Boric acid is an acid because its molecule

1. Contains replaceable H+ ion

2. Gives up a proton

3. Accepts OH- from water releasing proton

4. Combines with proton from water molecule


Hint: Boric acid, that is, H3BO3 is a lewis acid.

Lewis acids are substances whose octet is incomplete and accepts electrons.
Boric acid is a monobasic weak acid. It does not liberate H+ ion but accepts electrons from OH- ion i.e., behaves as Lewis acid.
or, B(OH)3+2H2O[B(OH)4]-+H3O+

The structure of H3BO3 is as shown below, where the octet of boron in H3BO3 is incomplete.