  The evolution of a universe modelled as a mixture of generalised Chaplygin
gas and ordinary matter field is studied for a Robertson Walker type of
spacetime. This model could interpolate periods of a radiation dominated,
matter dominated and a cosmological constant dominated universe. Depending on
the arbitrary constants appearing in our theory the instant of flip changes.
Interestingly we also get a bouncing model when the signature of one of the
constants changes. The velocity of sound may become imaginary under certain
situations pointing to a perturbative state and consequently the possibility of
structure formation. We also discuss the whole situation in the backdrop of
wellknown Raychaudhury equation and a comparison is made with the previous
results.
