10)It is not advisable to sleep under trees during the night.
CO2 is not poisonous. It is not advisable because it can cause dizziness. (Note: The only damage CO2 can cause is asphyxiation which happens when there is no oxygen to breath. I don't think it is possible to have 100 % CO2 under a tree :P)

100) Oil and water do not mix. Why?
I doubt your first reason because molecules of oil are indeed bigger than molecules of water. Write instead this:
I) The specific gravity of oil is less than 1
OR
I) the density of oil is less than that of water.

90) A man with a load his head jumps from a high building. What will be the load experienced by him.
The reason is that the weight (Man + his load) equals air resistance.

705 Mercury has the smallest diameter.
Atomic diameter? It's Hydrogen, not Mercury.