# Borax and Boric Acid > Borax and boric acid for JEE Chemistry with bead test colours, Lewis-acid behaviour of B(OH)3, layered hydrogen-bonded structure and glycerol titration logic. - Canonical URL: https://prepelephant.com/topics/jee/chemistry/borax-boric-acid - Exam / course: JEE · Subject: Chemistry - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Borax and Boric Acid", PrepElephant, https://prepelephant.com/topics/jee/chemistry/borax-boric-acid ## Direct answer Borax, Na2B4O7·10H2O, and boric acid, H3BO3, are group 13's laboratory workhorses. Heated, borax swells as it loses water, then fuses to a transparent glassy bead that dissolves metal oxides into coloured metaborates — the borax bead test, cobalt deep blue and chromium green among the readings. Boric acid is a solid of layered sheets, planar BO3 units hydrogen-bonded into greasy layers, and despite its three hydroxyl groups a monobasic Lewis acid: it accepts OH− from water, B(OH)3 + 2H2O ⇌ [B(OH)4]− + H3O+, rather than donating any proton of its own. ## What you must remember - **Borax identity:** properly Na2[B4O5(OH)4]·8H2O; on heating it swells, gives anhydrous Na2B4O7, and at red heat decomposes to the glassy bead mixture 2NaBO2 + B2O3. - **Bead test palette:** cobalt metaborate blue, chromium green, copper blue, iron yellow-brown — colour on glass, read after cooling from the oxidising flame. - **Borax in water:** hydrolysis gives an alkaline solution, Na2B4O7 + 7H2O → 2NaOH + 4H3BO3; its stability and purity made borax a classical primary standard for acid titrations. - **Boric acid acidity:** very weak, Ka about 5.8 × 10^-10 as commonly quoted; monobasic because one OH− is accepted per B(OH)3, not three protons donated. - **The polyol trick:** with glycerol or mannitol, cyclic borate esters form and release protons — the effective acidity jumps enough for a sharp phenolphthalein titration with NaOH that is impossible on the bare acid. - **Thermal ladder of H3BO3:** near 370 K metaboric acid HBO2, on stronger heating tetraboric acid H2B4O7, and at red heat the B2O3 glass. - **Preparations:** Na2B4O7 + 2HCl + 5H2O → 4H3BO3 + 2NaCl; also vigorous hydrolysis of BCl3 to boric acid and HCl. - **Uses:** boric acid as a mild antiseptic eyewash powder, borax as a soldering flux and laundry booster, borosilicate glass, and perborate bleaches. ## Why one proton-equivalent, and how titration gets around it The molecule owns three OH groups and not one ionisable proton — boron's empty p orbital does the chemistry. Water's own hydroxide completes boron's octet as [B(OH)4]−, and the proton the water loses makes the solution acidic; with Ka near 10^-10, boric acid is hundreds of times weaker than acetic acid, and a direct titration with sodium hydroxide runs into a flat, useless endpoint. Add glycerol and the picture changes: the cis-diol chelates boron into a cyclic ester, and in forming that complex releases protons stoichiometrically — the apparent acidity climbs by orders of magnitude, phenolphthalein snaps to a clean pink, and the titration works. The bead test is the same boron-accepts-oxygen story in the dry: the sodium borate glass dissolves metal oxides from the hot loop, and cobalt oxide, for instance, reports in as blue cobalt(II) metaborate. Both experiments teach one idea — boron completes its sextet by accepting, and every usefully acidic proton was borrowed from water or a diol, never its own. ## Boron questions that decide ranks JEE Main keeps to statements and structures: monobasic Lewis acid (never tribasic Bronsted), the layered hydrogen-bonded lattice with its greasy feel, the bead colours, hydrolysis giving a basic borax solution. Advanced pushes the reasoning — why glycerol precedes the titration (an NCERT-paragraph classic), why the layers slide like a solid lubricant (planar sheets over hydrogen bonds), why BCl3 hydrolyses violently while CCl4 does not (empty orbital versus saturated octet). Two slips cost marks every session: calling boric acid tribasic, and predicting a strongly acidic pH for its solution — saturated boric acid sits only near pH 5. Group 13 chemistry remains on both current syllabi in trimmed form, with borax and boric acid among the named-chemistry anchors the exam keeps returning to. ## Frequently asked questions ### Why is boric acid monobasic despite three OH groups? It donates no proton; boron accepts an OH− from water to form [B(OH)4]−, releasing one equivalent of H+ per molecule — Lewis acidity, so one equivalent of base suffices. ### How does the borax bead produce characteristic colours? The fused sodium borate glass dissolves metal oxides as coloured metaborates — cobalt blue, chromium green, copper blue — read on the glass loop after cooling. ### Why is glycerol added before titrating boric acid with NaOH? The cis-diol forms a cyclic borate ester that releases protons, raising the effective acidity enough for a sharp phenolphthalein endpoint the bare acid cannot give. ### What happens when boric acid is heated progressively? It condenses through metaboric and tetraboric acids to a B2O3 glass at red heat — water leaves and B–O–B bridges build between the BO3 units. ### Why is an aqueous borax solution alkaline? Hydrolysis of the tetraborate generates sodium hydroxide alongside the very weak boric acid, leaving excess base in solution.