1.2.1 Systems

Chemists often talk about systems without ever defining precisely what a system is. To a chemist a system is that part of the universe the chemist wishes to speak about. In general the universe is mentally divided into two parts, the system and the surroundings.

One should not take the term universe too literally. What that really means is a large enough chunk of the surroundings so that nothing that goes on inside the system can change the surroundings very much.

Usually what is meant by the system is obvious from context. Thus if I speak of a beaker of salt, the salt is the system. (The beaker is not usually considered part of the system, but is taken to be an ideal sort of beaker providing containment but little else.)

Systems come in various varieties. One classification depends on the properties of the container of the system. In this scheme systems can be isolated, which means that nothing can cross the boundaries of the system, nor can the system change its size. In plain terms an isolated system is completely cut off from its surroundings.

Or a system can be closed. None of the contents of a closed system can enter or leave. But its walls do allow both the conduction of heat and the volume of such a system to change. They don't have to change, but they can.

Or a system can be open. In an open system anything goes. Not only can the volume change and heat can enter or leave, but the contents of the system can enter and leave as well.

Just to make life interesting, physicists use two of these terms and define them slightly differently. To them a closed system is what a chemist would call isolated. And an open system is to them the same as a chemist's open system. I mention this in case one ends up talking to a physicist about some of these things.