Chemistry Q134 – AP-EAMCET 2026

Home Chemistry Periodicity & ElementsQuestion #492

A boron compound

X

is obtained when aqueous solution of borax is treated with

H C l

. The incorrect statement about

X

Clue 1 of 3: Formation of Boric Acid

Borax reacts with strong acids like $H C l$ to produce boric acid. Identify the chemical formula of boric acid.

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Step 1: Identify compound X✦ Active

Borax ( ${Na}_{2} B_{4} O_{7} \cdot 10 H_{2} O$ ) when treated with $H C l$ in aqueous solution forms boric acid. The reaction is:

{Na}_{2} B_{4} O_{7} \cdot 10 H_{2} O + 2 HCl + 5 H_{2} O \to 4 H_{3} {BO}_{3} + 2 NaCl

Therefore, compound $X$ is boric acid ( $H_{3} {BO}_{3}$ ).

💡 Teacher's Secret Hint

Remember that borax is a common source for preparing boric acid. The stoichiometry of water molecules may vary depending on how concentrated the borax solution is and if anhydrous borax is considered, but the primary product remains boric acid.

Step 2: Analyze the structure and physical properties of Boric acid (X)○ Expand

Boric acid ( $H_{3} {BO}_{3}$ ) has a layered structure where planar ${BO}_{3}$ units are linked together by hydrogen bonds. It is sparingly soluble in cold water but its solubility increases significantly in hot water due to the weakening of hydrogen bonds between layers at higher temperatures.

💡 Teacher's Secret Hint

The layered structure of boric acid contributes to its flaky feel and explains its variable solubility with temperature.

Step 3: Determine the acidic nature and basicity of Boric acid (X)○ Expand

Boric acid is a weak monobasic Lewis acid. It does not donate protons directly but accepts a hydroxyl ion from water to release a proton.

H_{3} {BO}_{3} + H_{2} O ⇌ [{B(OH)}_{4}]^{-} + H^{+}

Despite having three hydrogen atoms, it is a monobasic acid because it effectively releases only one $H^{+}$ ion per molecule. Thus, its basicity is 1, not 3.

💡 Teacher's Secret Hint

Don't confuse the number of hydrogen atoms in the formula ( $H_{3} {BO}_{3}$ ) with its basicity. Boric acid is unique in its acidic behavior as a Lewis acid.

Step 4: Examine the thermal decomposition of Boric acid (X)○ Expand

Upon heating, boric acid first loses water to form metaboric acid ( ${HBO}_{2}$ ), then tetraboric acid ( $H_{2} B_{4} O_{7}$ ), and finally, at higher temperatures, forms boron trioxide ( $B_{2} O_{3}$ ), also known as boric anhydride.

H_{3} {BO}_{3} \overset{100^{\circ} C}{\to} {HBO}_{2} \overset{140^{\circ} C}{\to} H_{2} B_{4} O_{7} \overset{red heat}{\to} B_{2} O_{3}

$B_{2} O_{3}$ is an acidic oxide, as it reacts with water to form boric acid.

💡 Teacher's Secret Hint

This sequence of dehydration products is important. $B_{2} O_{3}$ is indeed an acidic oxide, consistent with boron being a non-metal.

Step 5: Identify the incorrect statement○ Expand

Based on the analysis:

- Statement 1: "It has layer structure" - Correct.

- Statement 2: "Its basicity is 3" - Incorrect. Boric acid is monobasic (basicity 1).

- Statement 3: "On heating it gives an acidic oxide" - Correct ( $B_{2} O_{3}$ ).

- Statement 4: "It is highly soluble in hot water" - Correct.

Therefore, the incorrect statement is "Its basicity is 3".

💡 Teacher's Secret Hint

Always double-check the question to ensure you are selecting the *incorrect* statement, not the correct one.

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Chemistry Question 134 – AP-EAMCET 2026

Clue 1 of 3: Formation of Boric Acid

🧠 Full Solution Path