A new Lagrangian mixing model is presented that describes the turbulent mixing of reacting scalars as a cascade process from large to small scales. The model is derived by applying guidance of Eulerian multi-scale mixing models. Compared to these models, the essential advantage of the derived Lagrangian model is given by the fact that approximations are restricted to the simulation of mixing processes, i.e., chemical transformations are treated exactly. In contrast to previously applied Lagrangian methods for scalar fields, the model presented here is shown to be applicable to inhomogeneous reacting liquid-phase flows. This is of relevance to important practical applications and further developments of models for the scalar mixing in multi-phase flows. Evidence for the derived mixing model in its general formulation is provided for different flows through its full consistency with well-tested Eulerian transport equations. Applications to different homogeneous flows reveal essential features of the mixing model. Two new theoretical findings are presented: First, the appearance of scalar gradients may lead to a significant reduction of the composition frequency. Second, the model structure presented here permits the derivation of an algebraic model for the composition frequency by relatively weak assumptions. The good performance of that algebraic version of the general mixing model is demonstrated by simulating mixing and parallel chemical reactions in a turbulent pipe flow. It is shown that the error of conventional techniques that neglect Reynolds number effects may amount to 50% for the considered case. These errors may be larger if more complex reaction mechanisms have to be considered.