Fractional abundances of selected molecules with respect to total hydrogen as a function of linear time, in the regular model (panels a and c) and in the model including the C + H₂O reaction (panels b and d). The top axis indicates the dust temperature evolution, which takes a minimum value of 8 K. The solid lines indicate gas-phase abundances, and dotted lines of the same colour indicate the same species on the dust grains, in the same units.