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Choose the Best Answer · Q2

Q.Boric acid is an acid because its molecule (NEET)

a) contains replaceable H⁺ ion
b) gives up a proton
c) combines with proton to form water molecule
d) accepts OH⁻ from water, releasing proton
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✓ Free question

Step 1. An ordinary Brønsted acid is acidic because ITS OWN molecule donates a proton (H⁺) directly.

Step 2. Boric acid, B(OH)₃, has no dissociable H⁺ of its own to donate in that simple sense — rule out (a) and (b).

Step 3. Instead, the electron-deficient boron atom in B(OH)₃ accepts a hydroxyl ion (OH⁻) from a surrounding water molecule, forming the tetrahedral [B(OH)₄]⁻ ion: B(OH)₃ + 2H₂O ⇌ H₃O⁺ + [B(OH)₄]⁻.

Step 4. This equilibrium releases a proton from the water molecule that gave up its OH⁻, which is what actually makes the solution acidic — a Lewis-acid mechanism, not a classical Brønsted one. Option (c), 'combines with proton to form water,' describes basic behaviour, the opposite of what happens.

✓Final answer

(d) accepts OH⁻ from water, releasing proton — boric acid acts as a Lewis acid toward water's hydroxide, not as a classical proton donor.

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