Adiabatic process is the process in which change in pressure, volume and temperature takes place without any heat entering or leaving the system is called adiabatic process. So the total heat of the system, undergoing an adiabatic process always remains constant. in an adiabatic expansion since no heat is supplied from outside , therefore the energy required for the expansion of the gas is taken from the gas itself. The signifies that , the internal energy of an ideal gas undergoing in an adiabatic expansion decreases, and because the internal energy of an ideal gas depends only on the temperature, therefore its temperature must decreases. That is why the temperature of a gas drops in an adiabatic expansion.