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In borax, two boron atoms are in triangular geometry and two boron atoms are in tetrahedral geometry. The ion is [B4O5(OH)4]2− and the remaining eight water molecules are associated with the two sodium ions. Hence, the borax contains tetranuclear units [B4O5(OH)4]2− and therefore is formulated as Na2[B4O5(OH)4].8H2O.Check out Chemistry Formulas and NCERT Solutions for class 11 Chemistry prepared by Physics Wallah.
Borax occurs naturally as tincal (contains about 55% borax) in certain inland lakes of India, Tibet and California (U.S.A.).Borax can also be prepared from certain other minerals such as boracite, colemanite and boranatrocalcite. The minerals are powdered and boiled with sodium carbonate solution.
Ca2B6O11 + 2Na2CO3 → 2CaCO3 + Na2B4O7 + 2NaBO2
Borax is crystallised from the filtrate. Sodium metaborate, present in the mother liquor, is converted into borax by passing carbon dioxide through it
4NaBO2 + CO2 → Na2B4O7 + Na2CO3
Borax is also obtained by the action of Na2CO3 on orthoboric acid.
Two important hydrates of borax are known. These are,
The pentahydrate is obtained when the solution is crystallised at above 60°C, while the monoclinic variety is obtained when crystallisation is carried out below 60°C. Both the varieties on heating form anhydrous borax.
Borax is a colourless, crystalline solid sparingly soluble in cold water but dissolves readily in hot water.
For manufacturing enamels, glazes and optical glass.
As a flux for soldering and welding.
As an analytical reagent, e.g. in borax bead test.
In borax bead test
In purifying gold