How does a group 15 oxide react with water? Given the following reactants:


  $\ce{P2O5}$ and $\ce{H2O}$


How does one determine what product the two will form in a reaction?

I instantly tried to treat the problem as a double displacement reaction. However, the compounds that I got by doing so were absurd. Do double replacement reactions only occur with ionic compounds (e.g., the cations of two ions switch)?

The two elements listed above form $\ce{H3PO4}$. How do I even go about determining that? What indicates that such a product forms from those two reactants?
The reaction between the two substance you have mentioned above in synthesis 
it is an acid-base reaction .
and about the other question you asked above i can say that we have reactions where two ionic substances take part in oxidation reduction reactions instead of double replacement.