Question

What is the defining feature of a system?
A. properties that don’t change
B. collision
C. interaction
D. gravitational force

Answer 1

The system can be anything; it's the medium in which your chemical reaction is taking place. However the surrounds is everything's else that's not part of your reaction. There may be a better way to explain this.

Answer 2

I would assume that it has to do with the collisions between particles because that's what happens in a typical chemical reaction.