A system is the particular region of space in focus (the region of space in which reacting particles are contained)

The surroundings includes everything else not in the system

There are 3 types of reaction systems:

1. Open (both matter and energy can be exchanged between system and surroundings)

2. Closed (only energy can be exchanged between system and surroundings)

3. Isolated (nothing is exchanged between the system and surroundings)