[AndersHoveland]

White phosphorous is a highly reactive element, which is spontaneously oxidized in the presence of air. It can, however, be stored under water without any observable reaction. But despite this apparent unreactivity, I am going to make an argument that white phosphorous does indeed react with water.

First, in support of the argument, let us do a calculation to see if such a reaction would be energetically favorable.
2 P + 3 H2O --> PH3 + H3PO3
The heat of formation of solid crystals of phosphorous acid is apparently -964.4 kJ/mol, while a different source gave it as -972. The heat of formation of crystals of  phosphorous acid dissolved in an infinite quantity of water is -6.3 ±0.6 kJ/mol. The heat of formation of phosphine is +5.4 kJ/mol (PH3 prefers to decompose into its separate elements). The heat of formation of water is -285.8 kJ/mol. White phosphorous, as an element, has a value of 0, while red phosphorous is -17.5 kJ/mol.
-972 + (-6.3) = -285.8
0 + 3(-285.8 ) = -857.4
+5.4 - 964.4 = -959
Since one mole of PH3 and one mole of aqueous H3PO3 have have a net heat of formation lower than phosphorous and three moles of water, we would expect that such a reaction would be energetically favorable.


From the Literature

We can also look at some reactions involving phosphorous chemistry to provide support for the idea.

Phophorous can be stored under water, but when finely divided it decomposes water producing hydrogen phosphide. When boiled with water, phosphine and hypophosphorous acid are produced. (this information came from an online book, but unfortunately I neglected to record the reference when I put this information in my notes)

One old source says that the reaction of sodium phosphide with water generates small portions of phosphites and hydrogen gas, in addition to the main products of sodium hydroxide and phosphine, although this could potentially have been caused by impurities.
investigation of sodamide and of its reaction-products with phosphorus" William Phillips Winter p42-43

Aqueous alkali (KOH) dissolves red phopshorous, with the formation of phosphine. Interestingly, when hydrochloric acid was added to the solution, the phosphorous precipitated back into its elemental form. This could suggest an equilibrium, which shifts depending on pH.
Journal of the Chemical Society, Volume 75 (Great Britain) p.976


Explaining the Lack of Observed Reactivity

So why is there no obvious reaction when bulk white phoshorous is placed in water? There are several possible explanations. There could be a coat of something forming on the surface that prevents further reaction. For example, trying to dissolve large pieces of phosphorous pentoxide in water can be difficult if it has not been finely divided before. It has a tendency to form a protective viscous coating that inhibits further hydrolysis. In the case of elemental phosphorous reacting with water, it might be analogous to trying to hydrate silica. Si(OH)4 is quite stable, either as a solid or in aqueous solution, but it is essentially impossible to make bulk solid SiO2 react with water. Another possible reason could be activitation energy, although this seems less likely.

[dipesh747]

What experiments have you done? Have you put it in as a block of solid in water or have you broken it down into powder so there is a higher surface area to volume ratio ?

[cheese (MSW)]

Can no one point out the fundamental flaw in Anders' argument?  Hint: Given that the  ΔGf° for diamond is +2.9 kJ mol^-1 when was the last time you saw a diamond spontaneously crumble into pencil lead?
(The spelling of element 15 is phosphorus.)

[cheese (MSW)]

A thermodynamically disallowed process will never proceed in the direction written unless energy is added (sunlight in photosynthesis, electricity in electrolysis are well known examples).
To say a thermodynamically allowed process is spontaneous is however misleading because thermodynamics tells you nothing about the kinetic stability: the activation barrier that has to be overcome in order for the rxn to proceed.  For molecular cmpds the barrier is governed mainly by the interaction of the frontier orbitals as first put forward by Fukui (81 Nobel Prize). [Steric factors can play a part; e.g.,  inertness of SF6.] The H*OMO-LUMO interaction must match in symmetry and approximate energy for there to be a low activation barrier for rxn.   The frontier orbitals are governed by quantum mechanics.  The activation may be small (F2  + H2) or large diamond →graphite.  All living things (us!) are thermodynamically unstable wrt CO2 and H2O, but quantum mechanics prevents spontaneous combustion (whoa that’s lucky!).  Applying to P4/H2O: P4 HOMO four P lps; LUMO P3d AOs (access blocked?); H2O: H*OMO: O lp, LUMO O-H σ*(too high in energy). White P4 is stable indefinitely under water in a sealed container.   P4 does react with O2/H2O (that is accompanied by chemiluminescence) but O2 has a unique MO scheme (it’s H*OMO and LOMO are about the same).  Red P4 is stable in air.  OH^- does react with white P4 in boiling H2O to give PH3 (phosphine) and hypophosphites [1].   This can be explained by lp on OH^- having higher energy and to have significant overlap with the P 3d for rxn to occur at 100°C.     
[1] Encyclopedia of Inorganic Chemistry, R. B. King ed 1994, Vol 6, 3149 [chemistry not MO explanation which is mine].