  In this paper we show that a \emph{one-to-one} correspondence exists between
any dark energy model and an equivalent (from a cosmological point of view, in
the absence of perturbations) quartessence model in which dark matter and dark
energy are described by a single perfect fluid. We further show that if the
density fluctuations are small, the evolution of the sound speed squared,
$c_s^2$, is fully coupled to the evolution of the scale factor and that the
transition from the dark matter to the dark energy dominated epoch is faster
(slower) than in a standard $\Lambda$CDM model if $c_s^2 > 0$ ($c_s^2 < 0$). In
particular, we show that the mapping of the simplest quintessence scenario with
constant $w_Q \equiv p_Q/ \rho_Q$ into a unified dark energy model requires
$c_s^2 < 0$ at the present time (if $w_Q > -1$) contrasting to the Chaplygin
gas scenario where one has $c_s^2 > 0$. However, we show that non-linear
effects severely complicate the analysis, in particular rendering linear
results invalid even on large cosmological scales. Although a detailed analysis
of non-linear effects requires solving the full Einstein field equations, some
general properties can be understood in simple terms. In particular, we find
that in the context of Chaplygin gas models the transition from the dark matter
to the dark energy dominated era may be anticipated with respect to linear
expectations leading to a background evolution similar to that of standard
$\Lambda$CDM models. On the other hand, in models with $c_s^2 > 0$ the expected
transition from the decelerating to the accelerating phase may never happen.
