Strong heating of orthophosphoric acid yields

Phosphoric acid, also known as orthophosphoric acid, is a triprotic acid that exists as a dense liquid. It is an irritant or corrosive to the skin, eyes, and other mucous membranes of both humans and laboratory animals. Its salts, though, exhibit a significantly lower irritancy potential.

The reactions are ${\mathrm{H}}_3{\mathrm{PO}}_3\frac{{220}^{\circ }\mathrm{C}}{\text{ genle heat }}{\mathrm{H}}_4{\mathrm{P}}_2{\mathrm{O}}_7\frac{{320}^{\circ }\mathrm{C}}{\text{ strong heal }}{\left({\mathrm{HPO}}_3\right)}_n$.