface of the sun).
– Karsten
Jan 30, 2022 at 16:54

At any but the same temperature and pressure, both propane and butane are always denser than air.

As gas density (with ideal gas approximation) is $$\rho = \frac {pM}{RT},$$ where
$$p$$ is pressure,
$$M$$ is molar mass (mean one for air),
$$R$$ is the universal gas constant $$\pu{8.314 J K-1 mol-1}$$,
$$T$$ is absolute temperature.

Therefore, for the same temperature and pressure, density is proportional to molar mass.

$$\rho \propto M$$

As the molar masses of both gases are bigger than the mean air molar mass (44 resp. 58 versus 28.8 g/mol), the gases are always denser than air.

To address the recent question edits, eventually better dissipation of LPG gas in summer is not because changes of relative density, but due faster diffusion and possibly better thermal or forced convection.

At ambient pressure, the gas phase of a compound is of lesser density than their corresponding liquid (or solid) phase. The two latter are sometimes referred as condensed, too; molecules are much closer to each other than in the gaseous state and by consequence, there is a greater mass per unit of volume (i.e., an increase of density).

Thus, if the density of compound X as gas exceeds the density of air, then the density of compound X as liquid equally exceeds the density of air.