PCI₅ reacts with water to form

PCl₅ forms POCl₃, when PCl₅ reacts with H₂O. 

${\mathrm{PCl}}_5+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{POCl}}_3+2\mathrm{HCl}$

POCl₃ further reacts with water to form H₃PO₄.

${\mathrm{POCl}}_3+3{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_3{\mathrm{PO}}_4+3\mathrm{HCl}$