I heard you don’t need a perfect 800 but Chemistry is also the most difficult test. Is all this true? What’s a good score for a selective university?
General rule of thumb for SAT II tests is that 700+ is respectable and 750+ is optimal. Less than 700, maybe try again.
If you’re looking at a tech school, 780-800 for Math II.
No.
Difficulty is relative and depends on the student.
With the caveat, and it’s a huge caveat, that all these schools evaluate holistically, so no score will guarantee anything.