Time evolution of the abundances with respect to solid-state water of HNCO (solid line), HNCHO (dotted line) and NH₂CHO (dashed line) in a dense molecular cloud where solid HNCO density is arbitrarily fixed to be 5% of solid H₂O density at 3  ×  10⁵ years (arrow). No OCN⁻ is produced from the HNCO + H₂O thermal reaction at 10 K and OCN⁻ density is zero all the time. Almost all HNCO is consumed in hydrogenation reactions toward HNCHO and NH₂CHO, whose increases in density are directly correlated to HNCO decrease. In addition to add 5% of HNCO compared to the solid H₂O density, H and H₂ are reduced by 6 orders of magnitude (dot-dashed line): the HNCO abundance is only slightly decreasing. Inset: zoom of the evolution of the different abundances after t = 3  ×  10⁵ years.