Discussion
The mean molecular weight of AIR is nearly 28.9. It remains constant upto a height of ....................... and then ..................... .
Answer: C
The mean molecular weight stays near 28.9 up to about 80 km, and then decreases.
Below about 80 km lies the homosphere, where turbulent mixing keeps the proportions of nitrogen, oxygen and argon the same at every height, so the mean molecular weight of air does not change. Above it, in the heterosphere, mixing gives way to diffusive separation: heavier molecules settle and lighter ones dominate, and ultraviolet radiation splits molecular oxygen into atomic oxygen. Both effects lower the mean molecular weight with height.
An increase is impossible for that reason, which rules out every option ending in 'increases'. The 50 km and 60 km heights fall inside the well-mixed region.
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