Hydrogen bonding significantly affects the physical properties of molecules, particularly boiling points. In ortho-nitrophenol, the -OH group and the -NO_2 group are on adjacent carbon atoms in the benzene ring. This close proximity allows the O-H hydrogen to form an intramolecular hydrogen bond with one of the oxygen atoms of the -NO_2 group within the same molecule. Intramolecular H-bonding forms a six-membered ring and satisfies the H-bonding tendency of the -OH group internally. As a result, ortho-nitrophenol molecules are less available to form intermolecular hydrogen bonds with neighbouring molecules, leading to weaker intermolecular association and a lower boiling point (214 degrees C). In contrast, para-nitrophenol cannot form intramolecular H-bonds because the -OH and -NO_2 groups are too far apart (on opposite ends of the ring). The -OH group is therefore free to form intermolecular hydrogen bonds with -OH groups of adjacent molecules, creating an extended network of H-bonded molecules that requires more energy to break and thus results in a higher boiling point (279 degrees C). The nitro group itself does not block all intermolecular forces; it merely changes their nature in the ortho isomer. Para-nitrophenol does not have a different molar mass (both isomers have the same molecular formula C_6H_5NO_3). Ionic and covalent distinction does not apply here as both are covalent molecular compounds. This concept is a standard NCERT Class 11 application of intra- versus intermolecular hydrogen bonding. Plausibility: the ~65-degree-C boiling point difference is experimentally confirmed and is one of the most cited examples of intramolecular H-bonding effects in JEE chemistry.