Boric acid is an acid because its molecule:

accepts OH<sup>-</sup> from water

combines with proton from water molecule

contains replaceable H<sup>+</sup> ion

gives up a proton

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This question was previously asked in

UPSC NDA-2 – 2015

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Boric acid (H₃BO₃) is a weak monoprotic Lewis acid. It does not directly donate a proton (H⁺) from its own molecule in aqueous solution. Instead, it accepts a hydroxide ion (OH⁻) from a water molecule, releasing a proton from the water molecule in the process. The reaction is: H₃BO₃ + H₂O ⇌ [B(OH)₄]⁻ + H⁺. Therefore, it acts as an acid by accepting an OH⁻ ion from water.

Boric acid’s acidity is explained by its behavior as a Lewis acid, accepting an electron pair from the hydroxide ion of water.

Although boric acid contains three hydrogen atoms, they are not easily ionizable as protons in the way that typical Brønsted-Lowry acids like HCl or H₂SO₄ are. Its acidic nature arises from its ability to form a coordinate covalent bond with the hydroxide ion.

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