The continuously mixed tank reactor is an open system with an influent stream of reactants and an effluent stream of products. A lake can be regarded as a tank reactor, and lakes with long turnover times (e.g. with low flux-to-volume ratios) can for many purposes be regarded as continuously stirred (e.g. homogeneous in all respects). The mass balance then becomes
where 
* is the volumetric flow into the system;
* is the volumetric flow out of the system;
* is the concentration of A in the inflow;
* is the concentration of A in the outflow.
In an open system we can never reach a chemical equilibrium. We can, however, reach a steady state where all state variables (temperature, concentrations, etc.) remain constant ().