Open system

An open system is a thermodynamic system whose boundary is permeable to matter, so that particles (or chemical species) may cross between the system and the surroundings . Consequently, quantities like the particle number (and composition) are not fixed by definition.

This contrasts with a closed system , which exchanges energy but not matter, and an isolated system , which exchanges neither. The open/closed/isolated classification is about matter exchange; independently, the boundary may be an adiabatic wall or a diathermal wall with respect to heat flow.

Physical interpretation. Typical open systems include a control volume around a turbine with inlet/outlet flows, a container exchanging vapor with a large reservoir, or a membrane-permeable compartment exchanging solute. An open system may reach thermodynamic equilibrium if the environment fixes appropriate intensive conditions (e.g. temperature and chemical potential); otherwise it may sustain fluxes and remain out of equilibrium.

Equilibrium differential including matter exchange. For a simple compressible system that can exchange entropy, volume, and species amounts, the fundamental differential can be written as

where $U$ is internal energy , $S$ is entropy , $T$ is temperature , $P$ is pressure , $V$ is volume , and $\mu_i$ are chemical potentials for species amounts $N_i$. The term $\sum_i \mu_i\,dN_i$ is the energetic contribution carried by matter transfer (often grouped under “chemical work”; conventions are summarized under chemical-work convention ).

When an open system is simultaneously in contact with a thermal reservoir and particle reservoir(s) fixing $(T,\mu_i)$, the natural equilibrium potential is the grand potential .