Flexi Says: When the seedless plants died, became buried deep in the Earth, and were exposed to heat and pressure, coal formed. Coal is a highly complex heterogeneous rock consisting of mixture of polycyclic aromatic compounds such as benzene, toluene etc. their derivatives, some impurities, water, and volatile substances. The physical structure of coal is determined by its supramolecular structure, pore and size distribution. As per the experimental evidences from the X-ray studies, it has been observed that the pore size of coal varies from 5 to 1000 Angstroms.