Given a scientific passage and an answer, generate a question for the given answer.
Q: Passage: Dilution of the products with water, followed by filtration to remove calcium sulfate, gives a dilute acid solution contaminated with calcium dihydrogen phosphate, Ca(H2PO4)2, and other compounds associated with calcium phosphate rock. It is possible to prepare pure orthophosphoric acid by dissolving P4O10 in water. The action of water on P4O6, PCl3, PBr3, or PI3 forms phosphorous acid, H3PO3 (shown in Figure 18.53). The best method for preparing pure phosphorous acid is by hydrolyzing phosphorus trichloride: PCl 3(l) + 3H 2 O(l) ⟶ H 3 PO 3(aq) + 3HCl(g) Heating the resulting solution expels the hydrogen chloride and leads to the evaporation of water. When sufficient water evaporates, white crystals of phosphorous acid will appear upon cooling. The crystals are deliquescent, very soluble in water, and have an odor like that of garlic. The solid melts at 70.1 °C and decomposes at about 200 °C by disproportionation into phosphine and orthophosphoric acid: 4H 3 PO 3(l) ⟶ PH 3(g) + 3H 3 PO 4(l). Answer: phosphorus trichloride
A: 
The best method for preparing pure phosphorous acid is by hydrolyzing what?