Is gas from chimney more dense than pure gas and why?

Answer 1

Generally speaking, any gas emitted from a chimney, or any fire, is going to be denser than pure gas, that is gas that has not been burned. This is due to the impurities of the fuel being used to cause the flame. Impurities that are vaporized by the heat rise with the gas, making it more dense. Also, when burning something, especially wood for instance, there are many microscopic particulates that are emitted into the chimney (also, think diesel truck exhaust). This is soot. These particulates are not gases but rather solids, but they are suspended in the gas. If measuring the density of what comes out of your chimney, some of it would be due to these particulates. Pure gases do not have any particulates of course. Finally, when comparing the density of two gases (now neglecting the contribution of particulates), it is important to consider the molecular mass of the gases. For instance helium (He) gas is lighter than air (which is mostly nitrogen gas (N2) and oxygen gas (O2). Two samples (of the same volume) of gases at the same pressure and temperature will have nearly identical numbers of molecules (according to the Ideal Gas Law). So if those molecules have greater molecular mass, then the sample will be heavier. If the sample is heavier but the volume is the same, then the density is greater (density is after all just mass per unit volume). So, when you compare the emissions from a chimney to a "pure gas" it really depends on which gas you compare it too. Pretty much all gases are more dense than hydrogen gas (H2), but very few are as dense as say xenon gas (Xe) or radon gas (Rn) for instance. So, all that said, if chimney gases are more dense than say air, why does smoke rise? It rises because it is hotter than the air around it. Because it hot, it does have a lower density than air! The point is that if you compare them at the same temperature and pressure, the smoke will be more dense. But if you compare hot smoke with room-temperature air, the change in temperature makes enough of a difference such that the density of the smoke is lowered, and so it rises. This is why a chimney won't pull as well when it is hot outside. When it cold, there is a big difference in temperature between the air and the smoke, and so the smoke is much less dense. When it is hot out, the temperatures are closer to each other, and so the densities aren't so different, and the smoke doesn't rise as easily.