What is defined as a closed system in chemical reactions?

A closed system in chemical reactions is defined as one where nothing is lost or added to the system. This means that the total mass and the number of reactants and products remain constant; matter cannot enter or exit the system. In this scenario, chemical reactions can occur without any interactions with the surroundings, allowing for a precise study of the reactions and changes taking place within the system.

In the context of this definition, it is important to understand that while energy might still be exchanged (as heat might be released or absorbed during reactions), the closed system concept specifically focuses on the conservation of matter. This allows chemists to analyze reactions based on the reactants and products present without the complication of external inputs affecting the reaction process.