Q. Show properties of the Borax?

Salts of boric acids are known as borates. As said earlier, hydrated borates occur naturally, e.g., borax, Na₂B₆0₁₁. 5H₂0, kernite, Na₂ + 0₄H₂0. colemanite.  Ca₂B₆0₁₁,.5H₂0, etc. Anhydrous borates can be made by fusion of boric acid and metal oxides.

Sodium tetraborate  decahydrate, Na₂B₄+0₇+10H₂0, is commonly known as boric. It occurs in certain lakes in India, Tibet and U.S.A: It is obtained by extracting impure borax with water and concentrating the solution until crystals of borax separate out.

Borax can also be prepared from the mineral colemanite by boiling it with Na₂CO₃ solution:

Ca₂B₆O₁₁+ 2Na₂C0₃ -----------------------------------------------> Na₂B₄0₇ + 2NaB0₂ + 2CaC0₃

Borax is crystallised from the filtrate after removing insoluble CaC0₃, By passing   CO₂, NaB0₂ present in the mother liquor is converted into borax:

4NaB0₂ + C0₂---------------------------> Na₂B₄0₇ + Na₂C0₃

Borax is a white crystalline solid. It is hydrolysed by water to give an alkaline solution:

Na₂B₄O₇+ 7H₂O-----------------> 4H₃BO₃+ 2NaOH

On heating, borax loses water to become anhydrous. Anhydrous borax on strong heating with NH₄Cl gives boron nitride and boron trioxitde:

Na₂B₄0₇ + 2NH₄Cl ------------> 2BN'+ B₂O₃ + 2NaCl + 4H₂0

On heating alone, it decomposes to form NaBO₂ and B₂03

Na₂ B₄O₇------------------------> 2NaBO₂+ B₂O3

Borax possesses an anion [B₄O₅ (OH)₄] 2- having both  three and four coordinated boron as shown in fig. 6.8.