|What is heavier: oxygen or carbon dioxide?
|Question Date: 1998-03-17
At room temperature and atmospheric pressure,
oxygen and carbon dioxide are both gasses. In
fact, they are close to what scientists call ideal
Ideal gasses are much more easy to
understand than non-ideal gasses. For ideal
gasses, the density of the gas (the weight for a
given volume--basically the heaviness) is directly
proportional to the mass of an individual
For oxygen, a molecule consists of two
oxygen atoms, and has a weight of 2 x 16 = 32.
For carbon dioxide, there is one carbon atom
(weight 12) and two oxygen atoms (mass 16 x 2) for
a total of 44. This means that carbon dioxide
should be about 44/32 = 1.375 times as heavy as an
equivalent volume of oxygen.
In reality, oxygen gas has a density of 1.429 grams/liter at the so-called standard pressure and temperature (basically room temperature and atmospheric pressure) while carbon dioxide has a density of 1.977 grams/liter. If you do the math, you will find that carbon dioxide is 1.383 times as heavy as oxygen. This means that the "ideal gas model" is very good in this case.
Incidentally, if you cooled these gasses down and
pressurized them until they liquefied (liquid
oxygen is used as rocket fuel, liquid carbon
dioxide is used to carbonate soft drinks) you
would get a different answer. Liquid oxygen is
actually heavier than liquid carbon dioxide by a
little bit. 1.149 grams/ milliliter for liquid
oxygen at its boiling point versus 1.101 grams/
milliliter for liquid carbon dioxide.
If you cool even more and get solid oxygen and carbon dioxide, you would find that carbon dioxide is slightly heavier than a similar volume of solid
oxygen. Solid carbon dioxide is also known as dry
ice, and is used to keep thing cold when ordinary
ice is not good enough.
What you are really interested is the density of the gas, rather than the weight. For most gases at atmospheric pressure, a given volume contains the same number of molecules at a given temperature. This is known as the ideal gas law. The density (or how "heavy") something is depends on the amount of mass per volume - a pound of lead and a pound of feathers has the same mass, but take up quite different volumes, and hence have quite different densities, with the leak having a high density (heavy) and the feathers having a low density (light).
For an ideal gas, the density is
just the molecular weight of the gas, divided by
the volume of a given number of molecules. Since a
given number of molecules always takes up the same
volume for a gas, the higher the molecular weight,
the "heavier" the gas is, or the higher the
density. Carbon dioxide has one carbon atom and
two oxygen atoms, and a molecular weight of 44
grams per mole ( a certain number of molecules).
The oxygen in the air is actually O2,
or molecular oxygen, with a molecular weight of
32. Hence, carbon dioxide has a higher density,
or is heavier than oxygen. That is why you need
to be careful with carbon dioxide. It can
displace the oxygen in a room and lead to
Carbon has an atomic weight of 12, oxygen in diatomic form(O2) of 32 and CO2 of 44. This means that the mass of 6x1023 atoms of CARBON has a mass of 6 kilograms, the mass of 6x1023 diatomic oxygen molecules is 32 kg, and for CO2 it is 44kg. So carbon is the lightest and CO2 the heaviest.
Well, what makes up oxygen gas? Oxygen gas is made up of two oxygen atoms stuck together. That's why you'll sometimes see O2 when they mean oxygen (get it...two oxygens). Anyway, carbon dioxide is made up of two oxygen atoms and a carbon atom. So, which do you think is heavier?
The answer is carbon dioxide.
Bonus question: How much heavier is carbon
dioxide than oxygen?
Well, oxygen molecules have two oxygen atoms.
Carbon dioxide molecules have two oxygen atoms
plus one carbon atom. So, which molecule do you
think is heavier?
Oxygen has an atomic mass of 16 gram per mole. The gas oxygen that is in the "air" has the formula
O2. That means that there are two atoms
of oxygen. The formula for carbon dioxide is
CO2, that means one atom of carbon and
two atoms of oxygen. The atomic mass of carbon is
12 grams per mole.
So one mole of O2 weights : 2 x 16 = 32 grams
One mole of carbon dioxide : 12 + (2
x 16) = 44 grams.
Which one is heavier?
I am going to help you answer this one for
yourself. All gasses (like oxygen and carbon
dioxide) have about the same number of molecules
in, lets say a gallon of air. So the difference
in weight of the gasses is in proportion to the
difference in weight of the individual molecules.
Now, the each oxygen molecule has two oxygen
atoms. Each carbon dioxide molecule has two
oxygen atoms and one carbon atom. Even without knowing the weight of these atoms, can you tell now: which will be heavier? Write back with your answer!
That is an interesting question. Once, when I was at the Exploratorium, a science museum in San
Francisco, I saw a great exhibit that actually
answered your question perfectly! The exhibit had
you blow a single soap bubble into a large tub
with dry ice at the bottom. The soap bubble sank
towards the bottom of the tub at first, as you
would expect, but instead of hitting the bottom
and popping, the bubble gradually stopped falling
and finally sat suspended in the air! Dry ice
gives off carbon dioxide gas, and so there was a
thick layer of carbon dioxide in the bottom of the
Because carbon dioxide is heavier than air,
and the bubble was made of air, the bubble
actually floated above the carbon dioxide layer
because it was lighter. Eventually, the chill from
the dry ice froze the soap on the surface of the
bubble and the bubble sank to the bottom of the
tub and stayed there, a perfect sphere of hollow
If you've ever seen the smoke that comes off of dry ice, you probably already knew the answer to your question, as that "smoke" is actually carbon dioxide gas mixed with water vapor, and tends to "sink" and collect on the floor or on counter tops until it mixes with the air and disperses.
Here's an exercise: the chemical formula for molecular oxygen (the kind found in the atmosphere and which we breathe) is two oxygen atoms bound together (two atoms total),
while the chemical formula for carbon dioxide is
one carbon atom plus two oxygen atoms bound
together (3 atoms total). If a carbon atoms weighs
12 atomic mass units, and oxygen atoms each weigh
16 atomic mass units, how much heavier is carbon dioxide than molecular oxygen?
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