Q.An aqueous solution of borax is

Borax, Na₂B₄O₇.10H₂O, is the sodium salt of tetraboric acid, obtained from colemanite ore by boiling its solution with sodium carbonate: 2Ca₂B₆O₁₁ + 3Na₂CO₃ + H₂O → 3Na₂B₄O₇ + 3CaCO₃ + Ca(OH)₂.

Although conventionally formulated Na₂B₄O₇.10H₂O, borax's true structural unit is the tetranuclear [B₄O₅(OH)₄]²⁻ ion -- this is properly the prismatic form; borax also exists as jeweller's (octahedral) borax, Na₂B₄O₇.5H₂O, and as anhydrous borax glass, Na₂B₄O₇.

Borax is basic: in hot water it hydrolyses to boric acid and sodium hydroxide, making its hot solution alkaline (Na₂B₄O₇ + 7H₂O → 4H₃BO₃ + 2NaOH). On heating it first loses its water of hydration and then forms a clear, transparent bead (→ 2NaBO₂ + B₂O₃) -- the basis of the classic borax-bead test for identifying coloured metal ions. It reacts with acids to precipitate the sparingly soluble boric acid, and with ammonium chloride to give boron nitride.

Uses: identifying coloured metal ions via the borax bead; manufacturing optical and borosilicate glass, enamels and pottery glazes; and as a metallurgical flux and as a preservative.