  A method-of-moments scheme is invoked to compute the asymptotic, long-time
mean (or composite) velocity and dispersivity (effective diffusivity) of a
two-state particle undergoing one-dimensional convective-diffusive motion
accompanied by a reversible linear transition (``chemical reaction'' or
``change in phase'') between these states. The instantaneous state-specific
particle velocity is assumed to depend only upon the instantaneous state of the
particle, and the transition between states is assumed to be governed by
spatially-independent, first-order kinetics. Remarkably, even in the absence of
molecular diffusion, the average transport of the ``composite'' particle
exhibits gaussian diffusive behavior in the long-time limit, owing to the
effectively stochastic nature of the overall transport phenomena induced by the
interstate transition. The asymptotic results obtained are compared with
numerical computations.
