Schematic view of the process of gas condensation or trapping in the disc. The nearly horizontal line is the stability curve of pure condensate or clathrate of the species x. The nearly vertical line is the cooling curve of the protoplanetary disc (the evolution of the disc proceeds from high to low temperatures). Above the stability, the gas species remains in the gas phase of the disc. Below, the dashed area corresponds to the area of stability of ices or clathrates (area of gas condensation or trapping in water ice).