# What exactly does a “system” include in chemistry?

If we have reactants $\ce A$ and $\ce B$ and the reaction(s) between them haven't started yet, are those reactants our "system"?

After the reaction takes place, are products then also included in the "system"?

Basically, is it the case that the term "system" refers to all the chemicals we have in a reaction?

• There isn't one universal answer. "The" system is all the factors that are pertinent. Certainly the concentration of reactants would be one set of pertinent factors. But temperature, pressure, heat flow in or out of the reaction could be other factors too. Light might be a factor. So what is pertinent depends on the reaction. – MaxW Jan 27 '17 at 19:55
• You put your reagents in an imaginary box of impermeable matter. You calculate what would become of your reagents, considering the forces and influences which are inside the box. Those which are outside (air, ocean water, sun light, moon gravity, etc.) are disregarded altogether. Now, that box is your system. – Ivan Neretin Jan 27 '17 at 20:37
• I disagree with the VTC of this question. It indicates enough effort on the OP's side. – M.A.R. Jan 27 '17 at 22:08
• @IvanNeretin - I understand what you mean about the "imaginary box" but it isn't quite that simple of course. You might have to consider the heat flow in and out of the box. Also many gas reactions specify a constant pressure. So the volume of the box might need to expand or contract. – MaxW Jan 28 '17 at 0:20
• @MaxW can we just say that it includes the reactants and all the factors that can influence the reaction ? – Sam19KY Jan 28 '17 at 15:49

Note: This was not originally written by me. This was first written by @airhuff. I just thought it was relevant in this question as well.

According to this Wikipedia article:

...a physical system is a portion of the physical universe chosen for analysis. Everything outside the system is known as the environment. The environment is ignored except for its effects on the system.

The split between system and environment is the analyst's choice, generally made to simplify the analysis. For example, the water in a lake, the water in half of a lake, or an individual molecule of water in the lake can each be considered a physical system.

Every system has some relationship to it’s surroundings. As another way of stating the above, we can describe the surroundings, or environment, as all parts of the universe that is not part of the system. We can then generally categorize the system as being isolated, closed or open:

• Isolated system: Neither matter nor energy may pass into or out of the system.

• Closed system: The exchange of energy is allowed but matter may not pass into or out of the system.

• Open system: Both energy and matter can pass into and out of the system.

• I would say if you going to use someone's answer, even from another question, you should post it as community answer. – Tyberius Apr 3 '17 at 16:53
• @Tyberius, you can flag for mod attention for conversion to community wiki. Thanks anyway for pointing it out. – orthocresol Apr 3 '17 at 17:16