On the addition of mineral acid to an aqueous solution of borax, which of the following compound is formed?

The correct answer is: Orthoboric acid

Step 1: Understand the compound – Borax

Borax is a sodium borate salt with the chemical formula:

${\mathrm{Na}}_2{B}_4{O}_7·10H_{2}O$

It is also known as sodium tetraborate decahydrate. In aqueous solution, borax dissolves and creates a mildly alkaline (basic) environment.

Step 2: Identify the mineral acid

A mineral acid refers to any strong inorganic acid such as hydrochloric acid (HCl), sulfuric acid (H₂SO₄), or nitric acid (HNO₃). Let us use HCl as a representative example to write the balanced reaction.

Step 3: Write the chemical reaction

When a mineral acid like HCl is added to an aqueous solution of borax, the following reaction takes place:

${\mathrm{Na}}_2{B}_4{O}_7·10H_{2}O+2\mathrm{HCl}\to 2\mathrm{NaCl}+4H_{3}B{O}_3+5H_{2}O$

Step 4: Identify the boron-containing product

The boron-containing product formed in this reaction is H₃BO₃, which is called orthoboric acid (also commonly known as boric acid). It is a weak, monobasic acid.

Step 5: Why not the other options?

• Boron hydride (e.g., B₂H₆) – Boron hydrides are not formed by the reaction of borax with mineral acids. They require special reducing agents like NaBH₄ or LiAlH₄ for their preparation.

• Metaboric acid (HBO₂) – Metaboric acid is obtained when orthoboric acid is heated above 100°C, causing dehydration. It is not a direct product of the borax–acid reaction.

• Pyroboric acid (H₂B₄O₇) – Pyroboric acid is formed on further heating of metaboric acid. It is also not produced directly from the reaction of borax with a mineral acid in aqueous solution.

Conclusion: When a mineral acid is added to an aqueous solution of borax, the product formed is orthoboric acid (H₃BO₃), along with the corresponding sodium salt of the acid used.