Explain physical and chemical properties of borax. 

Borax : It is the most important compound of boron.

Formula : $\mathrm{Na}_{2} \mathrm{~B}_{4} \mathrm{O}_{7} \cdot 10 \mathrm{H}_{2} \mathrm{O}$

In fact it contains the tetra nuclear units of $\left[\mathrm{B}_{4} \mathrm{O}_{5}(\mathrm{OH})_{3}\right]^{2-}$ and

Correct formula : $\mathrm{Na}_{2}\left[\mathrm{~B}_{4} \mathrm{O}_{5}(\mathrm{OH})_{4}\right] \cdot 8 \mathrm{H}_{2} \mathrm{O}$

Borax dissolves in water to give an alkaline solution.

$\mathrm{Na}_{2} \mathrm{~B}_{4} \mathrm{O}_{7}+7 \mathrm{H}_{2} \mathrm{O} \rightarrow 2 \mathrm{NaOH}+4 \mathrm{H}_{3} \mathrm{BO}_{3}$

Orthoboric acid

Borax bead test : On heating, borax first loses water molecules and swells up. On further heating it turns into a transparent liquid, which solidifies in to glass like material known as borax bead.

$\mathrm{Na}_{2} \mathrm{~B}_{4} \mathrm{O}_{7} \cdot 10 \mathrm{H}_{2} \mathrm{O} \stackrel{\Delta}{\longrightarrow} \mathrm{Na}_{2} \mathrm{~B}_{4} \mathrm{O}_{7} \stackrel{\Delta}{\longrightarrow} 2 \mathrm{NaBO}_{2}+\mathrm{B}_{2} \mathrm{O}_{3}$

Sodium metaborate Boric anhydride

The metaborates of many transition metals have characteristic colours and therefore, borax bead test can be used to identify them in the laboratory. For example, when borax is heated in a Bunsen burner flame with $\mathrm{CoO}$ on a loop of platinum wire, a blue coloured $\mathrm{Co}\left(\mathrm{BO}_{2}\right)_{2}$ bead is formed.