A system is any region in space or quantity of matter set aside
for analysis. Everything not inside the system is in the surroundings.
The system boundary is an infinitesimally thin surface, real or
imagined, that separates the system from its surroundings.
Systems may be classified according to whether they exchange mass
with the surroundings. A closed system is a system that does not
exchange mass with its surroundings; thus, it has a fixed and unchanging
mass although its volume may change. An open system is a system
that may exchange mass with its surroundings; thus, its mass and
volume may change.
Processes and Interactions
Systems change state by interacting with their surroundings. A
process is the means by which a system changes state. An interaction
between a system and its surroundings occurs when an extensive property
crosses the system boundary.