Q.Ortho and para-nitrophenol can be separated by

Purifying LIQUIDS is built around distillation and its variants. Simple distillation boils the impure liquid, condenses its vapour, and works when a mixture's components differ widely in boiling point (roughly 40 K or more) and neither decomposes at ordinary pressure. Fractional distillation separates components whose boiling points sit much closer together, using a fractionating column (with a thermometer near the condenser to track the vapour temperature) so each component distils over near its own boiling point, repeated if needed for a sharper cut -- the industrial workhorse behind refining petroleum, coal-tar and crude oil.

Steam distillation purifies a compound (solid or liquid) that would decompose at its own boiling point, provided it has a reasonably high vapour pressure at 373 K, is insoluble in water, and its impurities are non-volatile -- steam is passed through the heated compound-plus-water mixture, carrying the compound's vapour over with it into the condenser at a temperature well below the compound's normal boiling point; used to recover essential oils and to manufacture aniline and turpentine oil. It is also the classic way to separate ortho- and para-nitrophenol: the ortho isomer's intramolecular hydrogen bonding leaves it comparatively volatile and readily steam-distillable, while the para isomer's intermolecular hydrogen bonding makes it far less volatile.

Distillation under reduced pressure lowers a liquid's boiling point below its decomposition temperature, and is specifically the method for a liquid that decomposes AT its normal boiling point. …