What NEET Asks
- Questions often focus on the correct structural formula of hydrated borax.
- Reactions of borax upon heating and with acids are frequently tested.
- Properties like the nature of its aqueous solution (alkaline) and the borax bead test are important.
Key Points
- Borax, Na₂B₄O₇·10H₂O, is correctly formulated as Na₂[B₄O₅(OH)₄]·8H₂O, indicating 8 water molecules of crystallization and 2 coordinated as hydroxyl groups.
- The anion [B₄O₅(OH)₄]²⁻ is a tetranuclear unit comprising two trigonal planar BO₃ units and two tetrahedral BO₄ units.
- It contains four B-O-B bridging linkages.
- Borax is a white crystalline solid, sparingly soluble in cold water but readily soluble in hot water.
- Its aqueous solution is alkaline due to hydrolysis.
- On heating, it loses water, swells, forms a transparent glassy bead (sodium metaborate and boric anhydride).
Must-Know Formula / Reaction
- Borax structural formula:
Na₂[B₄O₅(OH)₄]·8H₂O - Hydrolysis:
Na₂B₄O₇ + 7H₂O → 2NaOH + 4H₃BO₃NaOH: Strong base formed upon hydrolysis.H₃BO₃: Weak acid formed upon hydrolysis.
- Heating:
Na₂B₄O₇·10H₂O (heat) → Na₂B₄O₇ (red heat) → 2NaBO₂ + B₂O₃NaBO₂: Sodium metaborate (part of borax bead).B₂O₃: Boric anhydride (part of borax bead).
Common Mistakes
- Students often use Na₂B₄O₇·10H₂O as the structural formula instead of Na₂[B₄O₅(OH)₄]·8H₂O, leading to incorrect calculations of B-O bonds or coordination numbers.
- Don't confuse the anhydrous borax (Na₂B₄O₇) with the glassy mixture (NaBO₂ + B₂O₃) formed on strong heating.
- Misinterpreting the alkaline nature of borax solution, forgetting it's due to the strong base (NaOH) formed upon hydrolysis.
Rapid Revision
Remember borax is Na₂[B₄O₅(OH)₄]·8H₂O with 2 trigonal and 2 tetrahedral boron atoms. Its aqueous solution is alkaline due to hydrolysis. Strong heating yields a glassy mixture of sodium metaborate and boric anhydride, crucial for the borax bead test.