A closed system in chemistry refers to a type of a thermodynamic system in which mass is conserved inside the system but energy enters and leaves the system freely. When both energy and matter can be exchanged between the system and its surroundings, the system is known as an open system.
Only energy can exit or enter a closed system. When coal is burnt within a closed system, for instance, the states of the material change, but no matter leaves the system. Instead, stored energy is lost through radiation, conduction and convection. The drop in energy equals the amount of energy leaving the system.
A closed system should not be confused with an isolated system. A closed system differs from an isolated system with respect to the transfer of energy. An isolated system cannot exchange matter and energy outside its boundaries. A closed system is only closed to matter because energy can leave or enter the system.
The law of conservation is defined on the basis of a totally closed system (isolated system). It states that the mass of a totally closed system remains constant over time. The processes acting within the system cannot change the mass of the system. Therefore, mass can neither be destroyed nor created but may be reorganized in space and transformed into other types of particles.