Premise behind first conditional?

Hi. I think what could be said about the first conditional is that it is about possibility and I think it could be a possibility that is likely to be realized. But the way it looks to me, it looks me that it denotes a situation in which the possibility of it being realized is one hundred percent (as it looks to me) and sometimes I have difficulty trying to correctly use the second conditional when both the first and second conditional seem to be correct to use.

If he comes home, she wil leave.

From this sentence, what do you think is the possibility that she will leave once he comes home? I would say one hunded percent (if I am not mistaken). So it looks to me that we should not be talking about possibility, but a correlating result (I have used the word "correlating" to mean what I think it means but I am not sure I have used it correctly).