A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction.
Sawing a board in half is a physical change not a chemical change the board doesnt change wood or whatever material it is is still would still be wood
If it were burned it would be a chemical change but its properties stay the same atomically so thats why its not a chemical change.
<span>The "exosphere" is the most distant and tenuous "layer" of our atmosphere.</span>
The answer is CONDENSATION.
