word choice - "in case" vs. "if"

1. 
   

   “There are open fields over there,” he said, pointing, “in case it happens.”

   
   
   


2. 
   

   “There are open fields over there,” he said, pointing, “if it happens.”

   
   

Are "if" and "in case" generally interchangeable? If not, how is 1, the original version, different from 2?