I took 4 APs last year and my school only allows me to take 3, but I got special permission to take 4. One of them was AP Chem and I got a B+ in the class. I got As in every other class (my school doesn’t have A+s). Science is not one of my interests. I am totally a humanities person and do only humanities related extracurriculars. I did AP Lang, APUSH, and also AP Calc. How much will the B+ in AP Chem bring me down if I apply to high selective colleges?
If you get rejected from a college, it will not be because of a B+. But in the process, you found out the hard way why some high schools place limits. Taking more APs that you can handle (not that a B+ demonstrates this by any means) is never a good strategy.
@nanny123 - how did you do in the AP test and/or SAT subject test in Chem?
GPA and AP test scores are different parameters and doing well in one may not necessarily have any impact on the other one - but if AO’s start looking into your total profile, it may weigh a little bit (viz. AP score of 5 or 4 and a B+ in the course is indication of doing well overall). Irrespective of AP scores - I do feel that couple B’s in the full high school career are not going to preclude us if everything else is lined up (ps: I am a rising high school senior with couple B’s myself - so there is a bit of wishful thinking in my response - 
- take it fwiw)
In general, there are no offsets. A 5 on an AP exam show that the student can prepare for a 3 hour exam, but will not offset a grade in a 36 week course. Maybe if there are multiple applicants from this HS applying to the same “highly selective colleges” and the all have a B+ or below in AP Chem that will lead an AO to presume that the teacher is a hard grader. But one can’t bank on that.
A B+ in chem, for a humanities wannabe, is not the end of the world. But if you’re talking highly selectives, you need the full picture of strengths and the right range of ECs.