An important step in the chemical analysis of chemical sets Q is the establishment of the order among the elements in Q. An example of this appears in environmental chemistry where it is important to rank the objects of the set to try to establish priorities in the analysis or in the procedures to recover a contaminated place. We showed how a set Q of objects defined through their properties can be classified in order to find equivalence classes in the set. Such equivalence classes can be obtained through the use of cluster analysis where the classes are similarity classes in Q. We showed that it is possible to extract a representative of each class in order to rank the elements of the set and finally to obtain a Hasse diagram. We applied our methodology to a set of 59 regions of Baden Wuerttemberg, Germany, monitored with respect to Pb, Cd, Zn and S pollution in the herb layer. That example showed 26 equivalence classes (26 representatives) using cluster analysis. Thus, we built up a Hasse diagram that indicated the degree of contamination of the 26 representative regions.