Q.A liquid which decomposes at its boiling point can be purified 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. …