Chemical evolution of PrO in a two-phase model by considering various cloud densities. The upper two panels indicate the isothermal phase, the middle two panels indicate the warm-up, and the lower panel is for the gas phase peak abundance of PrO and final abundance of PrO at the warm-up phase for various density cloud. Different colors denote different cloud densities. In the upper and middle panels, the solid line represents the abundance in the gas phase whereas the dotted line represents the abundance in ice phase.