Question

compare the density of carbon dioxide with the density of air
CONTENT QUALITY REQUIRED
BEST ANSWER will be marked as BRINLIEST

Answer 1

PV = nRT 
n = mass / mw

PV = (mass / mw) RT 
mass / V = density = mw x P / (RT)

the higher the mw, the higher the density at the same T and P... 
mw CO2 = 44 
mw O2 = 32 
mw N2 = 28 
therefore.. CO2 is DENSER than O2 and N2

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why do you think a LIGHTER gas will form a LOWER layer?

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there are number of reasons that CO2 does NOT form a lower layer. they include

1) CO2 is in the ppm range. 300 molecules CO2 / 1,000,000 molecules air. Very dilute... brownian motion effects would prevent this seperation 
2) diffusion effects. 
3) our atmosphere is constantly in motion and constantly mixing. due to pressure differences cause by temperature changes. 
4) the are many structures on our planet also cause air to mix. mountains for example. 
etc