chornedsnorkack
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Is diphosphorus pentoxide hygroscopic?
Seriously.
Compare the example of phosphorus itself.
When phosphorus vapours are rapidly condensed, P4 condenses. P4 melts at 44 Celsius and boils at 280 Celsius.
But on prolonged heating, P4 is converted into P. Red P sublimes at 430 C without melting, and under pressure melts at 590 C.
Their reactivity is quite different!
P4 reacts with air to glow in dark at room temperature and readily ignites at room temperature - red P only burns above 200 C.
P4 is dismuted by hot aqueous alkali, like
P4+3KOH+3H2O=PH3+3KPH2O2
Red P is not.
P4 readily reacts with body to form poisonous amino bisphosphonates - red P does not.
Now compare phosphorus pentoxide...
On quick cooling, hexagonal P4O10 is condensed.
It sublimes at 360 C without melting, and under pressure melts at 423 C.
And P4O10 is notoriously hygroscopic.
But on prolonged heating, P4O10 converts into P2O5, which is orthorhombic, and melts at 580 C into a
viscous liquid which boils about 600 C.
Considering the vapour pressure difference between hexagonal P4O10 and orthorhombic P2O5, is their
chemical reactivity also different? Is P2O5 hygroscopic?
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njl
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Yeah
Reflux condenser?? I barely know her!
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Tsjerk
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Hygroscopicity is a physical property which happens via absorption or adsorption, so P2O5 (P4O10) can't be hygroscopic because it reacts with water.
https://en.m.wikipedia.org/wiki/Hygroscopy
Also P2O5 is not a molecule.
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karlos³
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P4O10, sorry 
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chornedsnorkack
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Indeed it is not.
The hexagonal form consists of distinct molecules - of P4O10.
However, the orthorhombic form is infinite covalent polymer that does not consist of molecules. Therefore it is P2O5, not
P4O10.
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Tsjerk
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No, it would be P2O5(n).
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