Examples of column density maps of three important chemical species in molecular clouds, CO, H₂O, and (from left to right). While is quite uniformly distributed in the box, H₂O forms in larger amounts in moderately higher density gas, and CO reaches typically observed values (N_CO ≳ 10¹⁷ cm⁻²) only in proto-filaments (where there is a huge difference, about four orders of magnitude, between the filaments and the background).