Expert: Dr. Henry Boyter, Jr. - 1/19/2005

Question
I didn't find this on google...could you settle a question amoung a couple of adults who play WAY too much trivial persuit...and graduated high school science WAY too long ago to remember...

is carbon monoxide heavier or lighter than air, and thus would it fill a structure top down or bottom up?

Answer
Look at the main consituents of air:

1. Nitrogen (N2) has a molecular weight of 28 (14x2)

2. Oxygen (O2) has a molecular weight of 32 (16x2)

3. Water (H2O) has a molecular weight of 18 (2x1 + 16)

4. Carbon dioxide(CO2)has a molecular weight of 44 (12 + 2x32)

Carbon Monoxide(CO)has a molecular weight of 28 (12+16)

So compared to the main constituents CO is lighter than O2 and CO2 and heavier than H2O.  It is the same as N2.  Since however, the nitrogen is equal and makes up almost 80% of the air, CO basically stays where you release it.  If you had a CO gun, it would really go where you point it.

Now since the O2 is heavier than CO and the second largest % in air, the CO will spread up as compared to the O2.  My guess is that this takes a long period of time, without outside forces.  So in a closed situation, with enough time, and no other air movement, the CO will drift up, not to all be at the top, just slightly more of it.  Same with O2, since more will be low, in a fire that is the place to be (also the smoke goes up also).

Take CO2 for example.  It is the heaviest of them all, but in a regular room, it would be almost impossible to find more CO2 on the floor, than on the ceiling.  There is just too much air movement that keeps things mixed up.

Now take CO coming off a poorly ventillated wood stove.  That CO is lighter than O2 by weight, but remember, it is also hot compared to the N2 in the room.  Hot gas is always lighter than cold, so CO in this case would be lighter than the N2, so it would go up due to the temperature difference.
By the same token, if you released cold CO, it would go down.

Not a simple answer, but if you only had the choice of up or down, the CO is lighter than O2, and the same as N2, so it would go up.  If you throw other variables such as temperature in, you can get different answers.