Definitions and Basic Concepts
Chemical-engineering thermodynamics: definitions and fundamental laws

A system is any collection of material elements bounded by an imaginary boundary generally referred to as an envelope. The envelope of a system has zero thickness, so that on either side of this boundary the properties of the material are exactly the same. It must never be confused with the actual wall of a container. The region of space located outside the chosen boundary is called the external environment.

The combination of the system and the external environment is called the universe. Thus, if the system under study is the entire universe, there is no longer an external environment.

There are only three ways in which energy can be transferred between a system...