Isothermal process is that process in which temperature is constant although the change in heat energy is taking place.Since temperature is constant so change in temperature is 0So all the energy absorb is used in doing work so system expands.Where as in adiabatic process the exchange of energy between system and surrounding doesn\’t take place so dQ =0 So system uses its own internal energy to do work And we know dQ= dU +WdQ=0So dU= -WIf system expands i.e.work is positive so internal energy is decreasesIf system contracts so work done is negative and internal energy increases