  In simulations of a water-like model (ST2) that exhibits a liquid-liquid
phase transition, we test for the occurrence of a thermodynamic region in which
the liquid can be modelled as a two-component mixture. We assign each molecule
to one of two species based on the distance to its fifth-nearest neighbor, and
evaluate the concentration of each species over a wide range of temperature and
density. Our concentration data compare well with mixture-model predictions in
a region between the liquid-liquid critical temperature and the temperature of
maximum density. Fits of the model to the data in this region yield accurate
estimates for the location of the critical point. We also show that the liquid
outside the region of density anomalies is poorly modelled as a simple mixture.
