Is phosphoric acid ionic or covalent?

Question

We are doing a project on household products, I chose phosphoric acid. There was a part that asked if it was covalent or ionic. Here is my answer:

3. It is a covalent compound because it is made of nonmetals.

 

Here is my professors feedback.

 

Hydrogen can act either as a metal or a nonmetal depending on what it's attached to. In this case, the hydrogen is attached to a polyatomic ion. Parts of this compound are covalent, and parts are ionic. Can you distinguish between them?

 

Can someone help here?

Answer 1

Hydrogen is generally considered to be a non metal, and may act as a metal at extremely high pressures (not likely to be found on earth).  I think your professor is mistaken, and the H bonded to the O in H3PO4 would be a covalent bond.  Hydrogen is listed in group I, but unlike the alkali metals, it does NOT readily form H⁺, as the ionization energy is almost 3x that for Li to form Li⁺.  I would say that phosphoric acid (H₃PO₄) is a molecular (not ionic) compound, and molecular is another way of saying it is covalent.

Answer 2

Phosphoric acid is 

H₃PO₄

The Phosphate, PO₄ is the polyatomic ion.  The bonds within phosphate are covalent.  However, the bonds between Hydrogen and Phosphate are ionic.