Closed System - in Thermodynamics

In Thermodynamics

In thermodynamics, a closed system can exchange energy (as heat or work) but not matter, with its surroundings. An isolated system cannot exchange any of heat, work, or matter with the surroundings, while an open system can exchange all of heat, work and matter. For a simple system, with only one type of particle (atom or molecule), a closed system amounts to a constant number of particles. However, for systems which are undergoing a chemical reaction, there may be all sorts of molecules being generated and destroyed by the reaction process. In this case, the fact that the system is closed is expressed by stating that the total number of each elemental atom is conserved, no matter what kind of molecule it may be a part of. Mathematically:

where is the number of j-type molecules, is the number of atoms of element i in molecule j and b_i is the total number of atoms of element i in the system, which remains constant, since the system is closed. There will be one such equation for each different element in the system.