The plot of compressibility factor, Z and P at constant temperature helps to predict the behaviour of a gas.The value of Z is greater than one at high temperature and high pressure. This can be easily explained by vanderWaal equation when:

According to vander W aal equation.   P+an2V2(V−nb)=nRT                     .....................(1)At high temperature, 'a' can be ignored as the pressure is high. Hence, equation (1) becomes:         P(V−b)=RTPV−Pb=RT   or   PV=Pb+RT Dividing both sides by RT, we get :    PVRT=PbRT+RTRT  or Z=PbRT+1 i.e., Z>1.  So, constant 'a' isnegligible and not 'b'.