4.4.27 Reaction of Phosphorous Pentoxide with Water

1. Phosphorous pentoxide is placed in a crucible 2. Addition of water. A strong reaction takes place
Phosphorous pentoxide (white) is placed in a crucible Water is squirted in. A strong reaction takes place
3. The reaction is strongly exothermic. Water vapor is formed 4. Addition of an indicator, showing the resulting solution is acidic
The reaction is highly exothermic. Water vapor forms An indicator is added. The resulting solution tests acidic
Animation of Experiment (GIF Format, 321 k)
Animation of Experiment (MPEG Format, 498 k)

Photo1: Experimental setup: 10 g phosphorous pentoxide (P2O5) is placed in a crucible.
Photo2: Adding water causes a strong reaction.
Photo3: The reaction is so strongly exothermic that a part of the water added evaporates immediately.
Photo4: Adding a universal indicator shows that the resulting solution is acidic.

An important property of phosphorous pentoxide, which normally exists as dimeric (P4O10) , is its high reactivity to water. The reaction produces first the intermediate product metaphosphoric acid, then polyphosphoric acid, and then finally the end product ortophosphoric acid (H3PO4):

P4O10water H4P4O12water 2 H4P2O7water 4 H3PO4

Phosphorous pentoxide is one of the strongest dehydrating agents, which is why it is used as a drying agent or for removing water from chemical bonds.